Interfacing DDR SDRAM with

Stratix & Stratix GX Devices

December 2005 ver. 2.0 Application Note 342

Introduction
Traditionally, systems featuring FPGAs used single data rate (SDR)
SDRAM, which transmits data on each rising edge of the clock signal. The
total amount of data an SDR memory device can send or receive is equal
to the clock speed multiplied by the bus width. To increase the data-rate
transmission, one of those parameters must increase. With dual-edge
clocking, double data rate (DDR) SDRAM can transmit data on both the
rising and falling edge of the clock signal. DDR SDRAM effectively
doubles the amount of data sent compared to SDR SDRAM without
increasing the clock speed or the bus width.

DDR SDRAM devices are widely used for a broad range of applications
such as embedded processor systems, image processing, storage,
communications, and networking. Stratix and Stratix GX devices can
interface with DDR SDRAM in component or module configurations up
to 200 MHz/400 Mbps. Tables 1 and 2 show the DDR SDRAM interface
support in Stratix and Stratix GX devices.

Table 1. DDR SDRAM Support in Stratix EP1S10 Through EP1S40 Devices & All Stratix GX Devices (Part
1 of 2)

Maximum Clock Rate

DDR -5 Speed
I/O -6 Speed Grade -7 Speed Grade -8 Speed Grade
Memory Grade
Standard
Type
Wire- Flip- Wire- Flip- Wire-
Flip-Chip Flip-Chip
Bond Chip Bond Chip Bond
DDR SDRAM SSTL-2 200 MHz (3) 167 MHz 133 MHz 133 MHz 100 MHz 100 MHz 100 MHz
(1), (2)

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Table 1. DDR SDRAM Support in Stratix EP1S10 Through EP1S40 Devices & All Stratix GX Devices (Part
2 of 2)

Maximum Clock Rate

DDR -5 Speed
I/O -6 Speed Grade -7 Speed Grade -8 Speed Grade
Memory Grade
Standard
Type
Wire- Flip- Wire- Flip- Wire-
Flip-Chip Flip-Chip
Bond Chip Bond Chip Bond
DDR SSTL-2 150 MHz 133 MHz 110 MHz 133 MHz 100 MHz 100 MHz 100 MHz
SDRAM, side
banks (2), (4)

Notes to Table 1:
(1) These maximum clock rates apply if the Stratix or Stratix GX device uses DQS phase-shift circuitry to interface with
DDR SDRAM. DQS phase-shift circuitry is only available in the top and bottom I/O banks (I/O banks 3, 4, 7, and
8).
(2) You should use the minimum drive strength setting in the Quartus II software for DDR SDRAM interfaces in Stratix
and Stratix GX devices.
(3) To achieve 200 MHz interface speed, you should use loading of 10pF or less for Class II termination.
(4) DDR SDRAM is supported on the Stratix device side banks (I/O banks 1, 2, 5, and 6) without dedicated DQS
phase-shift circuitry. The read DQS signal is ignored in this mode.

Table 2. DDR SDRAM Support in Stratix EP1S60 Through EP1S80 Devices

Maximum Clock Rate

DDR Memory Type I/O Standard -5 Speed Grade -6 Speed Grade -7 Speed Grade

Flip-Chip Flip-Chip Flip-Chip

DDR SDRAM (1), (2) SSTL-2 167 MHz 167 MHz 133 MHz
DDR SDRAM, side SSTL-2 150 MHz 133 MHz 133 MHz
banks (2), (3)

Notes to Table 2:
(1) These maximum clock rates apply if the Altera® Stratix or Stratix GX device uses DQS phase-shift circuitry to
interface with DDR SDRAM. DQS phase-shift circuitry is only available in the top and bottom I/O banks (I/O
banks 3, 4, 7, and 8).
(2) You should use the minimum drive strength setting in the Quartus II software for DDR SDRAM interfaces in
Stratix and Stratix GX devices.
(3) DDR SDRAM is supported on the Stratix device side banks (I/O banks 1, 2, 5, and 6) without dedicated DQS
phase-shift circuitry. The read DQS signal is ignored in this mode.

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 Functional Description

DDR SDRAM design requires both internal controller logic and a

physical interface (often called the data path). Altera’s DDR SDRAM
Controller MegaCore® function provides both the core and physical
interface portions. You should always ensure your system is designed
using the physical interface packaged with the IP Controller even if you
will be designing your own core logic. This enables you to use the same
physical interface that was tested by Altera and proven as robust. The
MegaCore function gives you open source code for the data path and
DQS postamble logic in addition to handling the timing analysis for the
system, resynchronization, and DQS postamble logic timings. With the
Altera DDR SDRAM Controller MegaCore function, you can easily
generate a design that can be tested on the board using OpenCore® Plus
hardware evaluation in minutes. You can then replace the encrypted part
of the MegaCore function with your own DDR SDRAM state machine
controller should you wish to do so.

This application note describes the DDR SDRAM interfacing in

Stratix and Stratix GX devices. It also describes the timing analysis for the
DDR SDRAM interface with Stratix and Stratix GX devices.

Functional DDR SDRAM is a 2n-prefetch architecture with two data transfers per
clock cycle. It uses a strobe, DQS, that is associated with a group of data
Description pins (DQ) for read and write operations. Both the DQS and DQ ports are
bidirectional. Address ports are shared for read and write operations.

Write and read operations are sent in bursts, and DDR SDRAM supports
burst lengths of two, four, and eight. You provide two, four, or eight
groups of data for each write transaction and receive two, four, or eight
groups of data for each read transaction. The interval between when the
read command is clocked into memory and the data is presented at the
memory pins is called the column address strobe (CAS) latency. DDR
SDRAM supports CAS latencies of 2, 2.5, and 3, depending on the
operating frequency. Both the burst length and CAS latency are set in the
DDR SDRAM mode register.

DDR SDRAM devices use the SSTL-2 class II I/O standard and can hold
between 64 Mb to 1 Gb of data, according to the JEDEC specification. Each
device is divided into four banks, and each bank has a fixed number of
rows and columns. Only one row per bank can be accessed at one time.
The ACTIVE command opens a row, and the PRECHARGE command
closes a row.

For data reads, a delay-locked loop (DLL) inside the DDR SDRAM edge-
aligns the DQ and DQS signals with respect to CK. The DLL must be
turned on for normal operation, but can be turned off to save power or for
debugging purposes. (All timing analyses in this document assume that

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the DLL is on.) DDR SDRAM also has adjustable output drive strength.
Altera recommends using a minimum drive strength setting on Altera
devices.

f For more information on DDR SDRAM specifications, go to

www.jedec.org.

Interface Pins Table 3 describes the DDR SDRAM interface pins and how to connect
them to Stratix and Stratix GX devices on the top and bottom I/O banks
whether or not you are using the DQS phase-shift circuitry. On the side
banks, connect DQ and DQS pins to Stratix and Stratix GX user I/O pins.

Table 3. DDR SDRAM Interface Pins

Stratix & Stratix GX

Pins Description
Pin Utilization
DQ Bidirectional read/write data DQ
DQS Bidirectional read/write data strobe DQS
CK System clock User I/O pin
CK# System clock User I/O pin
DM Optional write data mask, edge-aligned User I/O pin
to DQ during write
All other Addresses and commands User I/O pin

Clocks, Strobes & Data

The DDR SDRAM device uses the differential CK and CK# clock signals
to clock commands and addresses into the memory. The memory also
uses these signals to generate the DQS signal during a read via a DLL
inside the memory. The skew between CK or CK# and the SDRAM-
generated DQS signal is specified as tDQSCK in a DDR SDRAM data sheet.

The DQ and DQS signals are both bidirectional (the same signals are used
for both writes and reads). A group of DQ pins is associated with one
DQS pin. In 8 and 16 DDR SDRAM devices, one DQS pin is associated
with 8 DQ pins (Stratix and Stratix GX 8 mode definition). Use the DQS
pins and their associated DQ pins listed in the Stratix and Stratix GX pin
tables when interfacing with DDR SDRAM from Stratix and Stratix GX
I/O banks 3, 4, 7, or 8. When interfacing DDR SDRAM from Stratix and
Stratix GX I/O banks 1, 2, 5, and 6, use any of the user I/O pins in those
banks as DQS pins. I/O banks 1, 2, 5, and 6 do not have dedicated phase-
shift circuitry and can only support up to 150-MHz DDR SDRAM
interfaces.

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Table 4 and 5 show the number of DQS/DQ groups supported in

Stratix and Stratix GX devices, respectively.

Table 4. DQS & DQ Bus Mode Support in Stratix Devices Note (1)

Number of Number of Number of

Device Package
8 Groups 16 Groups 32 Groups
EP1S10 672-pin BGA 12 (2) 0 0
672-pin FineLine BGA
484-pin FineLine BGA 16 (3) 0 4
780-pin FineLine BGA
EP1S20 484-pin FineLine BGA 18 (4) 7 (5) 4
672-pin BGA 16 (3) 7 (5) 4
672-pin FineLine BGA
780-pin FineLine BGA 20 7 (5) 4
EP1S25 672-pin BGA 16 (3) 8 4
672-pin FineLine BGA
780-pin FineLine BGA 20 8 4
1,020-pin FineLine BGA
EP1S30 956-pin BGA 20 8 4
780-pin FineLine BGA
1,020-pin FineLine BGA
EP1S40 956-pin BGA 20 8 4
1,020-pin FineLine BGA
1,508-pin FineLine BGA
EP1S60 956-pin BGA 20 8 4
1,020-pin FineLine BGA
1,508-pin FineLine BGA
EP1S80 956-pin BGA 20 8 4
1,508-pin FineLine BGA
1,923-pin FineLine BGA

Notes to Table 4:
(1) See the Using Selectable I/O Standards in Stratix & Stratix GX Devices chapter in the Stratix Device Handbook, volume 2
for VREF guidelines.
(2) These packages have six groups in I/O banks 3 and 4 and six groups in I/O banks 7 and 8.
(3) These packages have eight groups in I/O banks 3 and 4 and eight groups in I/O banks 7 and 8.
(4) This package has nine groups in I/O banks 3 and 4 and nine groups in I/O banks 7 and 8.
(5) These packages have three groups in I/O banks 3 and 4 and four groups in I/O banks 7 and 8.

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Table 5. DQS & DQ Bus Mode Support in Stratix GX Devices Note (1)

Number of Number of Number of

Device Package
8 Groups 16 Groups 32 Groups
EP1SGX10 672-pin FineLine BGA 12 (2) 0 0

EP1SGX25 672-pin FineLine BGA 16 (3) 8 4

1,020-pin FineLine BGA 20 8 4

EP1SGX40 1,020-pin FineLine BGA 20 8 4

Notes to Table 5:
(1) See the Using Selectable I/O Standards in Stratix & Stratix GX Devices chapter in the Stratix Device Handbook, volume 2
for VREF guidelines.
(2) These packages have six groups in I/O banks 3 and 4 and six groups in I/O banks 7 and 8.
(3) These packages have eight groups in I/O banks 3 and 4 and eight groups in I/O banks 7 and 8.

The data signals (DQ) are edge-aligned with the DQS signal during a read
from the memory and center-aligned with the DQS signal during a write
to the memory. The memory controller shifts the DQS signal during a
write to center-align the DQ and DQS signals, and shifts the DQS signal
during a read so that the DQ and DQS signals are center-aligned at the
capture register. Stratix and Stratix GX devices use a phase-locked loop
(PLL) to center-align the DQS signal with respect to the DQ signals
during writes, and use dedicated DQS phase-shift circuitry to shift the
incoming DQS signal during reads. Figure 1 shows an example where the
DQS signal is center-aligned during a burst-of-two read. Figure 2 shows
an example of the relationship between the data and the data strobe
during a burst-of-two write.

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Figure 1. DQS Signal Shift During Burst-of-Two Read

Pin to register
delay

DQS at
FPGA Pin Preamble Postamble

DQ at
FPGA Pin

DQS at DQ
IOE registers

DQ at DQ
IOE registers

90 degree shift Pin to register

delay

Figure 2. Data & Data Strobe During Burst-of-Two Write

DQS at
FPGA Pin

DQ at
FPGA Pin

The memory device’s setup (tDS) and hold times (tDH) for the DQ and DM
pins during a write are relative to the edges of DQS write signals and not
the CK or CK# clock. These times are equal (tDS = tDH) and typically 0.4 ns
for a 200-MHz DDR SDRAM device.

The DQS signal is typically generated on the positive edge of the system
clock (because of the tDQSS requirement described below). The DQ and
data mask (DM) signals are clocked using a –90° shifted clock from the
system clock. The edges of DQS are centered on the DQ and DM signals
when they arrive at the DDR SDRAM.

The DQS, DQ, and DM board trace lengths should be similar to minimize
the skew in the arrival time of these signals.

The DDR SDRAM has a write-requirement tDQSS, that requires the

positive edge of DQS on writes to be within 25% (90°) of the positive edge
of the DDR SDRAM clock input. Therefore, use the IOE’s DDR register to
generate the CK and CK# signals to match the DQS signal and reduce any

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process, voltage, and temperature variations.

To improve resynchronization for the 200-MHz DDR SDRAM interfaces,

route the CK signal from the memory pin back to the Stratix or Stratix GX
device. You can also use a separate output pin for the feedback clock. See
“Read-Side Implementation Using the DQS Phase-Shift Circuitry” on
page 9 for resynchronization details.

DM & Optional ECC Pins

The DDR SDRAM uses the data mask (DM) pins during a write. Driving
the DM pins low marks that the write is valid. The memory masks the DQ
signals if the DM pins are driven high. You can use any of the I/O pins in
the same bank as the associated DQS/DQ pins to generate the DM signal.

The DM timing requirements at the DDR SDRAM input are identical to

those for DQ data. The DDR registers, clocked by the –90° shifted clock,
create the DM signals.

Some DDR SDRAM devices support error correction coding (ECC) to

detect and automatically correct errors in data transmission. 72-bit DDR
SDRAM modules contain 8 ECC pins in addition to 64 data pins. Connect
the DDR ECC pins to a Stratix or Stratix GX device’s DQS/DQ group. The
controller requires extra logic to encode and decode the ECC data.

Commands & Addresses

Commands and addresses in DDR SDRAM devices are clocked into the
memory using the CK and CK# signals at single data rate using only one
clock edge. DDR SDRAM devices have 12 to 14 address pins, depending
on the device capacity. The address pins are multiplexed, so two clock
cycles are required to send the row, column, and bank addresses. The CS,
RAS, CAS, and WE pins are DDR SDRAM command pins.

The DDR SDRAM address and command inputs require the same setup
and hold times with respect to the DDR SDRAM clock. The Stratix and
Stratix GX device’s address and command signals change at the same
time as the DQS write signal because they are both generated from the
system clock. The positive edge of the DDR SDRAM clock, CK, is aligned
with DQS to satisfy tDQSS. If the command and address outputs are
generated on the clock’s positive edge, they may not meet the hold time
requirements (Figure 3). Therefore, you should use the negative edge of
the system clock for the commands and addresses to the DDR SDRAM.
You can use any of the I/O pins for the commands and addresses.

Figure 3 shows the address and command timing and the DDR SDRAM
tDQSS, tDS, and tDH timing requirements.

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Figure 3. Address & Command Timing Notes (1), (2)

System Clock

CK at Stratix or Stratix GX
Device Pin

SDRAM Write Requirement tDQSS (SDRAM)

DQS Write (at FPGA Pin)

SDRAM Address/Command
tSU (SDRAM)
Input Timing to the DDR
SDRAM Device
tCO tH (SDRAM)
(FPGA)
Address/Command Pins
(Positive Edge)
Address/Command Pins
(Negative Edge)

tCO
(FPGA)

Notes to Figure 3:
(1) The address and command timing shown in Figure 3 is applicable for both read and write.
(2) If the board trace lengths for the DQS, CK, address, and command pins are the same, the signal relationships at the
Stratix and Stratix GX device pins are maintained at the DDR SDRAM pins.

Read-Side There is one DQS phase-shift circuit available on top of the device and one
on the bottom of the device. Each DQS phase-shift circuit requires an
Implementation input reference clock. The DQS phase-shift circuitry shifts the DQS signal
Using the DQS to center-align the signal with the DQ signal at the IOE register, ensuring
the data is latched at the IOE register. The DQS signal is then inverted
Phase-Shift before going to the DQ IOE clock ports, as described in the External
Memory Interfaces chapter of the Stratix Device Handbook, volume 2.
Circuitry
Figure 4 shows how the Stratix and Stratix GX devices generate the DQ,
DQS, CK, and CK# signals. The write PLL generates the system clock and
the –90° shifted clock (write clock). The write PLL’s input clock can be the
same or a different frequency as the DDR SDRAM frequency of operation.
If the frequencies are different, you must provide the input reference
clock to the DQS phase-shift circuitry from another input clock pin. For
details, see the External Memory Interfaces chapter in the Stratix Device
Handbook, volume 2. The system clock and write clock have the same
frequency as the DQS frequency. The write clock is –90° shifted from the
system clock.

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Figure 4. DDR SDRAM With DQS Phase-Shift Circuitry

LE IOE

DM

Length = l2
DDR

DQ "write" (1)

DDR Length = l2
Shifted -90˚

Write
Clock CK and CK#
input_clk System
Write PLL DDR
Clock Length = l1
Resynchronization DDR SDRAM
Clock

DQS "write" (1)

DDR Length = l2

DQS "read" (1)

Length = l2
DQS Phase-
Shift Circuitry
DQ "read" (1)

Length = l2

DDR
DDR DDR
(3) (2)

Notes to Figure 4:
(1) DQ and DQS signals are bidirectional. One DQS signal is associated with a group of DQ signals.
(2) The clock to the resynchronization register can be from the system clock, write clock, or an extra clock output from
the write PLL. Figure 4 shows the clock to the resynchronization register to be from an extra clock output from the
write PLL.
(3) The clock to this register can be either the system clock or another clock output of the write PLL. If another write
PLL clock output clocks the register feeding this register, another register is needed to transfer the data back to the
system clock domain.

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Figure 5 shows a more detailed picture of the Stratix and Stratix GX

device’s read data path for 8 mode. The DQS signal goes to the DQS
phase-shift circuitry and is shifted. The DQS local bus then inverts the
shifted DQS signal before it clocks the DQ at the input registers. The DQ
input register outputs then go to the resynchronization register in the
logic array. The resynch_clock signal clocks the resynchronization
register. The resynch_clock signal can come from the system clock, the
write clock, or another output from the write PLL. When selecting the
counter values for the PLL, use only even M and N counter values. Odd
counter values will use the falling edge of the clock which is subject to
duty cycle distortion. Even counter values only use rising-edge clock
edges within the PLL circuitry and will not be subject to duty-cycle
distortion.

Figure 5. DDR SDRAM Read Data Path in Stratix & Stratix GX Devices

LE IOE
dq_oe (1)

dq_out
dq[7..0]
A

E
D Q Q D

C
latch
D B
D Q Q D Q D
resynch_clock ena

dataout[15..0]

dqs_oe (1)
dqs_out dqs

DQS Phase
dqs
Shift Circuitry
Local
Bus

Note to Figure 5:
(1) The output enable registers are not shown here, but dqs_oe and dq_oe are active low in silicon. However, the
Quartus II software implements it as active high and adds the inverter automatically during compilation.

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DQS Phase-Shift Circuitry Use in DDR SDRAM Interfaces

System characterization with Stratix, Stratix GX, and DDR SDRAM
devices shows better timing margins when the DLL in the DQS phase-
shift circuitry is turned on only during initialization and refresh cycles.
This is to avoid any possibility of the DQS phase-shift circuitry changing
the phase shift of the DQS pin while a signal is coming into the Stratix or
Stratix GX device. The controller must turn the input reference clock to
the DLL on and off accordingly. If you are using an extra clock pin as the
input reference clock, you can add an enable signal to the register
generating the clock to turn the reference clock on and off (Figure 6).
Otherwise, you need some other mechanism to control the input
reference clock to the DLL.

Figure 6. Simple DDR Interface Example

Stratix

ENABLE
DLL ALT_DDIO

Clock
PLL
Generator

User DDR DDR

Logic Controller Memory

System characterization, proving detailed timing analysis, shows that

72° phase shift gives you better timing. This is because not all DQ signals
are edge-aligned to the DQS signal, as specified in the tDQSQ and tQHS
parameters in a DDR SDRAM data sheet. Altera highly recommends to
turn the DDR SDRAM DLL off during read operations and to turn the

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DLL on during initialization and refresh cycles only. See Table 7 on

page 19 for the detailed timing analysis for the DDR SDRAM read
operation.

DQS Postamble
The DDR SDRAM DQ and DQS pins use the SSTL-2 class II I/O standard.
If the Stratix, Stratix GX, or DDR SDRAM device do not drive the DQ and
DQS pins, the signals go to a high-impedance state. Because a pull-up
resistor terminates both DQ and DQS to VTT (1.25 V), the effective voltage
on the high-impedance line is 1.25 V. According to the JEDEC JESD 8-9
specification for SSTL-2 I/O standard, this is an indeterminate logic level,
and the input buffer can interpret this as either a logic high or logic low.
If there is any noise on the DQS line, the input buffer may interpret that
noise as actual strobe edges. Therefore, when the DQS signal goes to a
high-impedance state after a read postamble, you should disable the
clock to the input registers so that erroneous data is not latched in and all
the data from the memory is resynchronized properly.

Figure 7 shows a read operation example when the DQS postamble could
be a problem. Figure 5 on page 11 shows definitions of A, B, C, D, and E
waveforms. Waveform A shows the output of the active high IOE register.
Waveform B shows the active low register output of the Stratix and
Stratix GX IOE. The active low register output goes into the latch whose
output is illustrated in waveform C. Waveforms D and E show the output
signals after the resynchronization registers.

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Figure 7. Read Example With a DQS Postamble Issue

0 ns 5 ns 10 ns 15 ns 20 ns 25 ns

DQS
at the pin

DQ D0H D0L D1H D1L

at the pin

DQS
at the IOE

DQ D0H D0L D1H D1L

at the IOE

A D0L D1L

B D0H D1H

C D0H D1H

resynch_clock

D D0H

E D0L

The first falling edge of the DQS at the IOE register occurs at 10 ns. At this
point, data D0H is clocked in by the active low register (waveform B). At
12.5 ns, data D0L is sampled in by the active high register (waveform A)
and data D0H passes through the latch (waveform C). In this example, the
positive edge of the resynch_clock signal occurs at 16.5 ns, where both
D0H and D0L are sampled by the LE’s resynchronization registers.
Similarly, data D1H is clocked in by the active low register at 15 ns, data
D1L is clocked in by the active high register, and data D1H passes
through the latch at 17.5 ns. At 20 ns, noise on the DQS line causes a valid
clock edge at the IOE registers that changes the values of waveforms A,
B, and C. The next rising edge of the resynch_clock signal does not
occur until 21.5 ns, but data D1L and D1H are not valid anymore at the
output of the latch and the active-high input register. Therefore, the
resynchronization registers do not sample D1L and D1H and may sample
the wrong data instead.

To avoid this possibility, add one register clocked by the undelayed DQS
signal in the LE closest to the associated DQ group to act as an enable for
each DQS/DQ group (Figure 8). The output of this register, dq_enable,

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controls clock enable for the DQ IOE registers. The data input to the LE
register is set to GND, and the preset port of this register is connected to
the dq_enable_preset signal. The controller should generate
dq_enable_preset so that it is high when DQS is first detected low
(during read preamble) and low during the cycle prior to the last active
edge of DQS. This causes the dq_enable signal to go low with the last
active negative edge of the DQS signal. Register AI and BI are then
disabled before DQS goes into a high-impedance state. Latch CI has the
last data captured by register BI, whether or not it is in the transparent or
latched state.

Figure 8. Soft Logic to Avoid the DQS Postamble Issue

LE IOE
Register AI
DQ
Q D

EN

Latch CI Register BI
Q D Q D

Preset (asynchronous) EN EN
dq_enable_preset
D Q
postamble_clk dq_enable
D Q
Register 1
dq_capture_clk
(1) DQS
Delay
(2)

t1 path
t2 path

Notes to Figure 8:
(1) Invert combout of the IOE for the DQS pin before feeding into inclock of the IOE for the DQ pin. This inversion
is automatic if you use an altdq megafunction for the DQ pins.
(2) You can have 0, 1, or 2 LE buffers. These are required at lower frequencies to ensure that the capture registers are
not disabled too early.

Figure 9 shows the timing required for the signals in Figure 8.

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Figure 9. DQS Postamble Workaround Soft Logic Timing Diagram

DQS

dq_enable_preset

dq_enable

To analyze the timing of the circuit shown in Figure 8, assume that t1 is the
delay from the DQS pad through the compensated delay to registers AI
and BI, t2 is the delay from the DQS pad through register 1 (tCO) to the
enable pin of registers AI and BI, and T is the clock period. The timing
equations are then as follows:

t2 > t1 + micro hold time

t2 < t1 + 0.4T – micro setup time

Because t1 is actually the 72° phase shift plus some PVT variation, t1a is the
delay that varies with PVT so that t1 is either t1a + 0.2T so that the
equations above are now as follows:

t2 > t1a + 0.2T

t2 < t1a + 0.2T + 0.4T

The equation can be rearranged to 0.2T < t2 – t1a < 0.6T

The equation above shows that t2 and t1a vary the same way with PVT, so
when performing timing analysis, the maximum timing for t2 should be
considered with the maximum timing for t1a, and vice versa for minimum
timing. The equations in Table 6 show the timing requirements for a
specific frequency of operation when using the 72° phase shifts,
respectively:

Table 6. Timing Requirements Using 72° Phase Shift

Frequency Equation
200 MHz 1 ns < t2 – t1 a < 3 ns
166 MHz 1. 2 ns < t2 – t1 a < 3.6 ns
133 MHz 1.5 ns < t2 – t1 a < 4.5 ns

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Figure 10 shows the read timing waveform when the Stratix and
Stratix GX DQS postamble circuitry is used.

Figure 10. Stratix & Stratix GX DQS Postamble Circuitry Read Timing Waveform
0 ns 5 ns 10 ns 15 ns 20 ns 25 ns

DQS
at the Pin

DQ
D0H D0L D1H D1L
at the Pin

DQS
at the IOE

DQ D0H D0L D1H D1L

at the IOE

A D0L D1L

B D0H D1H

C D0H D1H

resynch_clock

D D0H D1H

E D0L D1L

Reset

EnableN

DDR SDRAM Controller MegaCore

The DDR SDRAM Controller MegaCore function allows you to
instantiate a simplified interface to industry-standard DDR SDRAM
memory. The DDR SDRAM Controller initializes the memory devices,
manages SDRAM banks, and keeps devices refreshed at appropriate
intervals. The MegaCore function translates read and write requests from
the local interface into all the necessary SDRAM command signals.

Altera Corporation 17
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Interfacing DDR SDRAM with Stratix & Stratix GX Devices

The DDR SDRAM Controller contains encrypted control logic as well as

an open source data path that you can use in your design without a
license. Download this MegaCore function (whether you plan to use the
Altera DDR SDRAM controller or not) to get the open source data path,
open source DQS postamble logic, placement constraints and timing
margin analysis.

The MegaCore function is accessible through the DDR SDRAM IP

Toolbench. When you parameterize your custom DDR SDRAM interface,
the DDR SDRAM IP Toolbench automatically decides the best phase-shift
and FPGA settings to give you the best margin for your DDR SDRAM
interface. It then generates an example instance that instantiates a PLL, an
example driver, and your DDR SDRAM Controller custom variation as
shown in Figure 11.

Figure 11. DDR SDRAM Controller MegaCore System Level Block Diagram

Stratix or Stratix GX Device

DDR SDRAM Controller

(1)
Local Control
Interface Logic
Pass Example Driver (Encrypted)
or fail
DDR SDRAM
Interface
DDR SDRAM

Input
Clock PLL Data Path
(Open
Source)

The example instance is a fully functional design that can be simulated,

synthesized, and used in hardware. The example driver issues read and
write commands to the controller and checks the read data to produce the
pass/fail and test-complete signals. If you do not want to use the DDR
SDRAM Controller encrypted control logic, you can replace it with your
own custom logic. This allows you to use the Altera data path with your
own logic.

1 For more information on the Altera DDR SDRAM Controller, see

the DDR SDRAM Controller MegaCore Function User Guide.

18 Altera Corporation
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 Read-Side Implementation Using the DQS Phase-Shift Circuitry

Read Timing Margin Analysis

Table 7 shows the worst case DDR SDRAM read timing margin analysis
at 200 MHz, when the board trace variations for the DQ and DQS pins is
20 ps (approximately 0.12 inches of FR4 trace length variations) for both
fast and slow corners of EP1S25F1020 device in –5 speed grade.

Table 7. Example 200MHz Read Timing Analysis When Using DQS Circuitry in an EP1S25F1020C5
Device (Part 1 of 2) Note (1)

Fast Corner Slow Corner

Parameter Specification Description
Model (ns) Model (ns)
Memory tH P 2.250 2.250 Half period as specified by the memory
Specifications data sheet
(2) tQ H S 0.500 0.500 Data hold skew factor as specified by
the memory data sheet
tD Q S Q 0.400 0.400 Skew between DQS and DQ from the
memory
FPGA tP a c k a g e 0.050 0.050 Package skew in Stratix
Specifications
tD L L J I T T E R (3) 0.000 0.000 Stratix and Stratix GX device’s DLL
jitter
tD Q S _ P S E R R 0.082 0.082 DLL phase-shift error
tD Q S I N T 0.140 0.140 Skew between DQ and DQS
Minimum clock delay 1.694 2.216 Minimum DQS pin to IOE register delay
(Input) (4) from Quartus II
Maximum clock delay 1.728 2.288 Maximum DQS pin to IOE register
(Input) (4) delay from Quartus II
Data delay 0.549 0.939 DQ pin to IOE register delay from
Quartus II
Minimum data delay 0.499 0.889 DQ pin to IOE register delay from
(Input) Quartus II - tPa c k a g e

Maximum data delay 0.599 0.989 DQ pin to IOE register delay from
(Input) Quartus II + tP a c k a g e
µtS U (4) 0.133 0.276 Intrinsic setup time of the IOE register

µtH (4) 0.032 0.068 Intrinsic hold time of the IOE register
Board tE X T 0.020 0.020 Board trace variations on the DQ and
Specification DQS lines

Altera Corporation 19
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Interfacing DDR SDRAM with Stratix & Stratix GX Devices

Table 7. Example 200MHz Read Timing Analysis When Using DQS Circuitry in an EP1S25F1020C5
Device (Part 2 of 2) Note (1)

Fast Corner Slow Corner

Parameter Specification Description
Model (ns) Model (ns)
Timing tE A R LY _ C L O C K 1.472 1.994 Earliest possible clock edge after DQS
Calculations phase-shift circuitry and uncertainties
(minimum clock delay – tD L L _ J I T T E R –
tD Q S _ P S E R R – tD Q S _ S K E W _ A D D E R )
tL AT E _ C L O C K 1.950 2.510 Latest possible clock edge after DQS
phase-shift circuitry and uncertainties
(maximum clock delay + tD L L _ J I T T E R +
tD Q S _ P S E R R + t D Q S _ S K E W _ A D D E R )
tE A R LY _ D ATA _ I N VA L I D 2.249 2.693 Time for earliest data to become invalid
for sampling at FPGA flop (tH P – tQ H S +
minimum data delay)
tL AT E _ D ATA _ VA L I D 0.999 1.389 Time for latest data to become valid for
sampling at FPGA flop (tD Q S Q +
maximum data delay)
Results Read setup timing 0.320 0.350 tE A R LY _ C L O C K – tL AT E _ D ATA _ VA L I D –
margin (4) µtS U – tE X T
Read hold timing 0.247 0.041 tE A R LY _ D ATA _ I N VA L I D – tL AT E _ C L O C K –
margin (5) µtH – tE X T
Total margin 0.567 0.350 Read setup margin + read hold margin

Notes to Table 7:
(1) This example uses 72° phase shift.
(2) The memory numbers used here come from Micron MT16VDDT3264A.
(3) This example assumes that DLL is on only during initialization and refresh cycles. tD L L J I T T E R specifications for
Stratix and Stratix GX devices are available in the DC & Switching Characteristics chapter in the Stratix Device
Handbook, volume 1.
(4) These numbers are from the Quartus II software, version 5.0 SP1 using the Altera IP core DDR SDRAM Controller
version 3.2.0. Altera recommends using the latest version of the Quartus II software for your design.
(5) PLL phase shift is adjustable if you need to balance the setup and hold time margin.

Table 7 is divided into five sections: memory specifications, FPGA

specifications, board specification, timing calculations, and results. The
memory specifications section lists the items from a memory data sheet
used in the calculation. The FPGA specifications section lists the items
required for the calculation from the FPGA. The board specification
section includes the board trace skew in the system. The timing
calculations section shows the calculations, and the results section shows
the final setup and hold-time margins.

20 Altera Corporation
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 Read-Side Implementation Using the DQS Phase-Shift Circuitry

Timing paths are analyzed by considering the data and clock arrival times
at the destination register. In Figure 12, the setup margin is defined as the
time between “earliest clock arrival time” and “latest valid data arrival
time” at the register ports. Similarly, hold margin is defined as the time be
tween “earliest invalid data arrival time” and the “latest clock arrival
time” at the register ports. These arrival times are calculated based on
propagation delay information with respect to a common reference point
(such as a DQS edge or system clock edge).

Figure 12. Data Valid Window Timing Waveform

Clock Arrival Time

Clock
Uncertainties

tH
Earliest Data Invalid

tSU

Latest Data Valid

Data Valid Window

You can perform a similar timing analysis for your interface with another
DDR SDRAM memory by replacing the tHP, tQHS, and tDQSQ values in
Table 7 on page 19 with those from your memory data sheet, and
Minimum_Clock_Delay, Maximum_Clock_Delay and Data_Delay, for
your device from the Quartus II software.

You can extract clock and data delay from the project folder with filename
of <core name>_extraction_data.txt. Data delay is obtained from
dq_2_ddio in the text file. Clock delay is obtained from the summation
of dqsclk_2_ddio and dqspin_2_dqsclk. The largest and smallest
summation will be the maximum and minimum clock delay respectively.

To obtain Quartus II software timing data for the target device, you
should instantiate and compile the DDR SDRAM Controller MegaCore.
If you are using your own controller logic, you should instantiate the
clear-text DDR SDRAM data path instead to obtain timing delays. For the
read interface, the MegaCore function extracts and reports timing delays

Altera Corporation 21
Preliminary
Interfacing DDR SDRAM with Stratix & Stratix GX Devices

associated with each DQ and DQS pin in the

<core_instance_name>extraction_data.txt file located in your project
directory. Using this data file and the extract.tcl utility, minimum and
maximum propagation delays on the clock and data path are extracted.
This timing extraction is done twice, once with each device model (fast
corner and slow corner). However, in Stratix and Stratix GX devices, there
is only one data delay being extracted. This is because package skews are
not modeled in Quartus II timing model for Stratix and Stratix GX
devices.

Round-Trip Delay Calculation

Read data is sent into the DDR registers using the DQS signal as a clock.
Therefore, data must be transferred from the DQS clock domain to the
system clock domain (resynchronization).

Figure 13 on page 23 shows the timing analysis and the round-trip delay
in Stratix and Stratix GX devices. The round-trip delay is the delay from
the FPGA clock to the DDR SDRAM and back to the FPGA (input to
register B). You can calculate whether the register outputs clocked by the
resynchronization clock need another resynchronization stage before
getting to the system clock domain. This analysis is required to reliably
transfer data from register A (in the IOE) to register B (in the LE).

1 You can also use a feedback clock and a second PLL for the
resynchronization clock.

22 Altera Corporation
Preliminary
 Read-Side Implementation Using the DQS Phase-Shift Circuitry

Figure 13. Round-Trip Delay Note (1)

dqs_ref_clk (2)
FPGA DDR
SDRAM
DQS Phase
Shift
Circuit

tPD (clk_ext to pin) tPD (Clock Trace)

e1

resynch_clk# (3) (B) clk_to _sdram (C)

clock_source c2 D Q

PLL FPGA CLK CLK

clk_shifted (4)
c1

clk (5) tPD (clk to pin)

c0
tDQSS
(Write)

(A) DQS Write (6)

D Q

tCQ (DQS Write) tDQSCK

(Read)
D Q
Address/
Command Pins
(7)

D Q

DQ Read DQ
(I) (H)
data_out Q D Q D
(8) (9)
B A
(G)
(F) DQS Read (D)
tPD (Routing) tCQ (Capture) DQS
Logic Block (E)
tPD (DQS Trace)
tSU (Resynchronization) tPD (Capture)
tH (Resynchronization)

Notes to Figure 13:

(1) The nodes for the round-trip delay analysis are marked with letters (A) through (I).
(2) The dqs_ref_clk input for Stratix and Stratix GX devices can only come directly from an input clock pin. To feed
the input from a PLL, route the PLL output outside the FPGA and loop it back to the input clock pin to the DLL.
(3) The resynch_clk signal is optional based on the round-trip delay of the system.
(4) The clk_shifted signal is shown for completeness, but it is not needed in the timing analysis for round-trip delay
or address/command timing.
(5) clk is the system clock.
(6) The DQS signal is bidirectional. DQS write and DQS read are shown as two separate pins for this timing analysis.
(7) You can clock the address/command register with either a rising or falling edge of the system clock signal.
(8) Register B’s clock input can be clk, clk_shifted, or resynch_clk. The clk and clk_shifted signals can also
be inverted at register B if needed.
(9) The DQS phase-shift reference circuit controls the 72° phase shift dynamically. The control path is not shown and
its operation is transparent to the user.

Altera Corporation 23
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Interfacing DDR SDRAM with Stratix & Stratix GX Devices

Register A in Figure 13 represents the DDR capture logic. The Q output

from register A represents the point at which the read data has been
converted from DDR to SDR. At the output of register A, the data is
already at single data rate, but is still in the DQS clock domain. DQH (DQ
data during DQS high) is sampled on the positive edge of the 72° phase-
shifted DQS pulse, but re-sampled on the negative edge of the 72° phase-
shifted DQS pulse to align it with DQL (DQ data during DQS low).

Once sampled by the negative edge of the 72° phase-shifted DQS pulse,
DQL and DQH are available for resynchronization.

To sample the Q output of register A into register B, you need the time
relationship between register B’s clock input and the D input, which
depends on the phase relationship between DQS and clock and involves
the following steps:

1. Calculate the system’s round-trip delay (described below).

2. Select a resynchronization phase of the system clock or other

available clock that reliably samples the Q output of register A,
based on the calculated safe resynchronization window. See
Figure 14 on page 26.

3. Apply the correct clock edge for the resynchronization logic in the
memory controller.

Use clk, clk_shifted, or resynch_clk signals as the clock input for

register B. You can also invert clk and clk_shifted if needed. To
determine the data timing at the D input of register B relative to clock,
consider the following timing-path dependencies:

■ The DDR SDRAM clock input arrives (a delayed version of clock)

■ DQS strobe from the DDR SDRAM arrives at the clock input of
register A
■ Data arrives at the Q output of register A
■ Data arrives at the D input of register B

There are three main parts to this path:

■ Clock delays between the FPGA global clock net and the DDR
SDRAM clock input.
■ DQS strobe delays between the DDR SDRAM clock input and DQS’s
arrival at the FPGA capture registers.
■ Read data delays between the output of register A and the input of
register B.

24 Altera Corporation
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 Read-Side Implementation Using the DQS Phase-Shift Circuitry

To determine the point at which the data can be reliably resynchronized,

calculate the minimum and maximum round-trip delay, and then
determine what resynchronization logic to use for your system.
Remember to take into account PVT variations.

Figure 13 shows the individual delays between points (A) and (I). The
sum of all these delays is the round-trip delay. Figure 14 shows the timing
relationship of the signals for the delays between points (A) to (I) for a
CAS latency of 2.5.

Altera Corporation 25
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Interfacing DDR SDRAM with Stratix & Stratix GX Devices

Figure 14. Round-Trip Delay Calculation Without a Feedback Clock Note (1)

Round-Trip Delay
Resynchronization
Phase
clk (A)

tPD (clk to pin)

Clock
FPGA CLK Pin (B)

tPD (Clock Trace)

SDRAM CLK Pin (C)

tDQSCK

SDRAM DQS Pin (D) (2)

(during Read
CL = 2.5) tPD (DQS Trace)

FPGA DQS Pin (E)

DQS 72˚
Strobe
72˚ DQS
phase shift (F)
tPD (Capture)

Clock input
at Register A (G) (3)
tCQ (Capture)

Q Output
of Register A (H) (4)
Captured tPD (Routing)
DQ Data
D Input
of Register B (I)

Notes to Figure 14:

(1) The letters in parenthesis refer to the letters in Figure 13.
(2) The DQS strobe edge can be anywhere within tD Q S C K of the DDR SDRAM clock pin edge. Figure 14 assumes the
DQS strobe occurs tD Q S C K time before the clock for minimum round-trip delay calculation and occurs tD Q S C K time
after the clock for the maximum round-trip delay calculation.
(3) The delays in the DQS path from the FPGA pin to the capture register are matched to the delays for the DQ path
with the exception of the DQS delay chain.
(4) Although data is initially sampled at a capture register on the positive edge of DQS, DQH and DQL are only
available on the negative edge in SDR at the Q outputs of the DDR capture logic.

26 Altera Corporation
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 Read-Side Implementation Using the DQS Phase-Shift Circuitry

Delay (A) to (B) is the clock-to-out time to generate the clock signals to the
DDR SDRAM device.

Delay (B) to (C) is the trace delay for the clock. If there are multiple
DIMMs or devices in the system, use the one furthest away from the
FPGA for the maximum calculation; use the one closest to the FPGA for
the minimum calculation.

Delay (C) to (D) is the relationship between the clock and the DQS strobe
timing during reads. This is tDQSCK in DDR SDRAM specifications,
nominally 0, but can vary by 0.75 ns, depending on the DDR SDRAM
device-speed grade. The DQS output strobe is only guaranteed to be
within tDQSCK of the clock input, so use tDQSCK (maximum), typically
+0.75 ns, for calculating the maximum round-trip delay; use tDQSCK
(minimum), typically –0.75 ns, for calculating the minimum delay.

Delay (D) to (E) is the trace delay for DQS, which typically matches the
trace delay for the DQ signals in the same byte group. To calculate the
maximum round-trip delay, use the byte group with the longest trace
lengths; to calculate the minimum round-trip delay, use the byte group
with the shortest. If there are multiple DIMMs or devices in the system,
use the one furthest from the FPGA for the maximum calculation and the
one closest to the FPGA for the minimum. Trace lengths between different
byte groups do not have to be tightly matched, but a difference between
the longest and shortest decreases the safe resynchronization window
where data can be reliably resynchronized.

PLL jitter and clock duty cycle also affect the round-trip delay. Add each
of these delays to the maximum value and subtract from the minimum
value. PLL jitter and clock duty cycles are not shown in Figure 13, but are
included in Table 8, which shows example round-trip delay calculations.

Table 8. Round-Trip Delay Calculation Example (Part 1 of 2) Note (1)

Example Example
Numbers in
Delay Minimum Maximum Comments
Figures 13 & 14
Values (ns) Values (ns)
tP D (clock to pin) (A) to (B) 2.00 3.00 Equal to tC Q (DQS write)

tP D (clock trace) (B) to (C) 0.33 0.50 2 to 3 inches at 166 ps per inch (2)

tD Q S C K (C) to (D) –0.60 + 0.60 See DDR SDRAM specifications

tP D (DQS trace) (D) to (E) 0.33 0.50 2 to 3 inches at 166 ps per inch (2)

Altera Corporation 27
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Interfacing DDR SDRAM with Stratix & Stratix GX Devices

Table 8. Round-Trip Delay Calculation Example (Part 2 of 2) Note (1)

Example Example
Numbers in
Delay Minimum Maximum Comments
Figures 13 & 14
Values (ns) Values (ns)
72° phase shift (E) to (F) 0.90 1.10 Include Stratix and Stratix GX DLL
jitter and phase-shift error

tP D (capture) (F) to (G) 0.50 1.00

tC Q (capture) (G) to (H) 0 0.16
tP D (routing) (H) to (I) 1.00 1.50
PLL jitter - –0.10 +0.10 PLL jitter specification

Clock duty cycle - –0.25 +0.25 45-55% duty at 200 MHz

Round-trip total (A) to (I) 4.11 8.71

Notes to Table 8:
(1) These numbers are not taken from a specific system or a specific device. The clock frequency in this example is
200 MHz.
(2) To know the exact delay for your system, perform a time domain reflectometry (TDR) analysis on your system.

Resynchronization Selections
When the DQS signal arrives at the Stratix or Stratix GX device, the
dedicated phase-shift circuitry shifts the signal to capture the DQ signals.
The DQ signals are then ready to be synchronized with the system clock.
The round-trip delay numbers vary depending on the board delay and
the device’s internal delay. Complete a timing analysis to decide whether
to use the falling or rising edge of the write clock’s system clock for the
synchronization registers. After calculating the maximum and minimum
round-trip delay, determine the equivalent number of system clock cycles
at your operating frequency to find the point at which the data becomes
valid relative to clock. The example maximum delay shown in Table 8
represents 1.7 cycles at 200 MHz; the minimum represents 0.8 cycles. If
the CAS latency is included, which is equal to three in this example, the
example represents a minimum delay of 3.8 cycles and a maximum delay
of 4.7 cycles.

The overlap of the minimum and maximum data-valid windows defines

the data-valid window, which comprises the safe resynchronization
window and tSU and tH of register B.

28 Altera Corporation
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 Read-Side Implementation Using the DQS Phase-Shift Circuitry

Figure 15 shows an example of the round-trip delay analysis. The extra

half-period shown is because the DDR IOE registers transfer the data to
the FPGA core on the falling edge of the shifted and inverted DQS signal.

Figure 15. Round-Trip Delay Diagram Example One Note (1)

time = 5.4T

time = 0 s time = 4.4T time = 5.3T

System clock

CK at the
FPGA
(maximum)

CK at the
memory
(maximum)

DQS at the
memory Read Command
(maximum) Latched Here

DQS at the
FPGA
(maximum)

DQS at the
IOE input ports
(maximum)

DQ at the
LE input ports
(minimum)
Minimum Round-Trip Delay + CAS Latency + 0.5 period
Safe Resynchronization Window
DQ at the
LE input ports
(maximum)
Maximum Round-Trip Delay + CAS Latency + 0.5 period
DQ after the
resynchronization

Note to Figure 15:

(1) T refers to the clock period of the system, which is 5 ns in this example.

The round-trip delay helps you determine the safe resynchronization

window and how to resynchronize the data. The timing analysis in this
section assumes that the clock domain transfer happens from the IOE
register to the LE register.

The read command is clocked into the DDR SDRAM once it receives the
rising edge of clk (at time 0) from the Stratix or Stratix GX device. You
can calculate the safe resynchronization window valid time as follows:

Minimum safe resynchronization window valid time = maximum

round-trip delay + CAS latency clock period + tSU

Maximum safe resynchronization window valid time = minimum

round-trip delay + (CAS latency + 1) clock period – tH

Altera Corporation 29
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Interfacing DDR SDRAM with Stratix & Stratix GX Devices

Table 8 on page 27 shows an example where the maximum round-trip

delay is 8.71 ns (~1.7 clock cycle), and the minimum round-trip delay is
4.11 ns (~0.8 clock cycle). Plugging these numbers into the above
minimum and maximum safe resynchronization window valid time
equations, the example system shows the minimum safe
resynchronization window valid time is 4.7 cycles and the maximum safe
resynchronization window valid time is 4.8 cycles (with tSU and tH
ignored).

The size of the safe resynchronization window in the example is then 0.1
cycle, calculated by the following equation:

Safe resynchronization window size =

maximum safe resynchronization window valid time –
minimum safe resynchronization window valid time

The size of your safe resynchronization window should be larger than

150 ps to accommodate worst case clock skew between two output clocks
of the PLL of 150 ps.

After you calculate the safe resynchronization window time, determine

how many half clock cycles elapse from time 0 to the minimum safe
resynchronization window valid time (numcycle) by calculating the
ceiling function of the minimum safe resynchronization window valid
time divided by half a clock cycle. To find out whether the safe
resynchronization window falls within a clock edge, multiply numcycle
by half a clock cycle. If the result is less than the maximum safe
resynchronization window valid time, a system clock edge falls within
the safe resynchronization window. Otherwise you need an extra PLL
output for your resynchronization clock.

The example in Table 8 on page 27, as depicted in Figure 15, shows that
numcycle is equal to 10 and that the safe resynchronization window
does not fall within a system clock edge.

If you do not need a resynchronization clock and numcycle is an even

number, the active system clock edge for resynchronization is the positive
edge. If numcycle is odd, the resynchronization system clock edge is the
negative edge, and you must determine the resynchronization phase
selection.

Figure 16 shows an example where the safe resynchronization window is

within a system clock edge. In the example, numcycle is equal to
9 (time = 4.5T), and the negative edge of the system clock is used for the
resynchronization clock.

30 Altera Corporation
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 Read-Side Implementation Using the DQS Phase-Shift Circuitry

Figure 16. Round-Trip Delay Diagram Clock Example Two Note (1)
time = 5.7T

time = 4.7T
time = 5.2T

time = 0 s
System Clock Edge to be
used for Resynchronization-

System Clock

CK at the
FPGA

CK at the
Memory

DQS at the
Read Command
Memory
Latched Here

DQS at the
FPGA

Safe Resynchronization Window

DQS at the
IOE Input Ports

Minimum Round-Trip Delay + CAS Latency + 0.5 period

DQ at the
LE Input Ports
(Minimum)
Maximum Round-Trip Delay + CAS Latency + 0.5 period
DQ at the
LE Input Ports
(Maximum)

DQ after the
Resynchronization
LE µtco

Note to Figure 16:

(1) T refers to the clock period of the system, which is 5 ns in this example.

If there is no clock edge available within the safe resynchronization

window, and you need an extra resynchronization clock, shift the system
clock from either edge. If numcycle is even in this case, the closest
system clock edge to the safe resynchronization window is negative. If
numcycle is odd, the closest clock edge is positive.

You can calculate the needed phase shift for the resynchronization clock
from the following equations:

Minimum phase shift = minimum safe resynchronization window

valid time – PLL clock skew (150 ps) – (numcycle – 1) tCK/2

Maximum phase shift = minimum safe resynchronization window

valid time + PLL clock skew (150 ps) + (numcycle – 1) tCK/2

The phase-shift calculation example in Table 8 on page 27 shows that the

minimum phase shift is 1.52 ns and the maximum phase shift is 1.61 ns.
This is because the safe resynchronization window is less than 300 ps.
You can still choose the median (1.565 ns) for the phase shift of the
resynchronization clock.

Altera Corporation 31
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Interfacing DDR SDRAM with Stratix & Stratix GX Devices

You then need to convert the results to the equivalent degree phase shifts.
If the closest clock edge to the safe resynchronization window is negative,
add or subtract 180° after the conversion to shift the clock from the
positive edge. For the example in Table 8 on page 27, the phase-shift
range is between 1.52 to 1.61 ns, based on the negative edge clock. The
median of this number is 1.565 ns, which equates to ~113° (from 200-MHz
clock). If you want to shift this clock from the positive edge of the system
clock, use either 293° (113° + 180°) or – 67° (113° – 180°).

The Altera DDR SDRAM Controller MegaCore function allows you to set
the resynchronization cycle and phase (Figure 17). The
0 resynchronization cycle starts at the first rising edge of the system clock
(clk) after the DQS signal’s first falling edge at the IOE register. Each
resynchronization cycle is one clock period. If there is no clock edge
within the safe resynchronization window, you must set the phase shift.
In the example shown in Figure 15 on page 29, you must choose the
0 resynchronization cycle with a 40° phase shift to set CAS latency to 2.0
or 3.0. Select the 0 resynchronization cycle with a 220° phase shift to set
CAS latency to 2.5.

32 Altera Corporation
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 Read-Side Implementation Using the DQS Phase-Shift Circuitry

Figure 17. Effect of Read Round-Trip Delay on the Choice of Resynchronization Phase for RTL
Example Note (1)

clk (PLL c0)

Resynchronization
Cycle 0 1 2

dq H L

dqs (72˚ Shifted

and Inverted)

Theoretical Q Output
H/L
of Register A at (H)

Theoretical Round-Trip Delay

Minimum Round-Trip Delay

H/L
CAS Latency
= 2.0 or 3.0
Actual Data Valid at
D Input of Register B
at (I) Maximum Round-Trip Delay

H/L

tSU (Register B) tH (Register B)

Safe Resynchronization Window

Resynchronization
Phase One Resynchronization
Cycle + 40˚

dq H L

dqs (72˚ Shifted

and Inverted)

Theoretical Q Output H/L

of Register A at (H)
Theoretical Round-Trip Delay
Minimum Round-Trip Delay

H/L
CAS Latency
= 2.5 Actual Data Valid at
D Input of
Register B at (I) Maximum Round-Trip Delay

H/L

tSU (Register B) tH (Register B)

Safe Resynchronization Window

Resynchronization
Phase One Resynchronization
Cycle + 220˚

Note to Figure 17:

(1) The letters in parenthesis refer to the letters in Figure 13 on page 23.

Altera Corporation 33
Preliminary
Interfacing DDR SDRAM with Stratix & Stratix GX Devices

Read-Side You can interface Stratix and Stratix GX devices with DDR SDRAM
devices without using the DQS phase-shift circuitry. This section
Implementation provides an example that uses two sets of PLLs (per I/O bank for best
Using a PLL performance), a write PLL and a read PLL, and a feedback clock between
the write PLL and the read PLL (Figure 18).

You can choose any of the user I/O pins for the DQ, DQS, and DM pins.
The board trace lengths for the DQ, DQS, and DM pins should be tightly
matched.

The write PLL generates the system clock, the –90° shifted clock, and the
feedback clock, FB_CLK. The feedback clock is routed outside the FPGA
and back into the FPGA. This board trace length should be equal to the
clock trace length from the FPGA to the memory, plus the DQ trace length
from the memory to the FPGA. If the clock trace length = l1 and the DQ
trace length = l2, the FB_CLK must have a trace length of l3 = (l1 + l2) (see
Figure 18).

The read PLL uses the feedback clock as the input clock and generates the
clock needed to capture the DQ during reads. The DQS signal entering
the FPGA is ignored in this scheme. The read PLL is in normal mode, so
the PLL output at an LE register is in phase with the PLL input at the clock
pin. Because the trace length of the feedback clock is the same as the
CK/CK# and DQS trace, FB_CLK coming into the FPGA looks like the
DQS signal with a little bit of skew. The read PLL can then be shifted to
compensate for the skew and create the 90° PLL phase shift to capture the
DQ signals during reads.

In the source synchronous mode, Enhanced PLLs compensate for clock

delay to top/bottom I/O registers and Fast PLLs compensate for clock
delay to side I/O registers. While implementing DDR SDRAM interfaces
without the DQS phase-shift circuitry, use the corresponding PLL type for
best matching between clock and data delays. The input pin to register
delay chain within the IOE should be set to zero in Quartus II for all data
pins.

34 Altera Corporation
Preliminary
 Read-Side Implementation Using a PLL

Figure 18. DDR SDRAM Implementation on Side I/O Pins Notes (1), (2), (3)

LE IOE

DQ "Write" (1)

Length = l2
DDR

Shifted −90˚

Write CK or CK#
input_clk Clock
Write PLL DDR Length = l1
System
Clock

DQS (2)

DDR
Length = l2

FB_CLK

DDR Length = l3 = l1 + l2
DDR SDRAM

DQ "Read" (1)
Read PLL
Length = l2

DDR

Notes to Figure 18:

(1) DQ is a bidirectional line.
(2) DQS is ignored during reads.
(3) Currently the Altera DDR SDRAM Controller MegaCore function in Stratix devices does not support this feedback
clock mode.

Altera Corporation 35
Preliminary
Interfacing DDR SDRAM with Stratix & Stratix GX Devices

Timing Margin Analysis

In the example, the Stratix and Stratix GX device’s side I/O pins in I/O
banks 1, 2, 5, and 6 can support up to 150-MHz DDR SDRAM (using two
PLLs per I/O bank for best performance).

Table 9 shows the timing analysis for this implementation based on

150 MHz.

Table 9. Example Read Timing Analysis for 150-MHz DDR SDRAM Interface in EP1S25F1020C5 Without
Using Dedicated DQS Circuitry (Part 1 of 2)

Fast Slow
Parameter Specification Corner Corner Description
Model (ns) Model (ns)
Memory tH P 3.000 3.000 Half period as specified by the memory data
Specifications sheet (including memory clock duty cycle
(1) distortion)
tA C 0.700 0.700 Data-hold skew factor specified by the
memory data sheet
FPGA tP L L J I T T E R 0.133 0.133 Output jitter specification for Stratix and
Specifications Stratix GX device’s fast PLL
tP L L P S E R R 0.080 0.080 Phase shift error of the fast PLL

tPA C K A G E 0.050 0.050 Package skew

PLL Phase Shift (2) 1.000 1.000 Extra PLL phase shift to capture data (This is
based on 54° PLL phase shift)
Minimum Clock Delay 0.904 1.627 Minimum DQS pin to IOE register delay from
(Input) (3) Quartus II
Maximum Clock Delay 0.958 1.738 Maximum DQS pin to IOE register delay
(Input) (3) from Quartus II
Data Delay (Input) (3) 0.590 1.037 DQ pin to IOE register delay from Quartus II

Minimum Data Delay 0.540 0.987 DQ pin to IOE register delay from Quartus II
(Input) – package skew
Maximum Data Delay 0.640 1.087 DQ pin to IOE register delay from Quartus II
(Input) + package skew
µtS U (4) 0.133 0.280 Intrinsic setup time of the IOE register

µtH (4) 0.032 0.276 Intrinsic hold time of the IOE register

Board tE X T 0.020 0.020 Board trace variations on the DQ and DQS

Specification lines

36 Altera Corporation
Preliminary
 Read-Side Implementation Using a PLL

Table 9. Example Read Timing Analysis for 150-MHz DDR SDRAM Interface in EP1S25F1020C5 Without
Using Dedicated DQS Circuitry (Part 2 of 2)

Fast Slow
Parameter Specification Corner Corner Description
Model (ns) Model (ns)
Timing tE A R LY _ C L O C K 1.691 2.414 Earliest possible clock edge after DQS
Calculation phase-shift circuitry and uncertainties
(minimum clock delay + PLL phase shift –
t P L L J I T T E R – tP L L P S E R R )
tL AT E _ C L O C K 2.171 2.951 Latest possible clock edge after DQS phase-
shift circuitry and uncertainties (maximum
clock delay + PLL phase shift + tP L L J I T T E R +
tP L L P S E R R )
tE A R LY _ D ATA _ I N VA L I D 2.840 3.287 Time for earliest data to become invalid for
sampling at FPGA flop (tH P – tA C + minimum
data delay)
tL AT E _ D ATA _ VA L I D 1.340 1.787 Time for latest data to become valid for
sampling at FPGA flop (tA C + maximum data
delay)
Results Read setup timing 0.198 0.327 tE A R LY _ C L O C K – tL AT E _ D ATA _ VA L I D – µtS U –
margin (2) tE X T
Read hold timing 0.617 0.331 tE A R LY _ D ATA _ I N VA L I D – tL AT E _ C L O C K – µtH –
margin (2) tE X T
Total margin 0.815 0.579 Setup margin + hold margin

Notes to Table 9:
(1) The memory numbers used here come from Micron MT16VDDT3264A clocked at 150 MHz.
(2) PLL phase shift is adjustable if you need to balance the setup and hold time margin.
(3) These numbers are from the Quartus II software, version 5.0 SP1 using the Altera IP core DDR SDRAM Controller
version 3.2.0. Altera recommends using the latest version of the Quartus II software for your design.
(4) These numbers are from the Quartus II software, version 5.0 SP1 using the Altera IP core DDR SDRAM Controller
version 3.2.0. Altera recommends using the latest version of the Quartus II software for your design.

To calculate the read PLL phase shift, add the 54° shift with the delay from
the DQ pin to the IOE (tDQ2IOE) register and account for the board trace
length skew. The calculation will have some error due to the skew
between DQ and CK from the memory itself. Figure 19 illustrates the DQ
and FB_CLK signal relationship on a 150-MHz operation in an ideal
situation.

Altera Corporation 37
Preliminary
Interfacing DDR SDRAM with Stratix & Stratix GX Devices

Figure 19. Example of DQ & FB_CLK Relationship for 150-MHz Operation

3.333 ns

FB_CLK at FPGA Pin

1.5 ns

DQ at FPGA Pin

tDQ2IOE (1) 54˚ Phase Shift

DQ at IOE Register

Ideal Clock at IOE Register

(Shifted and Inverted
Version of FB_CLK)
Actual Required Phase Shift

Note to Figure 19:

(1) The tDQ2IOE value is available in the Quartus II software.

Round-Trip Delay Calculation

You can use the same read PLL clock output used to capture the data to
resynchronize the data from the IOE register to the LE register. The
following factors contribute to the offset between the read PLL output
and the system clock (add these times together to determine the total
offset):

■ The tDQSCK
■ The skew between l3 and actual l1 + l2
■ The skew between tDQ2IOE used to compensate the PLL and the real
tDQ2IOE
■ The skew between the input to the PLL and the output of the PLL

38 Altera Corporation
Preliminary
 Write-Side Implementation

Write-Side Whether you are using the DQS phase-shift circuitry or the PLL to capture
data during a read operation from the DDR SDRAM device, there is only
Implementation one implementation for the write operation. As shown in Figure 4 on
page 10, the write side uses a PLL to generate the clocks listed in Table 10.

Table 10. Clocks in a DDR SDRAM Interface

Clock Description
System clock Use this clock for the memory controller and to
generate the DQS write and CK/CK# signals.
Write clock (–90° Use this clock in the data path to generate the DQ write
shifted from system signals.
clock)
Feedback clock Use this optional clock only if you are not using the
DQS phase-shift circuitry when reading from the DDR
SDRAM device.
Resynchronization Use this optional clock only if you are using the DQS
clock phase-shift circuitry and need a different clock phase
shift than available for resynchronization.

Figure 20 shows the data path for DDR SDRAM write operations.

Altera Corporation 39
Preliminary
Interfacing DDR SDRAM with Stratix & Stratix GX Devices

Figure 20. Stratix & Stratix GX DDR SDRAM Write Data Path

LE IOE

dqs_oe (1) D Q

VCC D Q

D Q

DQS

D Q

clk

dqs_in

dq_oe (1) D Q

datain[15..0] [15..8]
D Q D Q

DQ[7..0]

[7..0]
D Q D Q

Write Clock
dq_in

Note to Figure 20:

(1) The output enable registers are not shown here, but dqs_oe and dq_oe are active low in silicon. However, the
Quartus II software implements it as active high and adds the inverter automatically during compilation.

40 Altera Corporation
Preliminary
 Write-Side Implementation

Write Timing Margin Analysis

Table 11 shows the DDR SDRAM write timing margin analysis at
200 MHz, when the board trace variations for the DQ and DQS pins is
20 ps (about 0.12-inches of FR4 board trace variation) for both fast and
slow corners of EP1S25F1020 device in -5 speed grade. You can perform a
similar timing analysis for your interface with a different type of DDR
SDRAM memory; you only need the tDS and tDH values from the memory
data sheet and the tIOSKEW, Min_Clock_Delay, Max_Clock_Delay,
Min_Data_Delay, and Max_Data_Delay for your FPGA.

Table 11. Example 200MHz Write Timing Analysis for an EP1S25F1020C5 Device (Part 1 of 2)

Fast Slow
Parameter Specification Corner Corner Description
Model (ns) Model (ns)
Memory tD S 0.400 0.400 Memory Data Setup Requirement
Specifications
tD H (1) 0.400 0.400 Memory Data Hold Requirement

tH P 2.500 2.500 Nominal half clock period

FPGA tI O S K E W (2) 0.160 0.160 Absolute value of the difference in clock-to-

Specifications out times (tC O ) between any two output
registers on the top or bottom of the device
fed by a common clock
tC L K S K E W 0.150 0.150 Skew between two PLL outputs
tD C D 0.250 0.250 Duty cycle distortion (5% of clock period)

tP L L J I T T E R 0.000 0.000 Does not affect margin as the same PLL

generates both write clocks (0° and -90°)
Minimum Clock Delay 1.322 2.304 Minimum DQS tCO from Quartus II (0° PLL
(Output) (3) output clock)
Maximum Clock Delay 1.448 2.563 Maximum DQS tCO from Quartus II (0° PLL
(Output) (3) output clock)
Minimum Data Delay 0.072 1.054 Minimum DQ tCO from Quartus II (-90° PLL
(Output) (3) output clock)
Maximum Data Delay 0.180 1.275 Maximum DQ tCO from Quartus II (-90° PLL
(Output) (3) output clock)
Board tE X T 0.020 0.020 Board trace variations for the DQ and DQS
Specification lines

Altera Corporation 41
Preliminary
Interfacing DDR SDRAM with Stratix & Stratix GX Devices

Table 11. Example 200MHz Write Timing Analysis for an EP1S25F1020C5 Device (Part 2 of 2)

Fast Slow
Parameter Specification Corner Corner Description
Model (ns) Model (ns)
Timing tE A R LY _ C L O C K 1.172 2.154 Earliest possible clock edge seen by
Calculations memory device (minimum clock delay –
tC L K S K E W – tP L L J I T T E R )
tL AT E _ C L O C K 1.598 2.713 Latest possible clock edge seen by memory
device (maximum clock delay + tC L K S K E W +
tP L L J I T T E R )
tE A R LY _ D ATA _ I N VA L I D 2.162 3.144 Time for earliest data to become invalid for
sampling at the memory input pins (tH P -
tD C D + minimum data delay – tI O S K E W )
tL AT E _ D ATA _ VA L I D 0.340 1.435 Time for latest data to become valid for
sampling at the memory input pins
(maximum data delay + tI O S K E W )
Results Write setup timing 0.412 0.299 tE A R LY _ C L O C K – tL AT E _ D ATA _ VA L I D – tD S –
margin tE X T
Write hold timing 0.144 0.011 tE A R LY _ D ATA _ I N VA L I D – tL AT E _ C L O C K – tD H –
margin tE X T
Total margin 0.556 0.310 Setup margin + hold margin

Notes to Table 11:

(1) The memory numbers used here come from Micron MT16VDDT3264A.
(2) The output I/O skew specifications for different Stratix and Stratix GX devices are available in the DC & Switching
Characteristics chapter in the Stratix Device Handbook, volume 1.
(3) These numbers are from the Quartus II software, version 5.0 SP1 using the Altera IP core DDR SDRAM Controller
version 3.2.0. Altera recommends using the latest version of the Quartus II software for your design.

Table 11 is divided into five sections: memory specifications, FPGA

specifications, board specification, timing calculations, and results. The
memory specifications section lists the items from a memory data sheet
used in the calculation. The FPGA specifications section lists the items
required for the calculation from the FPGA. The board specification
section includes the board trace skew in the system. The timing
calculations section shows the calculations, and the results section shows
the final setup and hold-time margins. As shown in Table 11, the EP1S25
device has a write setup and hold-timing margin of 556 ps and 310 ps for
fast and slow corners respectively. In the EP1S60 and EP1S80, tIOSKEW is
500 ps, which prevents these devices from supporting 200 MHz DDR
SDRAM.

42 Altera Corporation
Preliminary
 Stratix & Stratix GX DDR Characterization Data

Stratix & The DDR SDRAM interface in Stratix and Stratix GX devices was
characterized under worst-case conditions. The Altera DDR SDRAM
Stratix GX DDR Controller MegaCore function was used to access the DDR SDRAM
Characterization module. For more information on the characterization setup, contact
Altera Applications.
Data

Board Design This section provides general guidelines for board design when using the
DDR SDRAM Controller MegaCore function and Stratix and Stratix GX
Guidelines devices. It also provides information about decoupling capacitance. The
following general guidelines apply when designing with Stratix and
Stratix GX devices and DDR SDRAM.

■ Keep the memory component or DIMM and the Stratix and

Stratix GX devices close together. The routing length between
Stratix and Stratix GX devices and the DIMM should be within 4.5
inches.

■ The locations of the series impedance-balancing resistors (RS) are

important. For address and control signals, place these series-
terminating resistors as close as possible to the Stratix and Stratix GX
device. For data, data strobe, and data mask signals, place the series-
terminating resistors as close as possible to the memory component
or DIMM socket for the best signal integrity results. Pull-up resistors
RT to VTT (1.25 V) are required for data, data strobe, data mask,
address, and control signals and should be located after the end of
the DIMM structure in a fly-by termination scheme. Routing length
to the pull-ups is less critical, but most designs require 0.5 to 1 inch
to route. Figure 21 shows this termination scheme.

1 These termination instructions are guidelines only. The best way

to predict that the termination arrangement meets your
requirements is to simulate your design, including the PCB and
device packages. For more information, visit www.micron.com
to obtain the Micron Technical Note TN-46-06: Termination for
Point-to-Point Systems.

Altera Corporation 43
Preliminary
Interfacing DDR SDRAM with Stratix & Stratix GX Devices

Figure 21. Termination Scheme

VTT

RT = 56 Ω
Address RS = 10 Ω
and Control 50 Ω
Signals

VTT
DIMM Pin

RT = 56 Ω
Data Strobe,
Data Mask, 50 Ω
and Data Signals RS = 10 Ω

■ Match routing for data byte-groups as closely as possible on the PCB.

For example, you should match the timing skews for data groups
dq0 to dq7, dm0, and dqs0 as closely as possible. These should be
17 ps (0.1 inch). Altera also recommends matching the timing skews
of different data byte-groups. These should also be 17 ps (0.1 inch) to
105 ps (0.5 inch). To match the routing, take the longest trace and
match the rest of the signals (DQ, DQM, DQS) with the longest trace.
Also, you should account for vias, which have electrical length, in all
trace balancing configurations. Proper routing topology is best
achieved when all point-to-point connections match not only in
physical length but also in electrical length.

■ Unbuffered address and control signals are generally noisier than

buffered signals because they create crosstalk. Therefore, you should
route these unbuffered signals on different layers or with greater
spacing than data, data mask, and data strobes. Do not route
differential clock and clock enable signals close to address signals.

■ Route differential clock pairs in parallel and match routing lengths

within 10 ps (0.0588 inch). The spacing between CK and CK# traces
should be the same. The spacing between one pair of CK and CK#
traces and another pair should be at least three times the amount of
space between the CK and CK# traces. See Figure 22 for more
information.

44 Altera Corporation
Preliminary
 Board Design Guidelines

Figure 22. CK & CK# Trace Spacing

CK Trace
d
CK# Trace

>3×d

CK Trace
d
CK# Trace

■ Avoid routing signals across split planes. Altera recommends

controlling returns at high frequencies. Also, avoid routing memory
signals any closer than 0.025 inches from PCI or system clocks. Avoid
routing memory signals close to system reset signals to reduce
crosstalk.

■ When using resistor networks, Altera recommends confining the

address and control signals to separate physical packages from data
signals. To eliminate crosstalk within R-pack resistors, the address,
control, and data lines (DQ, DQM, DQS) should not share R-pack
series resistors. Use series and pull-up resistors with 1 to 2% network
tolerances.

Decoupling Capacitance
Traditional methods for providing decoupling involve placing capacitors
in locations that are convenient based on the routing of the board, and
applying some predetermined ratio of capacitors to driver pins.
However, the higher switching speeds of DDR make typical ratios less
useful. Perform careful planning and analysis to ensure that sufficient
decoupling is provided. The amount of capacitance on a board is usually
not the critical limiting factor in designing a decoupling system. Typically,
the amount of inductance in the capacitor leads and the vias attaching the
capacitors to the power and ground planes creates limitations. Altera
recommends using 0.1-F capacitors in an 0603-sized package to provide
sufficient capacitance without adding too much inductance. Make VTT
voltage decoupling on the motherboard close to the parallel pull-up
resistors. Connect the decoupling capacitors between VTT and ground.
The Stratix and Stratix GX memory interface board has a 0.1- F capacitor
for every other VTT pin. The Stratix and Stratix GX memory interface
board also has 0.1- and 0.01- F capacitors for every VDD and VDDQ pin.

Altera Corporation 45
Preliminary
Interfacing DDR SDRAM with Stratix & Stratix GX Devices

Conclusion Stratix and Stratix GX devices have dedicated circuitry to interface with
up to 200-MHz DDR SDRAM with comfortable and consistent margins.
The circuitry dynamically adjusts with PVT variations and can be fine-
tuned for your system requirements. Stratix and Stratix GX devices use
dedicated circuitry when reading from the memory and use the PLL
when writing to the memory. This implementation simplifies board
layout and controller design.

References JEDEC Standard Publication JESD79C, DDR SDRAM Specification, JEDEC

Solid State Technology Association

MT16VDDT3264A, 184-Pin DDR SDRAM DIMMs Data Sheet, Micron

Technology, Inc.

DDR SDRAM Controller MegaCore Function User Guide, Altera

Corporation.

Copyright © 2005 Altera Corporation. All rights reserved. Altera, The Programmable Solutions Company,
the stylized Altera logo, specific device designations, and all other words and logos that are identified as
trademarks and/or service marks are, unless noted otherwise, the trademarks and service marks of Altera
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spective holders. Altera products are protected under numerous U.S. and foreign patents and pending
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San Jose, CA 95134 to current specifications in accordance with Altera's standard warranty, but reserves the right to make chang-
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arising out of the application or use of any information, product, or service described
www.altera.com herein except as expressly agreed to in writing by Altera Corporation. Altera customers
Applications Hotline: are advised to obtain the latest version of device specifications before relying on any pub-
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46 Altera Corporation
Preliminary