SPI
4-Wire
3-Wire
Real Time
Embedded Systems
www.atomicrhubarb.com/embedded
Lecture 1 – January 17, 2012
Topic
Section Topic
• Where in the books
– Catsoulis chapter/page
– Simon chapter/page
– Zilog UM197 (ZNEO Z16F Series Flash Microcontroller Contest Kit User Manual)
– Zilog UM171 (ZiLOG Developer Studio II—ZNEO User Manual)
– Zilog PS220 (ZNEO Z16F Series Product Specification)
– Zilog UM188 (ZNEO CPU Core User Manual)
– Assorted datasheets
Synchronous Serial Buses
1-wire
2-wire
3-wire
4-wire
… but not in this order
Serial Buses
Communication between components
Typically between integrated circuit components
CPU to peripherals
Typically short distances (at least that was
the plan)
Number of wires = number of SIGNAL wires
Power & ground not included in the counting
Usually synchronous (data on one line and
clock on another)
Lots of variety, lots of similarity.
The nice thing about standards is that there
are so many of them to choose from.
- Andrew Tannenbaum
(or was it Grace Hopper)
Why so many?
• 1,2,3,4 wire?
Why so many?
4-wire – SPI – Motorola
3-wire – Maxim IC
Microwire – National Semiconductor
2-wire – I2C – Phillips Semiconductor
1-wire – Dallas Semiconductor
SensorPath – National Semiconductor
UNI/O - Microchip
SPI (4 wire)
Serial Peripheral Interface
4 wires (really 3 + 1 for each device)
Low Cost
Simple
Intended for CPU to Peripheral
communication and control.
Defined by Motorola.
SPI Specification
The Specification is hidden inside the
HC08 microcontroller data sheets
Motorola Semiconductor is now
Freescale Semiconductor.
Defines 4 signals
SPI Signals
SCLK – Serial Clock
MISO – Master In, Slave Out
MOSI – Master Out, Slave In
SS – Slave Select
Based on shifting data between 2 shift
registers.
Whats a shift register?
Shift Register
A cascade of flip flops
sharing the same clock.
The output of any one
but the last flip-flop
connected to the input
of the next one in the
chain. Resulting in a
circuit that shifts by
one position the one-
dimensional "bit array"
stored in it.
SPI Signals
• Swap 2 bytes, 1 bit at at time using the BRG as our serial
clock.
• Multiple chips can be
connected to the master.
• Individual SS wires are
necessary to select the
one we want to talk to.
SPI Modes
4 timing modes
Based on clock polarity and clock
phase.
Timing
It is common to see a reference to the
SPI mode as CKPOL = 0, PHASE = 0
This is by far the most common SPI
transfer.
SPI data
Is device specific
Some need a single byte
Some need multiple bytes
Reads follow writes
A dummy write is necessary to initiate
a read.
What?
• A dummy write is necessary to initiate
a read.
To read THIS data we need to write something (anything)
to shift the slave data to the master.
What if...
• ... I only have
• one SS pin?
?
SPI Devices
IO Expanders
Clocks
Temperature Sensors
Non-volatile memory
... lots of things.
ESPI on the ZNEO
Enhanced Serial Peripheral Interface
ESPI
• The Enhanced Serial Peripheral Interface (SPI) is a synchronous
interface allowing several SPI-type devices to be
interconnected. SPI-compatible devices include EEPROMs,
Analog-to-Digital Converters, and ISDN devices. Features of the
SPI include:
• Full-duplex, synchronous, character-oriented communication
• Four-wire interface
• Data transfers rates up to a maximum of one-fourth the system
clock frequency
• Error detection
• Write and mode collision detection
• Dedicated Baud Rate Generator
ESPI Pins
• PC2 - /SS
• PC3 - SCK
• PC4 - MOSI
• PC5 - MISO
ESPI Block Diagram
What was that thing we
could do to access multiple
SPI devices with only
ONE /SS pin?
ESPI
Several Modes
Master controller of a single device
Slave Device
Master controller of multiple devices
ESPI Registers
ESPIDATA – Byte to be transmitted or byte
received
ESPITDCR – Transmit Data Command Register
ESPICTL – Configuration register
ESPISTAT – Status Register
ESPISTATE – State register
ESPIMODE – More configuration
ESPIBRH/L – Baud Rate
ESPIDATA
ESPICTL
SPISTAT
SPIMODE
SPIBRH,L
SPI recipe
SPI
1. Configure SPI pins (alternate function)
2. Configure ESPISTL (phase, pol, etc)
3. Configure ESPIMODE (ssio, etc)
4. Set ESPI BRG
5. Write the spi_read functions **
6. Write the spi_write functions **
7. Write device specific functions
8. Configure interrupts if desired
9. Write Interrupt Service Routine, set vector
10. Configure SPI to generate interrupts
(set DIRQE bit of ESPICTL)
10. Enable interrupts (EI)
11. Enable SPI (set bit 0 of SPICTL)
** non-trivial
SPI Details
Data read/written is device specific.
DS1722
This is on the ZNEO developer board
DS1722
Get the datasheet !
Note
CE = active HIGH
/SS = active LOW
Use the SSPO bit of the ESPIMODE
register to control this.
DS1722 Error
3-wire
3-Wire/Microwire
The MICROWIRE protocol is essentially a
subset of the SPI interface, specifically
CPOL = 0 and CPHA = 0.
The Maxim 3-wire interface is found on
some ICs from Maxim. The data flow to
and from the device is multiplexed on
one pin (DQ) while SPI needs two
separate signals (MOSI, MISO).
Example_
ESPI Example SpiDs1722
Read from the
DS1722 SPI
Temperature sensor
