The 8253 data sheets are list in the following:

Functional Description
General

The 8253 is a programmable Interval timer/counter specifically designed for use with
the Intel Micro computer systems. Its function is that of a general purpose,
multi-timing element that can be treated as an array of I/O ports in the system
software.

The 8253 solves one of the most common problems any microcomputer system. The
generation of accurate time delay under software control. Instead of selling up timing
loops in systems software. The programmer configures the 8253 to match his
requirements. Initializes one of the counters of the 8253 with the desired quantity,
then upon command the 8253 will count-out the delay and interrupt the CPU when it
has completed its tasks. It is easy to see that the software overhead is minimal and that
multiple delays can easily be maintained by assignment of priority levels.

Other counter/timer functions that are non-delay in nature but also common to most
microcomputers can be implemented with the 8253.
λ Programmable Rate Generator
λ Event Counter
λ Binary Rate Multiplier
λ Real Time Clock
λ Digital One-Shot
λ Complex Motor Controller

Data Bus Buffer

This 3-state, be directional,8-bit buffer is used to interface the 8253 to the systems
data bus. Data is transmitted or received by the buffer upon execution of Input and
Output CPU instructions. The Data Bus Buffer has three basic functions.

1. Programming the MODES of the 8253

2. Loading the count registers
3. Reading the count values
Read/Write Logic:

The Read/Write Logic accepts inputs from the system bus and in turn generates
control signals for overall device operation. It is enabled by CS so that no operation
can occur to change the function unless the device has been selected by the system
logic.

RD (Read)
A “low” on this input informs the 8253 that the CPU is inputting data in the form of a
counters value.

WR (Write)
A “low” on this input informs the 8253 that the CPU is outputting data in the form of
mode information or loading counters.

AO,A1

This inputs are normally connected to the address bus.

Their function is to select one of the three counters to be operated on and to address
the control word register for mode selection.

CS (Chip Select)

A ‘low’ on this input enables the 8253 . No reading or writing will occur unless the
device is selected. The CS Input has no elect upon the actual operation of the
counters.
__ __ __
CS RD WR A1 A0
0 1 0 0 0 Load Counter No. 0
0 1 0 0 1 Load Counter No. 1
0 1 0 1 0 Load Counter No. 2
0 1 0 1 1 Write Mode Word
0 0 1 0 0 Read Counter No. 0
0 0 1 0 1 Read Counter No. 1
0 0 1 1 0 Read Counter No. 2
0 0 1 1 1 No Operation 3-State
1 X X X X Disable 3 State
0 1 1 X X No Operation 3-State

Control Word Register

The Control Word Register is selected when AO.AI are 11 then accepts information
from the data bus buffer and stores it in a register. The information stored in this
register controls the operational NODE of each counter, selection of binary or BCD
counting and the loading of each count register.

The Control Word Register can only be written into no. read operation of its contents
is available.

Counter #0, Counter #1, Counter #2

These three functional blocks are identical in operation so only a single Counter
will be described. Each Counter consists of a single 16.bit,pre-sellable DOWN
counter. The counter can operate in either binary or BCD and its input, gate and
output are configured by the selection of MODES stored in the Control Word
Register.

The counters are fully independent and each can have separate Mode configuration
and counting operation, binary or BCD. Also there are special features in the control
word that handle the loading lf the count value so that software overhead can be
minimized for these functions.

The reading of the contents of each counter is available to the programmer with
simple READ operations for event counting applications and special commands and
logic are included in the 8253 so that the contents of each counter can be read “on the
fly” without havi ng to inhibit the clock input.

8253 SYSTEMS INTERFACE

The 8253 is a component of the Intel-Microcomputer Systems and Interfaces in the

same manner as all other peripherals of the family. It is treated by the systems
software as an array of peripheral I/O ports: three are counters and the fourth is a
control register for MODE programming.

Basically, the select Inputs AO, A1 connect to the AO, A1 connect to the A0, A1
address bus signals of the CPU. The CS can be derived directly from the address bus
using a linear select method or it can be connected to the output of a decoder, such as
an Intel 8205 for larger systems.
Figure 4. Block Diagram Showing Control Word and Register and Counter
Functions

Figure 5. 8253 System Interface

OPERATIONAL DESCRIPTION

General

The complete functional definition of the 8253 is programmed by the systems

software. A set of control words must be sent out by CPU to initialize each counter of
the 8253 with the desired MODE and quantity Information. Prior to Initialization. The
MODE count, and output of all counters is undefined. These control words program
the MODE, Loading sequence and selection of binary or BCD counting.

Once programmed, the 8253 is ready to perform whatever timing tasks it is assigned
to accomplish.

The actual counting operation of each counter is on-chip so that the usual problems
associated with efficient monitoring and management of external asynchronous events
or rates 10 the microcomputer systems have been eliminated.

Programming the 8253

All of the MODES for each counter are programmed by the systems software by
simple I/O operations.
Each counter of the 8253 is individually programmed by writing a control word into
the Control Word Register. (A0,A1=1I)

Control World Format

D7 D6 D5 D4 D3 D2 D1
D0
SC1 SC0 RL1 RL0 M2 M1 M0 BCD

Definition of Control

SC – Select Counter

SC1 SC0
0 0 Select Counter 0
0 1 Select Counter 1
1 0 Select Counter 2
 1 1 Illegal

RL – Read/Load

RL1 RL0
0 0 Counter Latching operation (see READ/WRITE Procedure section)
1 0 Read / Load most significant byte only
0 1 Read / Load least significant byte only
1 1 Read / Load least significant byte first, then most significant byte

M – MODE

0 0 0 Mode 0
0 0 1 Mode 1
X 1 0 Mode 2
X 1 1 Mode 3
1 0 0 Mode 4
1 0 1 Mode 5

BCD:
0 Binary Counter 16 bits
1 Binary Coded Decimal (BCD) Counter (4 decade)

Counter Loading

The count register is not loaded until the count value is written (one or two bytes,
depending on the mode selected by the RL bits), followed by a rising edge and a
falling clock edge may yield Invalid data.

MODE Definition

MODE O: Interrupt on Terminal Count. The output will be initially low after the
mode set operation. After the counts is loaded into the selected count register, the
output will remain low and the counter will count. When terminal count is reached the
output will go high and remain until the selected count register is reloaded with mode
or a new count is loaded. The counter continues to decrement after terminal count has
been reached.

Rewriting a counter register during counting results in the following:

(1) Write 1st byte stops the current counting

(2) Write 2nd byte starts the new count.

MODE 1: Programmable One-Shot. The output will go low on the count following
the rising edge of the gate input.
The output will go high on the terminal count. If a new count value is loaded while
the output is low it will not affect the duration of the one-shot pulse until the
succeeding trigger. The current count can be read at any time without affecting the
one-shot pulse.

The one-shot is retriggerable, hence the output. It remain low for the full count after
any rising edge of the gate input.

MODE 2: Rate Generator. Divide by N counter. The output will be low for one
period of the Input clock. The period from one output pulse to the next equals the
number of input counts in the count register. If the count register is reloaded between
output pulses the present period will not be affected, but the subsequent period will
reflect the new value.

The gate Input, when low, will force the output high. When the gate Input goes high,
the counter will start from the Initial count. Thus, the gate Input can be used to
synchronize the counter.

MODE 3: Square Wave Rate Generator. Similar to MODE 2 except that the output
will remain high until one half the count has been completed (for even numbers) and
go low for the other half of the count. This is accomplished by decrementing the
counter by two on the falling edge of each clock pulse. When the counter reaches
terminal count, the state of the output is changed and the counter is reloaded with the
full count and the whole process is repeated.
If the count is odd and output is high, the first clock pulse (after the count is loaded)
decrements the count by 1. Subsequent clock pulses decrement the clock by 2. After
timeout, the output goes low and the full count is reloaded. The first clock pulse
(following the reload decrements the counter by 3. Subsequent clock pulses
decrement the count by 2 until timeout. Then the whole process is repeated. In this
way. If the count is odd, the output will be high for (N + 1)/2 counts and low for
(N-1)/2 counts.

MODE 4: Software Triggered 8trobe. After the mode is set, the output will be high.
When the count is loaded, the counter will begin counting. On terminal count, the
output will go low for one input clock period, then will go high again.
If the count register is reloaded during counting, the new count will be loaded on
the next CLK pulse. The count will be inhibited while the GATE input is low.

MODE 5: Hardware Triggered Strobe. The counter will start counting after the
rising edge of the trigger input and will go low for one clock period when the terminal
count is reached. The counter is retriggerable. The output will not go low until the full
count after the rising edge of any trigger.

Signal Low
Status Or
Modes Going Low Rising High
0 Disables -------- Enables
Counting Counting
1 1) Initiates Counting --------
2) Resets output after
next clock
2 1) Disables Counting 1) Reloads counter Enables
2) Sets output 2) Initiates counting counting
immediately
high
3 1) Disables counting Initiates Enables
2) Sales output counting counting
immediately
high
4 Disables -------- Enables
counting counting
 5 Initiates --------
counting

Figure 8. Gate Pin Operations Summary

Write Operations

The systems software must program each counter of the 8253 with the mode and
quantity desired. The programmer must write out to the 8253 a MODE control word
and the programmed number of count register by (1, or 2) prior to actually using the
selected counter.

The actual order of the programming is quite flexible writing out of the MODEE
control word can be in any sequence of counter selection. E g , counter #0 does not
have to be first or counter #2 last. Each counter’s MODE control word register has a
separate address so that its loading is completely sequence independent (SCO,SC1).

The loading of the Count Register with the actual count value however mist be done
in exactly the sequence programmed in the MODEEE control word (RL0,RL1). This
loading of the counter’s count register’s still sequence independent like the MODE
control word loading, but when a selected count register is to be loaded. It must be
loaded with the number of bytes programmed in the MODE control word (RL0,RL1).
The one or two bytes to be loaded in the count register do not have to follow the
associated MODE control word. They can be programmed at any time following the
MODE control word loaded in order.

All counters are down counters. Thus, the value loaded into the count register will
actually be decromented. Loading all zeroes into account register will result in the
maximum count (2”for Binary or 10 for BCD). In MODE O the new count will not
restart until the load has been completed. It will accept one of two bytes depending on
how the MODE control words (RL0,RL1) are programmed. I then proceed with the
restart operation.

MODE Control Word

Counter n
LSB Count Register byte
 Counter n
MSB Count Register byte
Counter n
Figure 8. Programming Format

A1 A0
No. 1 MODE Control Word 1 1
Counter 0
No. 2 MODE Control Word 1 1
Counter 1
No. 3 MODE Control Word 1 1
Counter 2
No. 4 LSB Count Register Byte 0 1
Counter 1
No. 5 MSB Count Register Byte 0 1
Counter 1
No. 6 LSB Count Register Byte 1 0
Counter 2
No. 7 MSB Count Register Byte 1 0
Counter 2
No. 8 LSB Count Register Byte 0 0
Counter 0
No. 9 MSB Count Register Byte 0 0
Counter 0

Read Operations

In most counter applications it becomes necessary to read the value of the count. In
progress and make a computational decision based on this quantity. Event counters
are probably the most common application that uses this function. The 8253 contains
logic that will allow the programmer to easily read the contents of any of the three
counters without disturbing the actual count in progress.

There are two methods that the programmer can use to read the value of the counters.
The first method involves the use of simple I/O read operations of the selected counter.
By controlling the A0, A1 inputs to the 8253 the programmer can select the counter to
be read (remember that no read operation of the mode register is allowed A0, A1-11).
The only requirement with this method is that in order to assure a stable count reading
the actual operation of the selected counter must be inhibits either by controlling the
Gate input or by external logic that inhibits the clock input. The contents of the
counter selected will be available as follows.

First I/O Read contains the least significant byte (LSB: second I/O Reads contains the
most significant byte (MSB).

Reads Operational Charts

A1 A0 RD
0 0 0 Read Counter No. 0
0 1 0 Read Counter No. 1
1 0 0 Read Counter No. 2
1 1 0 Illegal

In order for the programmer to read the contents of any counter without electing or
disturbing the counting operation. The 8253 has special internal logic that can be
accessed using simple WR commands to the MODE register. Basically when the
programmer wishes to read the contents of a selected counter on the fly he loads the
MODE register with a special code which latches the present count value into a
storage register so that its contents contain an accurate stable quantity. The
programmer then issues a normal read command to the selected counter and the
contents of the latched register is available.

MODE Register for Latching Count

A0, A1 = 11
D7 D6 D5 D4 D3 D2 D1 D0
SC1 SC0 0 0 X X X X

SC1, SC0 – specify counter to be latched

D5, D4 – 00 designates counter latching operation
X – don’t care.
The same limitation applies to this mode of reading the counter as the previous
method. That is, it is mandatory to complete the entire read operation as programmed.
This command has no effect on the counter’s mode.

3 MHz 1.5 MHz

CLK 2 CLK

8285 8253-5

If an 8085 clock output is to drive an 8253-5 clock input, it must be reduced to 2 MHz
or less.

Figure 10. MCS-85 Clock Interface