1. Microprocessor & Interfacing
THE 8253 IC (PIT) Vatsal N Shah – IU Sharon Benedict – IU
Electronics and Communication Engineering Dept.
Indus University

2. Contents What is 8253? PIN-DIAGRAM OF 8253 ARCHITECTURE OF 8253
MODES OF 8253
Interfacing with 8085
References
M&I

3. What is 8253? The Intel 8253 is a programmable counter.
It includes 3 identical 16 bit counters.
It is packaged in a 24-pin DIP.
Six programmable modes.
Gate is used to enable or disable the counter.

4. ARCHITECTURE OF 8253 DATA BUS BUFFER COUNTER READ/ WRITE LOGIC COUNTER
CLK 0
DATA
BUS
BUFFER
COUNTER
GATE 0
D7-D0
OUT 0 RD WR A0 A1
CLK 1
READ/
WRITE
LOGIC
COUNTER 1
GATE 1
OUT 1 CS
CLK 2
CONTROL
WORD REGISTER
COUNTER 2
GATE 2
OUT 2

5. 8253 PIN DIAGRAM OF 8253 D7 VCC D6 WR D5 RD D4 CS D3 A1 D2 A0 CLK 2 D1
OUT 2 D0
CLK 0
GATE 2
OUT 0
CLK 1
GATE 0
OUT 1
GND
GATE 1

6. Data Bus Buffer: Control Logic:
This block contains the logic to buffer the data bus to the microprocessor and to the internal registers. It has 8 input pins, usually labelled as D7..D0, where D7 is the MSB.
Control Logic:
The control section has five signals: Read, Write, Chip select and the address lines A0 and A1.
In peripheral I/O mode read and write signals are connected to IOR and IOW while in memory mapped these are connected to MEMR and MEMW.

7. Control word Register:
The control word register and counters are selected according to signals on lines A0 and A1 as shown below:
CS A1 A0 Select
Counter 0
Counter 1
Counter 2
Control Reg.
Control word Register:
This register is accessed when lines A0 and A1 are at logic 1.
It is used to write command word which specifies the counter to be used, its mode and either read or write format.

8. 8253 Timer

9. MODES OF 8253 MODE 0: Interrupt on terminal count.
MODE 1: Programmable 1 shot.
MODE 2: Rate generator.
MODE 3: Square wave generator.
MODE 4: Software triggered strobe.
MODE 5: Hardware triggered strobe.

10. Mode 0: Interrupt on Terminal Count
The output will start off zero. The count is loaded and the timer will start to count down.
When the count has reached zero the output will be set high, and remain high until the next count has been reloaded.
This can be used as an interrupt.
Mode 1: Programmable One-Shot
The output will go low following the rising edge of the gate input.
The counter will count and the output will go high once the counter has reached zero.

11. Mode 2: Rate Generator
This mode is used to generate a pulse equal to the clock period at a given interval.
When count is loaded the OUT stays high until the count reaches 1 and the the OUT goes low for one clock period.
The count is reloaded automatically and the pulse is generated continuously.
Mode 3: Square Wave Generator
This mode is similar to mode 2. However, the duration of the high and low clock pulses of the output will be different from mode 2.

12. Mode 4: Software Triggered Pulse.
Suppose n is the number loaded into the counter (the COUNT message), the output will be:
high for n/2 counts, and low for n/2 counts, if n is even.
high for (n+1)/2 counts, and low (n-1)/2 for counts, if n is odd
Mode 4: Software Triggered Pulse.
The output will remain high until the timer has counted to zero, at which point the output will pulse low and then go high again.

13. Mode 5: Hardware Triggered Pulse
The counter will start counting once the gate input goes high, when the counter reaches zero the output will pulse low and then go high again.

14. Modes of Operation of 8253

15. 8253 Example

16. References http://www.circuitstoday.com/