1. Programmable Interval Timer
INTEL 8253 / 8254

2. The 8254 Programmable Interval Timer
3 independent 16-bit programmable counters
Counting in binary/BCD
Applications
Real-time clock
Event counter

3. 8254 I/O D7-D0: System data bus CLK0: Clock for counter 0
Gate0: Gate for counter 0
CLK1: Clock for counter 1
Gate1: Gate for counter 1
CLK2: Clock for counter 2
Gate2: Gate for counter 2
A1-A0: Address select
00: Counter 0
01: Counter 1
10: Counter 2
11: Control Register
Vcc: Power (+5V)
Ground:
RD: Read enable
WR: Write enable
CS: Chip select signal
OUT0: Output of counter0
OUT1: Output of counter1
OUT2: Output of counter2

4. 8254 Block Diagram

5. Control Register

6. Programming the 8254
Each counter is programmed individually by writing
A control word
The initial count
Control word selects
Counter
Mode of operation
Type of operation (read/write)
Type of count (binary/BCD)
Program sequence 1:
PROGRAM CONTROL WORD 1
PROGRAM CONTROL WORD 2
PROGRAM LSB 1
PROGRAM LSB 2
PROGRAM MSB 1
PROGRAM MSB 2
Program sequence 2:

7. 8254 Modes of Operation Mode 0: Events counter. Mode 1: One shot pulse
Mode 2: Continuous pulses, one clock pulse wide
Mode 3: Continuous square-wave as long as Gate is 1
Mode 4: Software triggered one-shot pulse
Mode 5: Hardware triggered one-shot pulse

8. Modes of Operation Mode 0: Events counter (used for system time)
Mode 1: One shot pulse

9. Modes of Operation Mode 2: Continuous pulses
Mode 3: Continuous square-wave

10. Modes of Operation Mode 4: Software triggered one-shot pulse
Mode 5: Hardware triggered one-shot pulse

11. Example
Connect an 8254 in a 8085 processor system so that it occupies the address range 4CH-4FH
Program counter 0 in mode 0 with a count of ABCDH and counter 1 in mode 3 and a count of 3456H
Program counter 0 to generate an interrupt every 10 ms using a 10MHz oscillator
Write an interrupt service routine that calls a sleep function every 1 min
Program counter 1 to produce a 1.25 MHz clock output with 50% duty cycle