1. Timers

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 8088 processor system so that it occupies the address range 4CH-4FH
Program counter 0 in mode 0 with a count of ABCD12H and counter 1 in mode 3 and a count of H
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

12. Example 2
Connect an 8254 in a 8088 processor system so that it occupies the address range 4CH-4FH through a PIC in the address range 6EH-6FH at IRQ1
Configure the PIC for vector 43H and unmask IRQ1 masking all others
Configure the timer to use one counter to count events on Gate0 and another to count time.
Write an ISR at vector 43H that reads the values of the two timers each time there is an interrupt.