Timers

The 8254 Programmable Interval Timer

• 3 independent 16-bit programmable counters

• Counting in binary/BCD

• Applications– Real-time clock– Event counter

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

8254 Block Diagram

Control Register

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:– PROGRAM CONTROL WORD 1– PROGRAM LSB 1– PROGRAM MSB 1– PROGRAM CONTROL WORD 2– PROGRAM LSB 2– PROGRAM MSB 2

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

Modes of Operation

• Mode 0: Events counter

(used for system time)

• Mode 1: One shot pulse

Modes of Operation

• Mode 2: Continuous pulses

• Mode 3: Continuous square-wave

Modes of Operation

• Mode 4: Software triggered one-shot pulse

• Mode 5: Hardware triggered one-shot pulse

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 345678H

• 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

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.