Programmable Interval Timer

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Presentation transcript:

Programmable Interval Timer INTEL 8253 / 8254

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:

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 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