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Appendix A 8254 Programmable Interval Timer • 69

Appendix A 8254 Programmable

Interval Timer

Note: The material of this section is adopted from

“Intel Microprocessor and Peripheral Handbook Vol. II --Peripheral”

A.1 The Intel (NEC) 8254

The Intel (NEC) 8254 contains three independent, programmable, multi-mode

16 bit counter/timers. The three independent 16 bit counters can be clocked at

rates from DC to 5 MHz. Each counter can be individually programmed with 6

different operating modes by appropriately formatted control words. The most

commonly uses for the 8254 in microprocessor based system are:

• programmable baud rate generator

• event counter

• binary rate multiplier

• real-time clock

• digital one-shot

• motor control

For more information about the 8254, please refer to the NEC Microprocessors

and peripherals or Intel Microprocessor and Peripheral Handbook.

A.2 The Control Byte

The 8254 occupies 8 I/O address locations in the PCI-7300A I/O map. As

shown in the following table:

Base + 0 LSB OR MSB OF COUNTER 0

Base + 4 LSB OR MSB OF COUNTER 1

Base + 8 LSB OR MSB OF COUNTER 2

Base + C CONTROL BYTE for Chip 0

1 2 ... 72 73 74 75 76 77 78 79 80 81 82

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