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28 • Programming

4.3 Programmable Interval Timer

Note: The material of this section is adopted from

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

4.3.1 The Intel (NEC) 8254

The 8254 contains three independent, programmable, and 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

common uses for the 8254 in microprocessor-based systems are:

• Programmable baud rate generator

• Event counter

• Binary rate multiplier

• Real-time clock

• Digital one-shot

• Motor control

4.3.2 The Control Byte

The 8254 occupies 8 I/O address locations in the ACL-7120A I/O map, as

shown below:

Base + 4 LSB OR MSB OF COUNTER 0

Base + 5 LSB OR MSB OF COUNTER 1

Base + 6 LSB OR MSB OF COUNTER 2

Base + 7 CONTROL BYTE for Chip 0

Base + 8 LSB OR MSB OF COUNTER 3

Base + 9 LSB OR MSB OF COUNTER 4

Base + 10 LSB OR MSB OF COUNTER 5

Base + 11 CONTROL BYTE for Chip 1

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ADLINK Technology ACL -6128 Guía de usuario Pagina 34