User`s guide

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