User`s guide

70 • Appendix A 8254 Programmable Interval Timer
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