Chapter 7: Programmable interval Timer
Manual Number: 00650-004-5
Page 7-1
Chapter 7: Programmable Interval Timer
The ADIO1600 contains a type 8254 programmable counter/timer which allows you to implement
such functions as a Real-Time Clock, Event Counter, Digital One-Shot, Programmable Rate Gener-
ator, Square-Wave Generator, Binary Rate Multiplier, Complex Wave Generator, and/or a Motor
Controller. The 8254 is a flexible but powerful device that consists of three independent, 16-bit,
presettable, down counters. Each counter can be programmed to any count between 1 or 2 and
65,535 in binary format, depending on the mode chosen.
On the ADIO1600 these three counters are designated Counter #0, Counter #1, and Counter #2.
Counter #0 is un-dedicated, with the gate, output and clock connections fully accessible via the I/O
connector. Counter #0 is enabled by a discrete input and uses either the internal 1MHz clock or an
external clock of up to 10MHz. Counters #1 and #2 are cascaded together to form a 32-bit counter.
This dual counter can be enabled (gated) by program control and is clocked by an 1MHz precision
crystal-controlled internal source.
Operational Modes
The 8254 modes of operation are described in the following paragraphs to familiarize you with the
versatility and power of this device. For those interested in more detailed information, a full descrip-
tion of the 8254 programmable interval timer can be found in the Intel (or equivalent manufacturers)
data sheets. The following conventions apply for use in describing operation of the 8254 :
Clock:
A positive pulse into the counters clock input.
Trigger:
A rising edge input to the counters gate input.
Counter Loading: Programming of a binary count into the counter.
Mode 0: Pulse on Terminal Count
After the counter is loaded, the output is set low and will remain low until the counter decrements to
zero. The output then goes high and remains high until a new count is loaded into the counter. A
trigger enables the counter to start decrementing. This mode is commonly used for event counting
with Counter #0.
Mode 1: Retriggerable One-Shot
The output goes low on the clock pulse following a trigger to begin the one-shot pulse and goes high
when the counter reaches zero. Additional triggers result in reloading the count and starting the
cycle over. If a trigger occurs before the counter decrements to zero, a new count is loaded. Thus,
this forms a re-triggerable one-shot. In mode 1, a low output pulse is provided with a period equal
to the counter count-down time.