Manual PCI-WDG-CSM
24
Appendix A: Programmable Interval Timer
This Appendix includes basic information about the type 8254 Counter/Timer chip. For those interested in
more detailed information, a full description can be found in the manufacturer's data sheets.
Please note: The information in this Appendix is for general reference, and may include features or
functions not applicable to this card.
Operation Modes
Modes of operation are described in the following paragraphs to familiarize you with the power and
versatility of this device. The following conventions apply in describing operation of type 8254 chips:
Clock:
A positive pulse into the counter's clock input.
Trigger:
A rising edge input to the counter's gate input.
Counter Loading:
Programming 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.
Mode 2: Rate Generator
This mode provides a divide-by-N capability where N is the count loaded into the counter. When
triggered, the counter output goes low for one clock period after N counts, reloads the initial count, and
the cycle starts over. This mode is periodic, the same sequence is repeated indefinitely until the gate
input is brought low. This mode also works well as an alternative to mode 0 for event counting.
Mode 3: Square Wave Generator
Like mode 2, this mode operates periodically. The output is high for half of the count and low for the other
half. If the count is even, then the output is a symmetrical square wave. If the count is odd, then the
output is high for (N+1)/2 counts and low for (N-1)/2 counts. Periodic triggering or frequency synthesis are
two possible applications for this mode. Note that, in this mode, to achieve the square wave, the counter
decrements by two for the total loaded count, then reloads and decrements by two for the second part of
the waveform.
Mode 4: Software Triggered Strobe
This mode sets the output high and, when the count is loaded, the counter begins to count down. When
the counter reaches zero, the output will go low for one input period. The counter must be reloaded to
repeat the cycle. A low gate input will inhibit the counter.
Mode 5: Hardware Triggered Strobe
In this mode, the counter will start counting after the rising edge of the trigger input and will go low for one
clock period when the terminal count is reached. The counter is retriggerable. The output will go low until
the full count after the rising edge of the trigger.
