ADLINK PCIe-7300A - Manual (Page 111)

Appendix 101
2. The count of the BCD counter is from 0 up to 99,999.
Mode Definition
In 8254, there are six different operating modes can be selected.
They are:
Mode 0: Interrupt on terminal count
The output will be initially low after the mode set operation.
After the count is loaded into the selected count register, the
output will remain low and the counter will count. When termi-
nal count is reached, the output will go high and remain high
until the selected count register is reloaded with the mode or a
new count is loaded. The counter continues to decrement after
terminal count has been reached.
Rewriting a counter register during counting results in the fol-
lowing:
(1) Write 1st byte stops the current counting.
(2) Write 2nd byte starts the new count.
Mode 1: Programmable One-Shot.
The output will go low on the count following the rising edge of
the gate input. The output will go high on the terminal count. If
a new count value is loaded while the output is low it will not
affect the duration of the one-shot pulse until the succeeding
trigger. The current count can be read at anytime without
affecting the one-shot pulse.
The one-shot is re-triggerable, hence the output will remain low
for the full count after any rising edge of the gate input.
Mode 2: Rate Generator.
Divided by N counter. The output will be low for one period of
the input clock. The period from one output pulse to the next
equals the number of input counts in the count register. If the
count register is reloaded between output pulses the present
period will not be affected, but the subsequent period will
reflect the new value.