ADLINK ACL-7120A/6 - Manual (Page 36)

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• Programming
4.3.3 Mode definition
There are six different selectable operating modes in the 8254:
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 begin counting. When terminal
count is reached, the output will go high and remain high until the
selected count register is reloaded with a mode or a new count is
loaded. The counter continues to decrement after terminal count
has been reached.
Rewriting a counter register during counting is done by:
(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
A Divide 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.
The gate input, when low, will force the output high. When the gate
input goes high, the counter will start form the initial count. Thus,
the gate input can be synchronized by software.
When this mode is set, the output will remain high until after the
count register is loaded. The output can also be synchronized by
software.