ADLINK PCI-7300A - Manual (Page 80)

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• Appendix A 8254 Programmable Interval Timer
A.3 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 terminal 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 following:
(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.
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 used to synchronized by software.
When this mode is set, the output will remain high until after the
count register is loaded. The output then can also be synchronized
by software.