ADLINK cPCI-7300 - Manual (Page 81)

Appendix A 8254 Programmable Interval Timer
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Mode 3: Square Wave Rate Generator.
Similar to MODE 2 except that the output will remain high until one
half the count has been completed (or even numbers) and go low
for the other half of the count. This is accomplished by decrement
the counter by two on the falling edge of each clock pulse. When
the counter reaches terminal count, the state of the output is
changed and the counter is reloaded with the full count and the
whole process is repeated.
If the count is odd and the output is high, the first clock pulse (after
the count is loaded) decrements the count by 1. Subsequent clock
pulses decrement the clock by 2 after time-out, the output goes low
and the full count is reloaded. The first clock pulse (following the
reload) decrements the counter by 3. Subsequent clock pulses
decrement the count by 2 until time-out. Then the whole process is
repeated. In this way, if the count is odd, the output will be high for
(N + 1)/2 counts and low for (N - 1)/2 counts.
In Modes 2 and 3, if a CLK source other then the system clock is
used, GATE should be pulsed immediately following Way Rate of a
new count value.
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Mode 4: Software Triggered Strobe.
After the mode is set, the output will be high. When the count is
loaded, the counter will begin counting. On terminal count, the
output will go low for one input clock period, then will g o high again.
If the count register is reloaded during counting, the new count will
be loaded on the next CLK pulse. The count will be inhibited while
the GATE input is low.
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Mode 5: Hardware Triggered Strobe.
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 re-triggerable. the output will not go low
until the full count after the rising edge of any trigger.
The detailed description of the 8254, please refer to the Intel Micro-system
Components Handbook.