Specifications

8254 Programmable Interval Time •• 79
• 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.
• 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 go
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.