2-19
You can program the 8254 counter/timer circuitry to operate in one of the
following counter/timer modes:
●
Pulse on terminal count (Mode 0) - This mode is useful for event
counting or for programming a time delay. The software forces the
output low. On the next clock pulse after the software writes the
initial count value, the counter is loaded. When the counter reaches
zero, the output goes high and remains high until the software writes
a new count value. Note that the output does not go high until n + 1
clock pulses after the initial count is written, where n indicates the
loaded count.
A high gate input enables counting; a low gate input disables
counting. The gate input has no effect on the output. Note that an
initial count value written while the gate input is low is still loaded on
the next clock pulse.
Figure 2-6 illustrates pulse on terminal count mode.
Figure 2-6. Pulse on Terminal Count Mode
Clock pulse
Output
Software forces
output low
Software writes initial
count value of 3
3
2
1
