AMD Elan SC520, User Manual, Page: 313 / 444

Programmable Interval Timer
Élan™SC520 Microcontroller User’s Manual 16-5
16.5.4.3 Mode 2: Rate Generator
When programmed in rate generator mode, the counters operate as divide by n counters,
where n is the initial count.
1. The output signal starts off High until the initial count is decremented to one.
2. The output then goes Low for one clock pulse and goes High again.
3. The counter is reloaded with the initial count, and the counting sequence is repeated.
There appears one clock pulse at the output for every n clock cycles.
By default, PC/AT-compatible systems program PIT Channel 0 for this mode.
16.5.4.4 Mode 3: Square Wave Mode
In square wave mode:
1. The output of the counter has a 50% duty cycle whenever the counter is loaded with an
even count. Initially the output is High.
2. The count decrements by two with each clock cycle when the gate is held High.
3. When the count reaches zero, the output toggles state, the initial count is reloaded, and
the sequence is repeated.
The period of the output signal is:
Period = Input clock period * Initial count loaded into the counter
If the initial count is an odd number, the output is High for (n+1)/2 cycles and is Low for
(n
±
1)/2 cycles.
By default, PC/AT-compatible systems program PIT channels 1 and 2 to use this mode to
drive DRAM refresh and the speaker, respectively.
16.5.4.5 Mode 4: Software-Triggered Strobe
In software-triggered mode:
1. The counter automatically begins to decrement one clock pulse after it is loaded with
the initial count through software. The output signal is initially High.
2. The count decrements at the rate set by the clock input signal.
3. At the moment the terminal count is reached, the counter generates a single strobe
pulse on the output for one clock pulse duration.
4. If the counter is loaded with a count of n, then a strobe pulse is produced at the output
after n+1 clock cycles.
16.5.4.6 Mode 5: Hardware-Triggered Strobe
In hardware-triggered mode:
1. Counting begins on a Low-to-High transition of the gate signal.
2. The output remains High until the count has expired.
3. The output goes Low for one clock cycle and goes High again.
4. After writing the control word and the initial count, the counter is loaded at the next clock
pulse after the trigger.
The strobe pulse occurs n+ 1 clock pulses after the Low to High transition (trigger) on the
Gate input. This count sequence is retriggerable.