82801CA Datasheet, PDF (104/521 Pages) Intel Corporation – I/O Controller Hub 3-S (ICH3-S)

Functional Description
Table 5-13 lists the six operating modes for the interval counters.
Table 5-13. Counter Operating Modes
Mode
0
1
2
3
4
5
Function
Out signal on end of count (=0)
Hardware retriggerable one-shot
Rate generator (divide by n counter)
Square wave output
Software triggered strobe
Hardware triggered strobe
Description
Output is 0. When count goes to 0, output goes to 1 and
stays at 1 until counter is reprogrammed.
Output is 0. When count goes to 0, output goes to 1 for
one clock time.
Output is 1. Output goes to 0 for one clock time, then
back to 1 and counter is reloaded.
Output is 1. Output goes to 0 when counter rolls over, and
counter is reloaded. Output goes to 1 when counter rolls
over, and counter is reloaded, etc.
Output is 1. Output goes to 0 when count expires for one
clock time.
Output is 1. Output goes to 0 when count expires for one
clock time.
5.6.5 Reading from the Interval Timer
It is often desirable to read the value of a counter without disturbing the count in progress. There
are three methods for reading the counters: a simple read operation, Counter Latch command, and
the Read-Back command. Each is explained below.
With the simple read operation and Counter Latch command methods, the count must be read
according to the programmed format; specifically, if the counter is programmed for two byte
counts, two bytes must be read. The two bytes do not have to be read one right after the other.
Read, write, or programming operations for other counters may be inserted between them.
5.6.5.1
Simple Read
The first method is to perform a simple read operation. The counter is selected through port 40h
(counter 0), 41h (counter 1), or 42h (counter 2).
Note: Performing a direct read from the counter will not return a determinate value, because the counting
process is asynchronous to read operations. However, in the case of counter 2, the count can be
stopped by writing to the GATE bit in port 61h.
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Intel^® 82801CA ICH3-S Datasheet