82801DB Datasheet, PDF (104/587 Pages) Intel Corporation – Intel 82801DB I/O Controller Hub 4 (ICH4)

Functional Description
If a counter is programmed to read/write two-byte counts, the following precaution applies: A
program must not transfer control between writing the first and second byte to another routine
which also writes into that same counter. Otherwise, the counter will be loaded with an incorrect
count.
The Control Word Register at port 43h controls the operation of all three counters. Several
commands are available:
• Control Word Command. Specifies which counter to read or write, the operating mode, and
the count format (binary or BCD).
• Counter Latch Command. Latches the current count so that it can be read by the system. The
countdown process continues.
• Read Back Command. Reads the count value, programmed mode, the current state of the
OUT pins, and the state of the Null Count Flag of the selected counter.
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.
5.6.2 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 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.
5.6.2.1 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^® 82801DB ICH4 Datasheet