82801FB Datasheet, PDF (130/786 Pages) Intel Corporation – Intel I/O Controller Hub 6 (ICH6) Family

Functional Description
5.8.2.1
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 does 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.
5.8.2.2
Counter Latch Command
The Counter Latch command, written to port 43h, latches the count of a specific counter at the time
the command is received. This command is used to ensure that the count read from the counter is
accurate, particularly when reading a two-byte count. The count value is then read from each
counter’s Count register as was programmed by the Control register.
The count is held in the latch until it is read or the counter is reprogrammed. The count is then
unlatched. This allows reading the contents of the counters on the fly without affecting counting in
progress. Multiple Counter Latch Commands may be used to latch more than one counter. Counter
Latch commands do not affect the programmed mode of the counter in any way.
If a Counter is latched and then, some time later, latched again before the count is read, the second
Counter Latch command is ignored. The count read is the count at the time the first Counter Latch
command was issued.
5.8.2.3
Read Back Command
The Read Back command, written to port 43h, latches the count value, programmed mode, and
current states of the OUT pin and Null Count flag of the selected counter or counters. The value of
the counter and its status may then be read by I/O access to the counter address.
The Read Back command may be used to latch multiple counter outputs at one time. This single
command is functionally equivalent to several counter latch commands, one for each counter
latched. Each counter's latched count is held until it is read or reprogrammed. Once read, a counter
is unlatched. The other counters remain latched until they are read. If multiple count Read Back
commands are issued to the same counter without reading the count, all but the first are ignored.
The Read Back command may additionally be used to latch status information of selected counters.
The status of a counter is accessed by a read from that counter's I/O port address. If multiple
counter status latch operations are performed without reading the status, all but the first are
ignored.
Both count and status of the selected counters may be latched simultaneously. This is functionally
the same as issuing two consecutive, separate Read Back commands. If multiple count and/or
status Read Back commands are issued to the same counters without any intervening reads, all but
the first are ignored.
If both count and status of a counter are latched, the first read operation from that counter returns
the latched status, regardless of which was latched first. The next one or two reads, depending on
whether the counter is programmed for one or two type counts, returns the latched count.
Subsequent reads return unlatched count.
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Intel^® I/O Controller Hub 6 (ICH6) Family Datasheet