Read Operations - Intel 460GX Software Developer’s Manual

PCI/LPC Bridge 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 count must always be completely loaded with both bytes.
15.4.1.3

Read Operations

It is often desirable to read the value of a counter without disturbing the count in progress. This is
easily done in the IFB timer unit. There are three possible methods for reading the counters: a
simple read operation, the Counter Latch Command, and the Read-Back Command.
15.4.1.4 Counter I/O Port Read
The first method is to perform a simple read operation. To read the counter, which is selected with
the A1, A0 inputs (port 040h, 041h, or 042h), the CLK input of the selected counter must be
inhibited by using either the GATE input or external logic. Otherwise, the count may be in the
process of changing when it is read, giving an undefined result. When reading the count value
directly, follow the format programmed in the control register: read LSB, read MSB, or read LSB
then MSB. Within the IFB timer unit, the GATE input on Counter 0 and Counter 1 is tied high.
Therefore, the direct register read should not be used on these two counters. The GATE input of
Counter 2 is controlled through I/O port 061h. If the GATE is disabled through this register, direct
I/O reads of port 042h will return the current count value.
15.4.1.5 Counter Latch Command
The Counter Latch Command latches the count 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 selected counter's output latch (OL) latches the count at the time the Counter Latch Command
is received. This count is held in the latch until it is read by the CPU (or until the Counter is
reprogrammed). The count is then unlatched automatically and the OL returns to "following" the
counting element (CE). 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. Each latched counter's OL holds its count until it is read. Counter Latch Commands do
not affect the programmed mode of the counter in any way. The Counter Latch Command can be
used for each counter in the IFB timer unit.
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 will be the count at the time the first Counter
Latch Command was issued.
With either method, 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.
Another feature of the IFB timer is that reads and writes of the same counter may be interleaved.
For example, if the Counter is programmed for two byte counts, the following sequence is valid:
1. Read least significant byte.
2. Write new least significant byte.
3. Read most significant byte.
4. Write new most significant byte.
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