82801AB datasheet(71/414 Pages) INTEL

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82801AA and 82801AB Datasheet

5-21

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 is 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 5-12 lists the six operating modes for the interval counters.

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.

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.

Table 5-12. Counter Operating Modes

Mode

Function

Description

Out signal on end of count (=0)

Output is ‘0’. When count goes to 0, output goes to ‘1’ and

stays at ‘1’ until counter is reprogrammed.

Hardware retriggerable one-shot

Output is ‘0’. When count goes to 0, output goes to ‘1’ for

one clock time.

Rate generator (divide by n counter)

Output is ‘1’. Output goes to ‘0’ for one clock time, then

back to ‘1’ and counter is reloaded.

Square wave output

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.

Software triggered strobe

Output is ‘1’. Output goes to ‘0’ when count expires for

one clock time.

Hardware triggered strobe

Output is ‘1’. Output goes to ‘0’ when count expires for

one clock time.

82801AB Datasheet(PDF) 71 Page - Intel Corporation

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