Functional Description
5.7.2
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.
5.7.2.1
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.
5.7.2.2
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.
5.7.2.3
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.
120
Datasheet
INTEL [ INTEL ]
