msm82c54-2rs Oki Semiconductor, msm82c54-2rs Datasheet - Page 17

• Manufacturer Part Number: msm82c54-2rs
• Description: Cmos Programmable Interval Timer
• Manufacturer: Oki Semiconductor

¡ Semiconductor
Reading Counter Values
All MSM82C54-2 counting is down-counting, the counting being in steps of 2 in mode 3.
Counter values can be read during counting by. (1) direct reading, (2) counter latching (“read
on the fly”), and (3) read back command.
(1) Direct reading
(2) Counter latching
Counter values can be read by direct reading operations.
Since the counter value read according to the timing of the RD and CLK signals is not
guaranteed, it is necessary to stop the counting by a gate input signal, or to interrupt the
clock input temporarily by an external circuit to ensure that the counter value is correctly
read.
In this method, the counter value is latched by writing counter latch command, thereby
enabling a stable value to be read without effecting the counting in any way at all. The
output latch (OL) of the selected counter latches the count value when a counter latch
command is written. The count value is held until it is read by the CPU or the control word
is set again.
If a counter latch command is written again before reading while a certain counter is latched,
the second counter latch command is ignored and the value latched by the first counter latch
command is maintained.
The MSM82C54-2 features independent reading and writing from and to the same counter.
When a counter is programmed for the 2-byte counter value, the following sequence is
possible:
1. Count value (LSB) reading
2. New count value (LSB) writing
3. Count value (MSB) reading
4. New count value (MSB) writing
An example of a counter latching program is given below.
Counter latching executed for counter #1 (Read/Load 2-byte setting)
MVI A
OUT n3
IN n1
MOV B, A
IN n1
MOV C, A
0 1 0 0 ¥ ¥ ¥ ¥
Denotes counter latching
Write in control word address (n3)
The counter value at this point is latching
Reading of the LSB of the counter value
latched from counter #1.
n1: Conter #1 address
Reading of MSB from counter #1
MSM82C54-2RS/GS/JS
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