Counter Operations - Advantech PCM-3718 Series User Manual

7.4 Counter Operations

Read/Write Operation
Before you write the initial count to each counter, you must first specify
the read/write operation type, operating mode and counter type in the
control byte and write the control byte to the control register (BASE+15).
Since the control byte register and all three counter read/write registers
have separate addresses, and each control byte specifies the counter it
applies to (set by SC1 and SC0), no instructions on the operating
sequence are required. Any programming sequence following the 8254
convention is acceptable.
There are three types of counter operations: read/load LSB, read/load
MSB and read/load LSB followed by MSB. It is important that you make
your read/write operations in pairs and keep track of the byte order.
Counter Read-back Command
The 8254 counter read-back command lets you check the count value,
programmed mode and current states of the OUT pin and Null Count flag
of the selected counter(s). You write this command to the control word
register. Format is as shown at the beginning of the chapter.
The read-back command can latch multiple counter output latches. Sim-
ply set the CNT bit to 0 and select the desired counter(s). This single
command is functionally equivalent to multiple counter latch commands,
one for each counter latched.
The read-back command can also latch status information for selected
counter(s) by setting the STA bit to 0. The status must be latched to be
read; the status of a counter is accessed by a read from that counter. For
the counter status format please see the beginning of the chapter.
Counter Latch Operation
Users often want to read the value of a counter without disturbing the
count in progress. You can enable this function by latching the count
value for the specific counter and then reading the value.
The 8254 supports the counter latch operation in two ways. The first way
is to set bits RW1 and RW0 to 0. This setting latches the count of the
selected counter in a 16-bit hold register. The second way is to perform a
latch operation under the read-back command. Set bits SC1 and SC0 to 1
PCM-3718 Series User Manual
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