Advantech PCI-1751U, User Manual

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Appendix A  

A.4  Counter Operations

A.4.1 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+27). 
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 (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 operation: 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.

A.4.2 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. The format is as shown at the beginning of this section.
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 sta-
tus information for selected counter(s) by setting STA bit = 0. The status 
must be latched to be read; the status of a counter is accessed by a read 
from that counter. 

A.4.3 Counter Latch Operation

Users often want to read the value of a counter without disturbing the 
count in progress. You do this by latching the count value for the specific 
counter then reading the value.
The 8254 supports the counter latch operation. The first way is to set bits 
RW1 and RW0 to 0. This 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 and CNT = 0. The second 
method has the advantage of operating several counters at the same time. 
A subsequent read operation on the selected counter will retrieve the 
latched value.