Advantech PCI-1751U Manual de usuario descargar pdf (Pagina 35)

31 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

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 STA bit = 0. The status must be latched to be read;

the status of a counter is accessed by a read from that counter. The

counter status format appears at the beginning of the chapter.

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 in two ways. 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

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Advantech PCI-1751U Manual de usuario Pagina 35