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Manual PCI-DIO-24DH

Programming the 8254

The counters are programmed by writing a control byte into the counter control register. Refer to the

previous register map for the base addresses of the counters and the counter control register. The control

byte specifies the counter to be programmed, the counter mode, the type of read/write operation, and the

modulus. The control byte format is as follows:

SC1

SC0

RW1

RW0

BCD

SC0-SC1: These bits select the counter that the control byte is destined for.

SC1

SC0

Function

Program Counter #0

Program Counter #1

Program Counter #2

Read/Write Cmd.*

* See section on Reading and Loading the Counters.

RW0-RW1: These bits select the read/write mode of the selected counter.

RW1

RW0

Counter Read/Write Function

Counter Latch Command

Read/Write LS Byte

Read/Write MS Byte

Read/Write LS Byte, then MS Byte

M0-M2: These bits set the operational mode of the selected counter.

Mode

BCD: Set the selected counter to count in binary (BCD = 0) or BCD (BCD = 1).

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