82374EB datasheet(149/208 Pages) INTEL

149 / 208 page

82374EB82374SB

The ReadWrite Logic selects the Control Word register during an IO write when address lines A1 A0e11

This condition occurs during an IO write to port addresses 043h and 04Bh the addresses for the Control

Word Register on Timer 1 and Control Word Register on Timer 2 respectively If the CPU writes to port 043h or

port 04Bh the data is stored in the respective Control Word Register and is interpreted as a Control Word

used to define the operation of the Counters

The Control Word Register is write-only Counter Status information is available with the Read-Back Com-

mand

Table 22 Counter Operating Modes

Mode

Function

Out signal on end of count (e0)

Hardware retriggerable one-shot

Rate generator (divide by n counter)

Square wave output

Software triggered strobe

Hardware triggered strobe

Because the timer counters come up in an unknown state after power up multiple refresh requests may be

queued up To avoid possible multiple refresh cycles after power up program the timer counter immediately

after power up

Write Operations

Programming the interval timer is a simple process

1 Write a control word

2 Write an initial count for each counter

3 Load the least andor most significant bytes (as required by Control Word Bits 5 4) of the 16-bit counter

The programming procedure for the ESC timer units is very flexible Only two conventions need to be ob-

served First for each Counter the Control Word must be written before the initial count is written Second the

initial count must follow the count format specified in the Control Word (least significant byte only most

significant byte only or least significant byte and then most significant byte)

Since the Control Word Register and the three Counters have separate addresses (selected by the A1 A0

inputs) and each Control Word specifies the Counter it applies to (SC0 SC1 bits) no special instruction

sequence is required Any programming sequence that follows the conventions above is acceptable

A new initial count may be written to a Counter at any time without affecting the Counter’s programmed Mode

in any way Counting will be effected as described in the Mode definitions The new count must follow the

programmed count format

If a Counter is programmed to readwrite two-byte counts the following precaution applies A program must

not transfer control between writing the first and second byte to another routine which also writes into that

same Counter Otherwise the Counter will be loaded with an incorrect count

Interval Timer Control Word Format

The Control Word specifies the counter the operating mode the order and size of the COUNT value and

whether it counts down in a 16-bit or binary-coded decimal (BCD) format After writing the control word a new

count may be written at any time The new value will take effect according to the programmed mode

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