82C54 datasheet(9/18 Pages) INTEL

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82C54

A1 A0

Write into Counter 0

Write into Counter 1

Write into Counter 2

Write Control Word

Read from Counter 0

Read from Counter 1

Read from Counter 2

No-Operation (3-State)

Figure 14 ReadWrite Operations Summary

Mode Definitions

The following are defined for use in describing the

operation of the 82C54

CLK PULSE a rising edge then a falling edge in

that order of a Counter’s CLK input

TRIGGER a rising edge of a Counter’s GATE in-

put

COUNTER LOADING the transfer of a count from

the CR to the CE (refer to

the ‘‘Functional Descrip-

tion’’)

MODE 0 INTERRUPT ON TERMINAL COUNT

Mode 0 is typically used for event counting After the

Control Word is written OUT is initially low and will

remain low until the Counter reaches zero OUT then

goes high and remains high until a new count or a

new Mode 0 Control Word is written into the Coun-

ter

GATE e 1 enables counting GATE e 0 disables

counting GATE has no effect on OUT

After the Control Word and initial count are written to

a Counter the initial count will be loaded on the next

CLK pulse This CLK pulse does not decrement the

count so for an initial count of N OUT does not go

high until N a 1 CLK pulses after the initial count is

written

If a new count is written to the Counter it will be

loaded on the next CLK pulse and counting will con-

tinue from the new count If a two-byte count is writ-

ten the following happens

1) Writing the first byte does not disable counting

OUT is set low immediately (no clock pulse re-

quired)

2) Writing the second byte allows the new count to

be loaded on the next CLK pulse

3) When there is a count in progress writing a new

LSB before the counter has counted down to 0

and rolled over to FFFFh WILL stop the counter

However if the LSB is loaded AFTER the counter

has rolled over to FFFFh so that an MSB now

exists in the counter then the counter WILL NOT

stop

This allows the counting sequence to be synchroniz-

ed by software Again OUT does not go high until N

1 CLK pulses after the new count of N is written

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