8254 データシートの表示（PDF） - Intel

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8254

PROGRAMMABLE INTERVAL TIMER

Intel 

8254

MODE 4 SOFTWARE TRIGGERED STROBE

OUT will be initially high When the initial count ex-

pires OUT will go low for one CLK pulse and then

go high again The counting sequence is ‘‘triggered’’

by writing the initial count

GATE e 1 enables counting GATE e 0 disables

counting GATE has no effect on OUT

After writing a Control Word and initial count the

Counter 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 strobe low until N a

1 CLK pulses after the initial count is written

If a new count is written during counting 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 has no effect on counting

2) Writing the second byte allows the new count to

be loaded on the next CLK pulse

This allows the sequence to be ‘‘retriggered’’ by

software OUT strobes low N a 1 CLK pulses after

the new count of N is written

Figure 19 Mode 4

231164 – 11

15

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