8254 データシートの表示（PDF） - Intel

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8254

PROGRAMMABLE INTERVAL TIMER

Intel 

8254

NULL COUNT bit D6 indicates when the last count

written to the counter register (CR) has been loaded

into the counting element (CE) The exact time this

happens depends on the Mode of the counter and is

described in the Mode Definitions but until the count

is loaded into the counting element (CE) it can’t be

read from the counter If the count is latched or read

before this time the count value will not reflect the

new count just written The operation of Null Count

is shown in Figure 12

This Action

Causes

A Write to the control word register (1) Null Count e 1

B Write to the count register (CR) (2) Null Count e 1

C New Count is loaded into

Null Count e 0

x CE (CR CE)

NOTE

1 Only the counter specified by the control word will

have its Null Count set to 1 Null count bits of other

counters are unaffected

2 If the counter is programmed for two-byte counts

(least significant byte then most significant byte) Null

Count goes to 1 when the second byte is written

Figure 12 Null Count Operation

If multiple status latch operations of the counter(s)

are performed without reading the status all but the

first are ignored i e the status that will be read is

the status of the counter at the time the first status

read-back command was issued

Both count and status of the selected counter(s)

may be latched simultaneously by setting both

COUNT and STATUS bits D5 D4 e 0 This is func-

tionally the same as issuing two separate read-back

commands at once and the above discussions ap-

ply here also Specifically if multiple count and or

status read-back commands are issued to the same

counter(s) without any intervening reads all but the

first are ignored This is illustrated in Figure 13

If both count and status of a counter are latched the

first read operation of that counter will return latched

status regardless of which was latched first The

next one or two reads (depending on whether the

counter is programmed for one or two type counts)

return latched count Subsequent reads return un-

latched count

CS RD WR A1 A0

0 1 0 0 0 Write into Counter 0

0 1 0 0 1 Write into Counter 1

0 1 0 1 0 Write into Counter 2

0 1 0 1 1 Write Control Word

0 0 1 0 0 Read from Counter 0

0 0 1 0 1 Read from Counter 1

0 0 1 1 0 Read from Counter 2

0 0 1 1 1 No-Operation (3-State)

1 X X X X No-Operation (3-State)

0 1 1 X X No-Operation (3-State)

Figure 14 Read Write Operations Summary

Command

D7 D6 D5 D4 D3 D2 D1 D0

Description

1 1 0 0 0 0 1 0 Read back count and status of

Counter 0

Result

Count and status latched

for Counter 0

1 1 1 0 0 1 0 0 Read back status of Counter 1 Status latched for Counter 1

1 1 1 0 1 1 0 0 Read back status of Counters 2 1 Status latched for Counter

2 but not Counter 1

1 1 0 1 1 0 0 0 Read back count of Counter 2

Count latched for Counter 2

1 1 0 0 0 1 0 0 Read back count and status of

Counter 1

Count latched for Counter 1

but not status

1 1 1 0 0 0 1 0 Read back status of Counter 1

Command ignored status

already latched for Counter 1

Figure 13 Read-Back Command Example

9

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