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
CE (CR
x
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 A
1
A
0
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
Description Result
D
7
D
6
D
5
D
4
D
3
D
2
D
1
D
0
11000010Read back count and status of Count and status latched
Counter 0 for Counter 0
11100100Read back status of Counter 1 Status latched for Counter 1
11101100Read back status of Counters 2, 1 Status latched for Counter
2, but not Counter 1
11011000Read back count of Counter 2 Count latched for Counter 2
11000100Read back count and status of Count latched for Counter 1,
Counter 1 but not status
11100010Read back status of Counter 1 Command ignored, status
already latched for Counter 1
Figure 13. Read-Back Command Example
9
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