Counter/Timers (8254)
NULL COUNT bit D6 indicates when the last count written to the
Count 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 "Mode Definitions" on page 11-16, 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 11-6.
This Action
Causes
A. Write to the Control Word register;
1
Null Count = 1
B. Write to the Count register (CR);
2
Null Count = 1
C. New Count is loaded into CE
(CR
> CE);
Null Count = 0
Notes:
1. Only the counter specified by the Control Word has 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 11–6. Null Count Operation.
11-13
