10

Commands, one for each Counter latched. Each Counter’s

latched count is held until it is read (or the Counter is

reprogrammed). That Counter is automatically unlatched

when read, but other Counters remain latched until they are

read. If multiple count Read-Back Commands are issued to

the same Counter without reading the count, all but the first

are ignored; i.e., the count which will be read is the count at

the time the first Read-Back Command was issued.

The Read-Back Command may also be used to latch status

information of selected Counter(s) by setting STATUS bit

D4 = 0. Status must be latched to be read; status of a

Counter is accessed by a read from that Counter.

The Counter status format is shown in Figure 13. Bits D5

through D0 contain the Counter’s programmed Mode exactly

as written in the last Mode Control Word. OUTPUT bit D7

contains the current state of the OUT pin. This allows the

user to monitor the Counter’s output via software, possibly

eliminating some hardware from a system.

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 14.

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 = 0. This is functionally the same as issuing two

separate Read-Back Commands at once, and the above

discussions apply 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 15.

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 byte counts) return latched count. Subsequent

reads return unlatched count.

D7

D6

D5

D4

D3

D2

D1

D0

OUT

PUT

NULL

COUNT

RW1

RW0

M2

M1

M0

BCD

D7 1 = Out Pin is 1

0 = Out pin is 0

D6 1 = Null count

0 = Count available for reading

D5-D0 = Counter programmed mode (See Figure 5)

FIGURE 13. STATUS BYTE

THIS ACTION:

CAUSES:

A. Write to the Control Word Register: (Note 17)

Null Count = 1

B. Write to the Count Register (CR): (Note 18)

Null Count = 1

C. New count is loaded into CE (CR

→ CE):

Null Count = 0

NOTES:

17. Only the Counter specified by the Control Word will have its Null

Count set to 1. Null Count bits of other Counters are unaffected.

18. 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 14. NULL COUNT OPERATION

COMMAND

DESCRIPTION

RESULT

D7

D6

D5

D4

D3

D2

D1

D0

11000010

Read back count and status of

Counter 0

Count and status latched for Counter 0

11100100

Read-back status of Counter 1

Status latched for Counter 1

11101100

Read-back status of Counters 2, 1

Status latched for Counter 2, but not

Counter 1

11011000

Read-back count of Counter 2

Count latched for Counter 2

11000100

Read-back count and status of Counter 1

Count latched for Counter 1, but not status

11100100

Read-back status of Counter 1

Command ignored, status already latched

for Counter 1

FIGURE 15. READ-BACK COMMAND EXAMPLE

HS-82C54RH