10

82C54

the counters selected by setting their corresponding bits D3,

D2, D1 = 1.

The read-back command may be used to latch multiple

counter output latches (OL) by setting the COUNT bit D5 = 0

and selecting the desired counter(s). This signal command is

functionally equivalent to several counter latch 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 6. 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 counter 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 below.

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

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.

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 7.

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 unlatched count.

A0, A1 = 11; CS = 0; RD = 1; WR = 0

D7

D6

D5

D4

D3

D2

D1

D0

1

1

COUNT

STATUS

CNT 2CNT 1CNT 0

0

D5:0=Latch count of selected Counter (s)

D4:0=Latch status of selected Counter(s)

D3:1=Select Counter 2

D2:1=Select Counter 1

D1:1=Select Counter 0

D0:Reserved for future expansion; Must be 0

FIGURE 5. READ-BACK COMMAND FORMAT

D7

D6

D5

D4

D3

D2

D1

D0

OUTPUT

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 Control Word Formats)

FIGURE 6. STATUS BYTE