FN3043 Rev 2.00

Page 9 of 17

Aug 2000

HS-82C54RH

A new initial count may be written to a Counter at any time

without affecting the Counter’s programmed Mode in anyway.

Counting will be affected as described in the Mode definitions.

The new count must follow the programmed count format.

If a Counter is programmed to read/write two-byte counts, the

following precaution applies: A program must not transfer

control between writing the first and second byte to another

routine which also writes into that same Counter. Otherwise,

the Counter will be loaded with an incorrect count.

Read Operations

It is often desirable to read the value of a Counter without

disturbing the count in progress. This is easily done in the HS-

82C54RH.

There are three possible methods for reading the Counters.

The first is through the Read-Back Command, which is

explained later. The second is a simple read operation of the

Counter, which is selected with the A1, A0 inputs. The only

requirement is that the CLK input of the selected Counter must

be inhibited by using either the GATE input or external logic.

Otherwise, the count may be in process of changing when it is

read, giving an undefined result.

Counter Latch Command

The other method for reading the Counters involves a special

software command called the “Counter Latch Command”. Like

a Control Word, this command is written to the Control Word

Register, which is selected when A1, A0 = 11. Also, like a

Control Word, the SC0, SC1 bits select one of the three

Counters, but two other bits, D5 and D4, distinguish this

command from a Control Word.

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

The selected Counter’s Output Latch (OL) latches the count

when the Counter Latch Command is received. This count is

held in the latch until it is read by the CPU (or until the Counter

is reprogrammed). The count is then unlatched automatically

and the OL returns to “following” the Counting Element (CE).

This allows reading the contents of the Counters “on the fly”

without affecting counting in progress. Multiple Counter Latch

Commands may be used to latch more than one Counter. Each

latched Counter’s OL holds its count until read. Counter Latch

Commands do not affect the programmed Mode of the Counter

in any way.

If a Counter is latched and then, some time later, latched again

before the count is read, the second Counter Latch Command

is ignored. The count read will be the count at the time the first

Counter Latch Command was issued.

With either method, the count must be read according to the

programmed format; specifically, if the Counter is programmed

for two byte counts, two bytes must be read. The two bytes do

not have to be read one right after the other; read or write or

programming operations of other Counters may be inserted

between them.

Another feature of the HS-82C54RH is that reads and writes of

the same Counter may be interleaved; for example, if the

Counter is programmed for two byte counts, the following

sequence is valid.

1. Read least significant byte.

2. Write new least significant byte.

3. Read most significant byte.

4. Write new most significant byte.

If a Counter is programmed to read or write two-byte counts,

the following precaution applies: A program MUST NOT

transfer control between reading the first and second byte to

another routine which also reads from that same Counter.

Otherwise, an incorrect count will be read.

Read-Back Command

The Read-Back Command allows the user to check the count

value, programmed Mode, and current state of the OUT pin

and Null Count flag of the selected Counter(s).

The command is written into the Control Word Register and

has the format shown in Figure 12. The command applies to

the Counters selected by setting their corresponding bits D3,

D2, D1 = 1.

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

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 single 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

D7

D6

D5

D4

D3

D2

D1

D0

SC1

SC0

0

0

X

X

X

X

SC1, SC0 - specify counter to be latched

SC1

SC1

Counter

0

0

0

0

0

1

1

1

2

1

1

Read-Back Command

D5, D4 = 00 designates Counter Latch Command

X = Don’t Care

NOTE: Don’t Care bits (X) should be 0 to insure compatibility with

future products.

FIGURE 11. COUNTER LATCH COMMAND FORMAT

D7

D6

D5

D4

D3

D2

D1

D0

1

1

COUNT

STATUS

CNT 2 CNT 1 CNT 0

0

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

D4: 0 = Latch status of selected Counters(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 12. READ-BACK COMMAND FORMAT