9

82C54

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

without affecting the Counter’s programmed Mode in any way.

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

82C54.

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.

.

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 82C54 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 5. The command applies to

POSSIBLE PROGRAMMING SEQUENCE

A1

A0

Control Word - Counter 1

1

1

Control Word - Counter 0

1

1

LSB of Count - Counter 1

0

1

Control Word - Counter 2

1

1

LSB of Count - Counter 0

0

0

MSB of Count - Counter 1

0

1

LSB of Count - Counter 2

1

0

MSB of Count - Counter 0

0

0

MSB of Count - Counter 2

1

0

NOTE:

In all four examples, all counters are programmed to

Read/Write two-byte counts. These are only four of many

programming sequences.

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

D7

D6

D5

D4

D3

D2

D1

D0

SC1

SC0

0

0

XX

XX

SC1, SC0 - specify counter to be latched

SC1

SC0

COUNTER

00

0

01

1

10

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.