4-5

Operational Description

General

After power-up, the state of the 82C54 is undefined. The

Mode, count value, and output of all Counters are undefined.

How each Counter operates is determined when it is pro-

grammed. Each Counter must be programmed before it can

be used. Unused counters need not be programmed.

Programming the 82C54

Counters are programmed by writing a Control Word and

then an initial count.

All Control Words are written into the Control Word Register,

which is selected when A1, A0 = 11. The Control Word spec-

ifies which Counter is being programmed.

By contrast, initial counts are written into the Counters, not

the Control Word Register. The A1, A0 inputs are used to

select the Counter to be written into. The format of the initial

count is determined by the Control Word used.

FIGURE 4. 82C54 SYSTEM INTERFACE

Write Operations

The programming procedure for the 82C54 is very flexible.

Only two conventions need to be remembered:

1. For Each Counter, the Control Word must be written

before the initial count is written.

2. The initial count must follow the count format specified in the

Control Word (least significant byte only, most significant byte

only, or least significant byte and then most significant byte).

Since the Control Word Register and the three Counters have

separate addresses (selected by the A1, A0 inputs), and each

Control Word specifies the Counter it applies to (SC0, SC1 bits),

no special instruction sequence is required. Any programming

sequence that follows the conventions above is acceptable.

Control Word Format

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

D7

D6

D5

D4

D3

D2

D1

D0

SC1

SC0

RW1

RW0

M2

M1

M0

BCD

ADDRESS BUS (16)

CONTROL BUS

DATA BUS (8)

I/OR I/OW

WR

RD

CS

A0

A1

A1

A0

8

COUNTER

0

OUT GATE CLK

COUNTER

1

COUNTER

2

OUT GATE CLK

OUT GATE CLK

D0 - D7

82C54

SC - Select Counter

SC1

SC0

0

0

Select Counter 0

0

1

Select Counter 1

1

0

Select Counter 2

1

1

Read-Back Command (See Read Operations)

RW - Read/Write

RW1 RW0

0

0

Counter Latch Command (See Read Operations)

0

1

Read/Write least significant byte only.

1

0

Read/Write most significant byte only.

1

1

Read/Write least significant byte first, then most

significant byte.

M - Mode

M2

M1

M0

0

0

0

Mode 0

0

0

1

Mode 1

X

1

0

Mode 2

X

1

1

Mode 3

1

0

0

Mode 4

1

0

1

Mode 5

BCD - Binary Coded Decimal

0

Binary Counter 16-bit

1

Binary Coded Decimal (BCD) Counter (4 Decades)

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

future products.

Possible Programming Sequence

A1

A0

Control Word - Counter 0

1

1

LSB of Count - Counter 0

0

0

MSB of Count - Counter 0

0

0

Control Word - Counter 1

1

1

LSB of Count - Counter 1

0

1

MSB of Count - Counter 1

0

1

Control Word - Counter 2

1

1

LSB of Count - Counter 2

1

0

MSB of Count - Counter 2

1

0

Possible Programming Sequence

A1

A0

Control Word - Counter 0

1

1

Control Word - Counter 1

1

1

Control Word - Counter 2

1

1

LSB of Count - Counter 2

1

0

82C54