13

82C54

MODE 2: RATE GENERATOR

This Mode functions like a divide-by-N counter. It is typically

used to generate a Real Time Clock Interrupt. OUT will

initially be high. When the initial count has decremented to 1,

OUT goes low for one CLK pulse. OUT then goes high

again, the Counter reloads the initial count and the process

is repeated. Mode 2 is periodic; the same sequence is

repeated indefinitely. For an initial count of N, the sequence

repeats every N CLK cycles.

GATE = 1 enables counting; GATE = 0 disables counting. If

GATE goes low during an output pulse, OUT is set high

immediately. A trigger reloads the Counter with the initial

count on the next CLK pulse; OUT goes low N CLK pulses

after the trigger. Thus the GATE input can be used to

synchronize the Counter.

After writing a Control Word and initial count, the Counter will

be loaded on the next CLK pulse. OUT goes low N CLK

pulses after the initial count is written. This allows the

Counter to be synchronized by software also.

Writing a new count while counting does not affect the current

counting sequence. If a trigger is received after writing a new

count but before the end of the current period, the Counter will

be loaded with the new count on the next CLK pulse and

counting will continue from the end of the current counting

cycle.

MODE 3: SQUARE WAVE MODE

Mode 3 is typically used for Baud rate generation. Mode 3 is

similar to Mode 2 except for the duty cycle of OUT. OUT will

initially be high. When half the initial count has expired, OUT

goes low for the remainder of the count. Mode 3 is periodic;

the sequence above is repeated indefinitely. An initial count

of N results in a square wave with a period of N CLK cycles.

GATE = 1 enables counting; GATE = 0 disables counting. If

GATE goes low while OUT is low, OUT is set high

immediately; no CLK pulse is required. A trigger reloads the

Counter with the initial count on the next CLK pulse. Thus

the GATE input can be used to synchronize the Counter.

After writing a Control Word and initial count, the Counter will

be loaded on the next CLK pulse. This allows the Counter to

be synchronized by software also.

Writing a new count while counting does not affect the

current counting sequence. If a trigger is received after

writing a new count but before the end of the current half-

cycle of the square wave, the Counter will be loaded with the

new count on the next CLK pulse and counting will continue

from the new count. Otherwise, the new count will be loaded

at the end of the current half-cycle.

NN

N

N

0

2

0

1

0

3

0

2

0

1

0

3

0

3

NNN

N

0

2

0

2

0

3

0

2

0

1

0

3

0

3

NN

N

N

0

3

0

2

0

1

0

5

0

4

0

3

0

4

WR

CLK

GATE

OUT

CW = 14

LSB = 3

WR

CLK

GATE

OUT

CW = 14

LSB = 3

WR

CLK

GATE

OUT

CW = 14

LSB = 4

LSB = 5

FIGURE 11. MODE 2

NN

N

N

0

2

0

4

0

2

0

4

0

2

0

4

0

2

0

4

0

4

0

2

NN

N

N

0

4

0

2

0

5

0

2

0

5

0

4

0

2

0

5

0

5

0

2

NN

N

N

0

2

0

4

0

2

0

2

0

2

0

4

0

2

0

4

0

4

0

2

WR

CLK

GATE

OUT

CW = 16 LSB = 4

WR

CLK

GATE

OUT

WR

CLK

GATE

OUT

CW = 16 LSB = 5

CW = 16 LSB = 4

FIGURE 12. MODE 3