4-9

Mode 1: Hardware Retriggerable One-Shot

OUT will be initially high. OUT will go low on the CLK pulse

following a trigger to begin the one-shot pulse, and will remain

low until the Counter reaches zero. OUT will then go high and

remain high until the CLK pulse after the next trigger.

After writing the Control Word and initial count, the Counter is

armed. A trigger results in loading the Counter and setting

OUT low on the next CLK pulse, thus starting the one-shot

pulse N CLK cycles in duration. The one-shot is retriggerable,

hence OUT will remain low for N CLK pulses after any trigger.

The one-shot pulse can be repeated without rewriting the

same count into the counter. GATE has no effect on OUT.

If a new count is written to the Counter during a one-shot

pulse, the current one-shot is not affected unless the

Counter is retriggerable. In that case, the Counter is loaded

with the new count and the one-shot pulse continues until

the new count expires.

FIGURE 10. MODE 1

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 ini-

tially 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 syn-

chronize 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 count-

ing will continue from the end of the current counting cycle.

FIGURE 11. MODE 2

WR

CLK

GATE

OUT

WR

CLK

GATE

OUT

WR

CLK

GATE

OUT

NNNN

0

3

0

2

0

1

0

0

FF

FF

0

3

0

2

N

CW = 12

LSB = 3

CW = 12

LSB = 3

CW = 12

LSB = 2

LSB = 4

NNNN

0

2

0

1

0

0

FF

FF

FF

FE

0

4

0

3

N

NNNN

0

3

0

2

0

1

0

3

0

2

0

1

0

0

N

NNN

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

NNN

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

82C54