82C54 Datasheet, PDF (9/17 Pages) Intel Corporation – CHMOS PROGRAMMABLE INTERVAL TIMER

82C54
Mode 1: Hardware Retriggerable One-Shot
Mode 2: Rate Generator
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.
CW = 12 LSB = 3
WR
CLK
GATE
OUT
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.
CW = 14
WR
LSB = 3
N
N
N
N
N
0
3
0
2
0 0 FF 0 0
1 0 FF 3 2
CLK
CW = 12 LSB = 3
WR
CLK
GATE
GATE
OUT
N
N
N
N
0
3
00
21
00
32
00
13
CW = 14
WR
LSB = 3
OUT
N
N
N
N
N
0
3
0
2
00 0
13 2
00
10
CLK
GATE
CW = 12 LSB = 2
WR
LSB = 4
CLK
GATE
OUT
N
N
N
N
N
0
2
0
1
0 FF FF 0 0
0 FF FE 4 3
FIGURE 10. MODE 1
OUT
N
N
N
N
0
3
0
2
0
2
CW = 14
WR
LSB = 4
LSB = 5
00
32
00
13
CLK
GATE
OUT
N
N
N
N
0
4
00
32
00
15
00
43
FIGURE 11. MODE 2
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