82C54 Datasheet, PDF (9/18 Pages) Intel Corporation – CHMOS PROGRAMMABLE INTERVAL TIMER

82C54
CS RD WR A₁ A₀
0 1 0 0 0 Write into Counter 0
0 1 0 0 1 Write into Counter 1
0 1 0 1 0 Write into Counter 2
0 1 0 1 1 Write Control Word
0 0 1 0 0 Read from Counter 0
0 0 1 0 1 Read from Counter 1
0 0 1 1 0 Read from Counter 2
0 0 1 1 1 No-Operation (3-State)
1 X X X X No-Operation (3-State)
0 1 1 X X No-Operation (3-State)
Figure 14 Read Write Operations Summary
Mode Definitions
The following are defined for use in describing the
operation of the 82C54
CLK PULSE a rising edge then a falling edge in
that order of a Counter’s CLK input
TRIGGER a rising edge of a Counter’s GATE in-
put
COUNTER LOADING the transfer of a count from
the CR to the CE (refer to
the ‘‘Functional Descrip-
tion’’)
GATE ^e 1 enables counting GATE ^e 0 disables
counting GATE has no effect on OUT
After the Control Word and initial count are written to
a Counter the initial count will be loaded on the next
CLK pulse This CLK pulse does not decrement the
count so for an initial count of N OUT does not go
high until N ^a 1 CLK pulses after the initial count is
written
If a new count is written to the Counter it will be
loaded on the next CLK pulse and counting will con-
tinue from the new count If a two-byte count is writ-
ten the following happens
1) Writing the first byte does not disable counting
OUT is set low immediately (no clock pulse re-
quired)
2) Writing the second byte allows the new count to
be loaded on the next CLK pulse
3) When there is a count in progress writing a new
LSB before the counter has counted down to 0
and rolled over to FFFFh WILL stop the counter
However if the LSB is loaded AFTER the counter
has rolled over to FFFFh so that an MSB now
exists in the counter then the counter WILL NOT
stop
This allows the counting sequence to be synchroniz-
ed by software Again OUT does not go high until N
a _{1 CLK pulses after the new count of N is written}
MODE 0 INTERRUPT ON TERMINAL COUNT
Mode 0 is typically used for event counting After the
Control Word is written OUT is initially low and will
remain low until the Counter reaches zero OUT then
goes high and remains high until a new count or a
new Mode 0 Control Word is written into the Coun-
ter
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