¡ Semiconductor
(2) Method using RESET
t
SU
5V
4.5 V
2.2 V
V
CCH
RESET
V
CC
Standby Mode
t
R
MSM81C55-5RS/GS/JS
GND
Note:
In this case, the C/S register is reset, the port is set into the input mode, and the timer stops.
PIN FUNCTION
Symbol
RESET
ALE
AD
0 - 7
CE
IO/M
RD
WR
PA
0 - 7
(PB
0 - 7
)
Function
A high level input to this pin resets the chip, places all three I/O ports in the input mode, resets all
output latches and stops timer.
Negative going edge of the ALE (Address Latch Enable) input latches AD
0 - 7
, IO/M, and
CE
signals into
the respective latches.
Three-state, bi-directional address/data bus. Eight-bit address information on this bus is read into
the internal address latch at the negative going edge of the ALE. Eight bits of data can be read from
or written to the chip using this bus depending on the state of the WRITE or READ input.
When the
CE
input is high, both read and write operations to the chip are disabled.
A high level input to this pin selects the internal I/O functions, and a low level selects the memory.
If this pin is low, data from either the memory or ports is read onto the AD
0 - 7
lines depending on
the state of the IO/M line.
If this pin is low, data on lines AD
0 - 7
is written into either the memory or into the selected port
depending on the state of the line IO/M line.
General-purpose I/O pins. Input/output directions can be determined by programming the command/
status (C/S) register.
Three pins are usable either as general-purpose I/O pins or control pins for the PA and PB ports.
When used as control pins, they are assigned to the following functions:
PC
0
: A INTR (port A interrupt)
PC
1
: A BF (port A full)
PC
2
: A STB (port A strobe)
PC
3
: B INTR (port B interrupt)
PC
4
: B BF (port B buffer full)
PC
5
: B STB (port B strobe)
Input to the counter/timer
Timer output. When the present count is reached during timer operation, this pin provides
a square-wave or pulse output depending on the programmed control status.
3–6V power supply
GND
PC
0 - 5
TIMER IN
TIMER OUT
V
CC
GND
9/19
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