¡ Semiconductor
MSM80C31F/80C51F
PIN DESCRIPTION
Symbol
V
SS
V
CC
Port 0.0
- 0.7
Port 1.0
- 1.7
Port 2.0
- 2.7
Port 3.0
- 3.7
Ground potential
Supply voltage during Normal, Idle and Power Down operation
Port 0 is an 8-bit open-drain bidirectional I/O port. It is also the mutiplexed low-order address
and data bus during accesses to external memory.
Port 1 is an 8-bit bidirectional I/O port with internal pull-ups. It can drive CMOS inputs without
external pull-ups.
Port 2 is an 8-bit bidirectional I/O port with internal pull-ups. It outputs the high-order address
byte during accesses to external memory. It can drive CMOS inputs without external pull-ups.
Port 3 is an 8-bit bidirectional I/O port with internal pull-ups. It also provides various special
features, as shown below:
Alternate Function
Port Pin
(serial input port)
RXD
P3.0
(serial output port)
TXD
P3.1
(external interrupt)
INT0
P3.2
(external interrupt)
INT1
P3.3
(Timer 0 external input)
T0
P3.4
(Timer 1 external input)
T1
P3.5
(external data memory write strobe)
WR
P3.6
(external data memory read strobe)
RD
P3.7
Port 3 can drive CMOS inputs without external pull-ups.
Reset input pin. A reset is accomplished by holding the RESET pin high for at least 1ms.
even if the oscillator has been stopped. The CPU responds by executing an internal reset. An
internal pull-down resistor permits Power-On reset using only a capacitor connected to V
CC
.
This pin does not receive the power down voltage since the function has been transferred to the
V
CC
pin.
Address Latch Enable. This output latches for latching the low byte of the address during
accesses to external memory. For this purpose, ALE is activated twice every machine cycle or
at a constant rate of 1/6th the oscillator frequency, except during an external memory access at
which time one ALE pulse is skipped. ALE can drive CMOS inputs without an external pull-up.
PSEN
Program Store Enable output. This output is the read strobe to external program memory.
For this purpose,
PSEN
is activated twice every machine cycle. (However, when executing out
of external program memory, two activations of
PSEN
are skipped during each access to
external data memory.)
PSEN
is not activated during fetches from internal program memory.
It can drive CMOS inputs without an external pull-up.
External Access input pin. When
EA
is held high, the CPU executes out of internal program
memory (unless the program counter exceeds 0FFFH).
When
EA
is held low, the CPU executes only out of external program memory.
EA
must not be floated.
XTAL1
XTAL2
Crystal 1 pin. It is an input to the inverting amplifier which forms the internal oscillator.
Crystal 2 pin. It is an output of the inverting amplifier that forms the internal oscillator.
Description
RESET
ALE
EA
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