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AT89C51AC3

4383B–8051–01/05

P3.0:7

I/O

Port 3:

Is an 8-bit bi-directional I/O port with internal pull-ups. Port 3 pins that have 1’s written to them are pulled high by the internal

pull-up transistors and can be used as inputs in this state. As inputs, Port 3 pins that are being pulled low externally will be a

The output latch corresponding to a secondary function must be programmed to one for that function to operate (except for

TxD and WR). The secondary functions are assigned to the pins of port 3 as follows:

P3.0/RxD:

Receiver data input (asynchronous) or data input/output (synchronous) of the serial interface

P3.1/TxD:

Transmitter data output (asynchronous) or clock output (synchronous) of the serial interface

P3.2/INT0:

External interrupt 0 input/timer 0 gate control input

P3.3/INT1:

External interrupt 1 input/timer 1 gate control input

P3.4/T0:

Timer 0 counter input

P3.5/T1/SS:

Timer 1 counter input

SPI Slave Select

P3.6/WR:

External Data Memory write strobe; latches the data byte from port 0 into the external data memory

P3.7/RD:

External Data Memory read strobe; Enables the external data memory.

It can drive CMOS inputs without external pull-ups.

P4.0:4

I/O

Port 4:

Is an 2-bit bi-directional I/O port with internal pull-ups. Port 4 pins that have 1’s written to them are pulled high by the internal

pull-ups and can be used as inputs in this state. As inputs, Port 4 pins that are being pulled low externally will be a source of

current (IIL, on the datasheet) because of the internal pull-up transistor.

The secondary functions are assigned to the 5 pins of port 4 as follows:

P4.0: Regular Port I/O

P4.1: Regular Port I/O

P4.2/MISO:

Master Input Slave Output of SPI controller

P4.3/SCK:

Serial Clock of SPI controller

P4.4/MOSI:

Master Ouput Slave Input of SPI controller

It can drive CMOS inputs without external pull-ups.

Pin Name

Type

Description