The 8051 contains UART – universal asynchronies receiver transmitter

• The serial port is full duplex

• It can transmit and receive simultaneously

• 2 port and 3 pins are used to provide the serial interface

P 3.0 is the receiver pin (RXD) P 3.1 is the transmitter pin (TXD)

Both synchronous and asynchronous transmission supported

A synchronous transmission sends a clock signal with the data adjustable baud rate and data frame size supported in asynchronous mode

Serial port mode:

Mode 0:

• Only synchronous mode

• Data transferred on RXD clock on TXD

• Clock is fixed at 1/12 of the oscillator frequency

Mode 1:

• Asynchronous mode

• 10 bit data frame (start bit, 8 data bit and stop bit)

• Variable baud rate

• Stop bit is placed in RB 8 bit of SCON register on reception of a frame

Mode 2:

• Asynchronous mode

• 11 bit data frame (start bit, 8 data bit, programmable 9 bit and stop bit)

• On transmission 9th bit is TB 8 bit of SCON

• On reception 9th bit is placed in RB 8 bit of SCON

• 9th bit may be used for data or as a parity bit

• Baud rate may be 1/32 or 1/64 of oscillator frequency

Mode 3:

• Same as module 2 but with programmable baud rate