The 8051 Microcontroller Chapter 5 SERIAL PORT OPERATION

The 8051 Microcontroller Chapter 5 SERIAL PORT OPERATION

• The essential function : parallel to serial conversion for output data and serial to parallel conversion for input data • Full duplex • Receive buffer • The serial port buffer (SBUF) is really 2 buffers • Writing to SBUF loads data to be transmitted, write-only register • Reading SBUF accesses received data, read-only register • SCON – serial port control register, control bits and status bits • Baud rate can be fixed or variable 2

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• Mode of operation of serial port is set by writing to the SCON at address 99 H 5

8 -bit Shift Register (Mode 0) • The RXD line for input and output data • The TXD line serves as the clock 6

• Reception: reception enable bit REN=1 and receive interrupt bit RI=0 7

8 -bit Shift register (mode 0) • Possible application is to expand the output capability of the 8051 8

8 -bit UART with Variable Baud Rate (Mode 1) • Universal Asynchronous Receiver/Transmitter • Receives and transmits serial data with each data character preceded by a start bit (low) and followed by a stop bit (high) • For a receive operation, the stop bit goes into RB 8 in SCON 9

8 -bit UART (Mode 1) • Clocking and synchronizing the serial port shift registers in mod 1, 2, 3 is established by a 4 -bit divide‑by-16 counter, the output of which is the baud rate clock 10

8 -bit UART (Mode 1) • Transmission is initiated by writing ti SBUF but does not actually start until the next rollover of the divide-by-16 counter supplying the serial port baud rate • Reception is initiated by a 1 to 0 transition on RXD • The receiver includes “false start bit detection” 11

9 -bit UART with Fixed Baud Rate (Mode 2) • Eleven bits are transmitted or received: a start bit, 8 data bits, a programmable ninth data bit, and a stop bit 12

9 -bit UART with Variable Baud Rate (Mode 3) • Modes 1, 2 and 3 are very similar • The differences lie in the baud rates (fixed in the 2, variable in modes 1 and 3) and in the number of data bits( 8 in mode 1, 9 in modes 2 and 3) 13

Initialization and accessing serial port registers • Receiver Enable : REN in SCON must be set by software to enable the reception of characters • The ninth data bit : The ninth bit transmitted in modes 2 and 3 must be loaded into TB 8 by software, received bit is placed in RB 8 • Interrupt Flags : the receive and transmit interrupt flags (R 1 and T 1) in SCON, set by hardware, must be cleared by software 14

Multiprocessor Communication • Modes 2 and 3 have a special provision • When the master want to send some data to slaves, it first sends out an address byte of target slaves • The trick is not to use the ninth data bit after a link has been established 15

Serial Port Baud Rates • The baud rate is fixed in modes 0 and 3 • In mode 0 it is always the on-chip oscillator freq. divided by 12 • By default following a system reset, the mode 2 baud rate is the osci. freq. divided by 64 • Baud rates in modes 1 and 3 are determined by the timer 1 overflow rate 16

Using timer 1 as the Baud Rate Clock • The baud rate is the timer 1 overflow rate divided by 32 • The formula for modes 1 and 3 Baud rate=timer 1 overflow +32 17

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