The 8051 Microcontroller Chapter 5 SERIAL PORT OPERATION

2/21 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

3/21

4/21

5/21 Mode of operation of serial port is set by writing to the SCON at address 99H

6/21 8-bit Shift Register (Mode 0) The RXD line for input and output data The TXD line serves as the clock

7/21 Reception: reception enable bit REN=1 and receive interrupt bit RI=0

8/21 8-bit Shift register (mode 0) Possible application is to expand the output capability of the 8051

9/21 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 RB8 in SCON

10/21 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

11/21 8-bit UART (Mode1) 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”

12/21 Eleven bits are transmitted or received: a start bit,8 data bits, a programmable ninth data bit, and a stop bit 9-bit UART with Fixed Baud Rate (Mode 2)

13/21 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)

14/21 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 TB8 by software, received bit is placed in RB8 Interrupt Flags : the receive and transmit interrupt flags (R1 and T1) in SCON, set by hardware, must be cleared by software

15/21 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

16/21 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

17/21 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

18/21

19/21

20/21

21/21