8051f.ppt microcontroller serial port operation
- 1. The 8051 Microcontroller Chapter 5 SERIAL PORT OPERATION
- 2. 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
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- 5. 5/21 • Mode of operation of serial port is set by writing to the SCON at address 99H
- 6. 6/21 8-bit Shift Register (Mode 0) • The RXD line for input and output data • The TXD line serves as the clock
- 7. 7/21 • Reception: reception enable bit REN=1 and receive interrupt bit RI=0
- 8. 8/21 8-bit Shift register (mode 0) • Possible application is to expand the output capability of the 8051
- 9. 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. 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. 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. 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. 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. 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. 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. 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. 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
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