The 8051 Microcontroller

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The 8051 Microcontroller

• Chapter 5
• SERIAL PORT OPERATION

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

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8-bit Shift Register (Mode 0)

• The RXD line for input and output data
• The TXD line serves as the clock

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• Reception reception enable bit REN1 and receive
  interrupt bit RI0

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8-bit Shift register (mode 0)

• Possible application is to expand the output
  capability of the 8051

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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

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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

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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

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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

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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)

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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

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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

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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

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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 ratetimer 1
  overflow 32

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