The document discusses serial communication in the 8051 microcontroller, which includes an internal serial port capable of four modes and programmable baud rates using timer-1. It details the components required for operation, such as memory, level converters, and how data can be transmitted and received in both serial and parallel formats. The document also covers the use of buffers for data storage during transmission and reception, as well as baud rate configurations based on specific operating modes.

1. SERIAL
COMMUNICATION IN
8051

2. 8051 has an internal serial port Which can operated 4 modes.
Baud rates for serial communication programmable using internal
timer-1.
8051 can be used as a full-duplex serial communication devices.
A system Requires:
Internal program memory,
RS232 level converter like MAX 232.
A quartz crystal and a reset circuit should be connected to controller.
Program for serial communication  stored permanently in the
internal ROM.

3. Serial bus formed by using TXD, RxD and Vss.
If any additional signals required  generated by software and output/input
through port pins.
TxD, RxD TTL logic levels. converted to standard RS232 logic levels
using bi directional level converter MAX 232.
RS232 level serial port signal can be terminated on a standard 9-pin D-type
Connector.(any serial device connected to 8051)
Parallel data transfer can be achieved through ports.
Parallel input data  received through port-2
Parallel output data  sent through port-1.
Example:
Controller receive parallel data from an ADC. convert it to serial.
transmit via the serial port to another serial devices.
Also controller receive  serial data  from other serial devices via serial
port, convert to parallel…. Then outputs through port-1 to a parallel devices
like DAC.
Supports full duplex communication transmission and reception perform
simultaneously.

4. Serial Data Buffer (SBUF) register
SBUF used to hold Parallel data during transmission and
reception.
During Serial Reception:
Serial data  received  via RxD pin.
Converted to parallel data  stored in receive buffer.
During serial transmission:
parallel data  stored in transmit buffer
converted to serial data to transmit via TxD pin.
Transmit and receive buffers  assigned the same internal address
99H.
Transmit buffer only for  write operation
Receive buffer only for  Read operation.

5. Power Control Register(PCON)
SMOD  used decide the baud rate in serial port operating modes
1,2,3
In Mode 2:
If SMOD=0 baud rate=1/64 of the oscillator frequency
If SMOD=1 Baud rate=1/32 of the oscillator frequency.
In Mode 1 & 3: baud rate depends on SMOD and timer-1 overflow
rate.Baud rate(mode 1 or 3)=(2^SMOD/32)*timer-1 overflow rate.
Timer-1  auto reload mode.
Timer-1 over flow rate= oscillator frequency/12*[256-(TH1)^2]
TH1 reload count value -8 bit in higher order timer-1 count register.

6. Half duplex
Full duplex
10 bit, 11 bit