How to interface computers Serial Port (RS232) with Atmel AT89S51/89S52 Microcontroller?

Atmel AT89S51/52 microcontroller has an integrated UART module for carrying serial communication. Serial communication makes use of asynchronous mode of operation. Serial port is defined as an interface between the PC and a device for transfer of data. AT89S51/52 with a serial port will allow reading and writing values to and from computer. Also note that the transfer of data through a serial port is bit by bit.

How to interface computers Serial Port (RS232) with Atmel AT89S51/89S52 Microcontroller?

Block Diagram

                            Block diagram for serial reception and serial transmission with microcontroller

                                                                                                            Block diagram showing Interfacing of 8051 Microcontroller with Serial Port

The block diagram consists of:

  1. RS232
  2. MAX232
  3. AT89S51/52
  • RS232

DB-9 RS-232 is a serial I/O standard, used commonly in PCs and other devices.

                                                                     RS232 DB-9

  • MAX232

The output of RS232 is not compatible with the TTL. Inorder to connect RS232 to AT89S51/52 microcontroller, a converter is required. Here we make use of MAX232. This can convert the output of the microcontroller to the RS232 output level and vice versa.  Usually, MAX232 consists of two line drivers for the transmission and reception of data.

AT89s51/52 uses SCON and SBUF registers for serial communication. SBUF is basically a 8-bit serial communication register. For serial transmission of datas, the data is first placed in SBUF . Also when a data is received serially, it comes in the SBUF register.

Program

1. Serial Transmission

Serial data transmission

                       Program to transmit the character 'B' serially

2. Serial Reception

Serial Data Reception

            Program to receive the character 'B' serially

Program Description

Serial communication between the computer and the microcontroller follows the below steps:

Step1: Set the serial port mode.

Step2: Set the serial port baud rate.

Step3: Write and read serial port.

Step1

Inorder to configure the serial port, AT89S51/52 uses the SCON register. The SCON register consists of the following registers as in the figure: In the program for serial transmission and reception, SCON is entered with the value 0x40.

                                                           SCON

BIT

NAME

EXPLANATION

7

SM0

Serial port mode bit0

6

SM1

Serial port mode bit1

5

SM2

Multiprocessor Communication Enable

4

REN

Receiver Enable

3

TB8

Transmit Bit

2

RB8

Receiver Bit

1

TI

Transmit Flag

0

RI

Receive Flag

 

 

The bits SM0 and SM1 will determine the baud rate to be selected. Table below shows how the baud rate is selected.

SM0

SM1

BAUD RATE

EXPLANATION

0

0

8-bit shift register

Oscillator Frequency/12

0

1

9 bit UART

Set by Timer1

1

0

9 bit UART

Oscillator Frequency/64

1

1

9 bit UART

Set by Timer1

Step2

Baud Rate

Baud rate is defined as the rate at which the data is transferred in bits per second. Usually for serial communication the baud rate is set according to SM0 and SM1. If the value of SM0 and SM1 are “00” or “10”, then baud rate depends upon the oscillator frequency. In other cases, the baud rate depends upon the Timer 1 of the microcontroller.

The value that has to be placed in TH1 to generate the required baud rate depends upon the following equation.

TH1 = 256 - ((Crystal / 384) / Baud)

In the program, TH1 is set to the value, 0xFD such that the baud rate is 9600bps according to the above equation for a crystal frequency of 11.059MHz.

Step3

After step 1 and 2, the serial port can be used for transmission and reception of data. Hyper terminal is used for the reception and transmission of data through RS232.

  • To open Hyper Terminal, go to Start Menu, Programs and Accessories.
  • Click Communications.
  • Select Hyper Terminal.
  • Go to File.
  • Click new connection.
  • Give name to connection.
  • Select Baud rate 9600bps.
  • Parity as none, Data bits 8, Flow control none Stop bit 1.

In the program of serial transmission, the letter ‘B’ is transmitted through SBUF of the controller and is displayed on the hyper terminal of PC. While, in case of serial reception, data is entered on the hyper terminal and it is serially received through the SBUF of microcontroller. Inorder to view the output on the microcontroller, in case of serial reception, LCD interfacing has to be done.

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