SERIAL PORT (RS232) INTERFACING TO 8051

• Data transferred by computer occurs in two ways. They are parallel and serial communication.
• Parallel Communication has 8 or more lines. It is used for transfer of data to a device that is only a short distance.
• while serial communication is used to transfer data over a long distance.
• The serial communication uses two techniques for communication. They are
• Synchronous- transfer block of data(characters) at a time Asynchronous- transfer single byte at a time
• This communication can be done by the following hardware: USART (Universal Synchronous-Asynchronous Receiver-Transmitter)
• UART (Universal Asynchronous Receiver Transmitter)

RS232 standards

• It is the most widely used serial I/O interfacing standard for serial communication.
• The input and output voltage levels are not TTL compatible.
• The bit-value of 1 is represented as -3 to -25 V and the value0 is represented as +3 to +25 V, while -3 to +3 is undefined.
• To connect RS232 to a microcontroller system, it must use voltage converters such as MAX232 to convert the TTL logic levels to the RS232 voltage levels, and vice versa.
• MAX232 IC chips are commonly referred to as line drivers.

8051 connections

• To RS232 8051 has two pins used for transferring and receiving data serially.
• They are TxD and RxD pins. TxD and RxD are part of the port 3 group (P3.0 and P3.1), while pin 11 (P3.1) of 8051 is assigned to TxD.
• The pin 10 (P3.0) of 8051 is assigned to RxD. These pins are TTL compatible.
• They require a line driver to make them RS232 compatible.
• The driver used for this is MAX232 chip.

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Serial Communication Registers

• SBUF Register: It is an 8-bit register used for serial communication. To transmit a byte of data via the TxD line, it must be placed in the SBUF register.
• SCON (serial control) register: It is an 8-bit register used to program start bit, stop bit, and data bits of data for framing, among other things. In this SM0 and SM1 are used to determine the mode.

BAUD RATE

• Relationship between the crystal frequency and the baud rate in the 8051 is as follows:
• 8051 divides the crystal frequency by 12 to get the machine cycle frequency.
• Load FD/FA/F4/E8 in TH1 to get 9600/4800/2400/1200 baud rate.

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Steps to send data serially in serial port communication

• 1. Set baud rate by loading TMOD register with the value 20H; this indicates timer 1 in mode 2 (8-bit auto-reload) to set baud rate.
• 2. The TH1 is loaded with proper values to set baud rate for serial data transfer.
• 3. The SCON register is loaded with the value 50H, indicating serial mode 1, where an 8- bit data is framed with start and stop bits.
• 4. TR1 is set to 1 to start timer 1.
• 5. Transmit Interrupt(TI) is cleared by CLR TI instruction.
• 6. The character byte to be transferred serially is written into SBUF register.
• 7. The TI flag bit is monitored to see if the character has been transferred completely. 8. To transfer the next byte, go to step 5.
• Check the TI flag bit, we know whether or not 8051 is ready to transfer another byte.
• TI flag bit is raised by the 8051 after transfer of data. ● TI flag is cleared by the programmer by instructions like “CLR TI”.
• Writing a byte into SBUF before the TI flag bit is raised, may lead to loss of a portion of the byte being transferred.

Steps to receive data serially in serial port communication

• 1. Set baud rate by loading TMOD register with the value 20H; this indicates setting of timer 1 in mode 2 (8-bit auto-reload) to set baud rate.
• 2. The TH1 is loaded with proper values to set baud rate.
• 3. The SCON register is loaded with the value 50H, indicating serial mode 1, where an 8- bit data is framed with start and stop bits.
• 4. TR1 is set to 1 to start timer 1.
• 5. Receive Interrupt(RI) is cleared by CLR RI instruction.
• 6. The RI flag bit is monitored to see if an entire character has been received yet.
• 7. When RI is raised, SBUF has the byte, its contents are moved into a safe place. 8. To receive next character, go to step 5.
• It receives the start bit, next bit is the first bit of the character about to be received.
• When the last bit is received, a byte is formed and placed in the SBUF register.
• When the stop bit is received, it makes RI = 1 indicating that the entire character byte has been received and can be written before being overwritten.
• When RI=1, received byte is in the SBUF register, copy SBUF contents to a safe place.
• After the SBUF contents are copied, the RI flag bit must be cleared to 0.

PROGRAMMING FOR SERILA PORT-8051

• #include void main(void)
• {

TMOD=0x20; //use Timer 1, mode 2 TH1=0xFA; //4800 baud rate SCON=0x50; TR1=1; while (1)

{

SBUF=‘A’; //place value in buffer while (TI==0); TI=0;

}

}

Program to send data serially

#include<reg51.h>

void SerTx(unsigned char);

void main(void)

{

TMOD=0x20; //use Timer 1, mode 2

TH1=0xFD; //9600 baud rate

SCON=0x50;

TR1=1; //start timer

while (1) {

SerTx(‘Y’);

SerTx(‘E’);

SerTx(‘S’);

}

}

void SerTx(unsigned char x){

SBUF=x; //place value in buffer

while (TI==0); //wait until transmitted

TI=0;

}

BLOCK DIAGRAM

CIRCUIT DIAGRAM

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