Serial Communication:
BASICS OF SERIAL COMMUNICATION:
· serial communication uses single data line making it much cheaper
· enables two computers in different cities to communicate over the telephone
· byte of data must be converted to serial bits using a parallel-in-serial-out shift register and transmitted over a single data line
· receiving end there must be a serial-in-parallel-out shift register
· if transferred on the telephone line, it must be converted to audio tones by modem
· for short distance the signal can be transferred using wire
· how PC keyboards transfer data to the motherboard
· 2 methods, asynchronous and synchronous
· synchronous method transfers a block of data (characters) at a time
· asynchronous method transfers a single byte at a time
· Uses special IC chips called UART (universal asynchronous receiver-transmitter) and USART (universal synchronous asynchronous receiver-transmitter)
· 8051 chip has a built-in UART
· Half- and full-duplex transmission
· if the data can be transmitted and received, it is a duplex transmission
· simplex transmissions the computer only sends data
· duplex transmissions can be half or full duplex
· depends on whether or not the data transfer can be simultaneous
· If one way at a time, it is half duplex
· If can go both ways at the same time, it is full duplex
· full duplex requires two wire conductors for the data lines (in addition to the signal ground)
· Asynchronous serial communication and data framing
· data coming in 0s and 1s
· to make sense of the data sender and receiver agree on a set of rules
· Protocol
· how the data is packed
· how many bits/character
· when the data begins and ends
· Start and stop bits
· asynchronous method, each character is placed between start and stop bits
· called framing
· start bit is always one bit
· stop bit can be one or two bits
· start bit is always a 0 (low)
· stop bit(s) is 1 (high)
· in modern PCs one stop bit is standard
· when transferring a text file of ASCII characters using 1 stop bit there is total of 10 bits for each character
· 8 bits for the ASCII code (1 parity bit), 1 bit each for the start and stop bits
· for each 8-bit character there are an extra 2 bits, which gives 20% overhead
· Data transfer rate
· rate of data transfer bps (bits per second)
· widely used terminology for bps is baud rate
· baud and bps rates are not necessarily equal
· baud rate is defined as the number of signal changes per second
· RS232 standards
· most widely used serial I/O interfacing standard
· input and output voltage levels are not TTL compatible
· 1 bit is represented by -3 to -25 V
· 0 bit is +3 to +25 V
· -3 to +3 is undefined
· to connect RS232 to a microcontroller system 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
·
Data communication classification
· DTE (data terminal equipment)
· DCE (data communication equipment)
· DTE - terminals and computers that send and receive data
· DCE - communication equipment responsible for transferring the data
· simplest connection between a PC and microcontroller requires a minimum of three pins, TxD, RxD, and ground
· Examining RS232 handshaking signals
· many of the pins of the RS-232 connector are used for handshaking signals
· they are not supported by the 8051 UART chip
· PC/compatible COM ports
· PC/compatible computers (Pentium) microprocessors normally have two COM ports
· both ports have RS232-type connectors
· COM ports are designated as COM 1 and COM 2 (replaced by USB ports)
· can connect the 8051 serial port to the COM 2 port
8051 CONNECTION TO RS232:
· RxD and TxD pins in the 8051
· 8051 has two pins used for transferring and receiving data serially
· TxD and RxD are part of the port 3 group
· pin 11 (P3.1) is assigned to TxD
· pin 10 (P3.0) is designated as RxD
· these pins are TTL compatible
· require a line driver to make them RS232 compatible
· driver is the MAX232 chip
· MAX232
· converts from RS232 voltage levels to TTL voltage levels
· uses a +5 V power source
· MAX232 has two sets of line drivers for transferring and receiving data
· line drivers used for TxD are called T1 and T2
· line drivers for RxD are designated as R1 and R2
· T1 and R1 are used together for TxD and RxD of the 8051
· second set is left unused
· MAX233
· MAX233 performs the same job as the MAX232
· eliminates the need for capacitors
· much more expensive than the MAX232
8051 SERIAL PORT PROGRAMMING IN ASSEMBLY:
· Baud rate in the 8051
· serial communications of the 8051 with the COM port of the PC
· must make sure that the baud rate of the 8051 system matches the baud rate of the PC's COM port
· can use Windows HyperTerminal program
· Baud rate in the 8051
· baud rate in the 8051 is programmable
· done with the help of Timer 1
· relationship between the crystal frequency and the baud rate in the 8051
· 8051 divides the crystal frequency by 12 to get the machine cycle frequency
· XTAL = 11.0592 MHz, the machine cycle frequency is 921.6 kHz
· 8051's UART divides the machine cycle frequency of 921.6 kHz by 32 once more before it is used by Timer 1 to set the baud rate
· 921.6 kHz divided by 32 gives 28,800 Hz
· Timer 1 must be programmed in mode 2, that is 8-bit, auto-reload
Ex-1 (a):
With XTAL = 11.0592 MHz, find the TH1 value needed to have the following baud rates. (a) 9600 (b) 2400 (c) 1200
· machine cycle frequency
= 11.0592 MHz / 12 = 921.6 kHz
· Timer 1 frequency provided by 8051 UART
= 921.6 kHz / 32 = 28,800 Hz
(a) 28,800 / 3 = 9600 where -3 = FD (hex)
(b) 28,800 / 12 = 2400 where -12 = F4 (hex)
(c) 28,800 / 24 = 1200 where -24 = E8 (hex)
· SBUF (serial buffer) register
· a byte of data to be transferred via the TxD line must be placed in the SBUF register
· SBUF holds the byte of data when it is received by the RxD line
· can be accessed like any other register
MOV SBUF,#'D' ;load SBUF=44H, ASCII for 'D‘
MOV SBUF,A ;copy accumulator into SBUF
MOV A,SBUF ;copy SBUF into accumulator
· when a byte is written, it is framed with the start and stop bits and transferred serially via the TxD pin
· when the bits are received serially via RxD, it is deframe by eliminating the stop and start bits, making a byte out of the data received, and then placing it in the SBUF
· SCON (serial control) register
· to program the start bit, stop bit, and data bits
· SM0 and SM1 determine the mode
· only mode 1 is important
· when mode 1 is chosen, the data framing is 8 bits, 1 stop bit, and 1 start bit
· compatible with the COM port of PCs
· mode 1 allows the baud rate to be variable and is set by Timer 1 of the 8051
· for each character a total of 10 bits are transferred, where the first bit is the start bit, followed by 8 bits of data, and finally 1 stop bit.
· REN (receive enable)
· REN=1, allows 8051 to receive data on the RxD
· if 8051 is to both transfer and receive data, REN must be set to 1
· REN=0, the receiver is disabled
· SETB SCON.4 and CLR SCON.4,
· TI (transmit interrupt)
· when 8051 finishes the transfer of the 8-bit character, it raises the TI flag to indicate that it is ready to transfer another byte
· RI (receive interrupt)
· when the 8051 receives data serially via RxD, it places the byte in the SBUF register
· then raises the RI flag bit to indicate that a byte has been received and should be picked up before it is lost
· Program to transfer data serially
· TMOD register is loaded with the value 20H
· TH1 is loaded with value to set the baud rate
· SCON register is loaded with the value 50H
· TR1 is set to 1 to start Timer1
· TI is cleared by the "CLR TI" instruction
· transmit character byte is written into the SBUF register
· TI flag bit is monitored to see if the character has been transferred completely.
Serial Data Transmission Procedure:
Ex1(b):
Write a program to transfer letter "A" serially at 4800 baud, continuously.
#include<reg51.h>
unsigned char i;
int j;
void main()
{
while(1)
{
TMOD=0X20;
SCON=0X50;
TH1=0XFD;
TR1=1;
SBUF=A;
while(TI==0);
TR1=0;
TI=0;
}
Ex 1(c):
Write a program to transfer the message "Hello" serially at 9600 baud, 8-bit data, 1 stop bit. Do this continuously.
#include<reg51.h>
unsigned char i;
int j;
unsigned char arr[]={"Welcome To All"};
void main()
{
while(1)
{
int z;
TMOD=0X20;
SCON=0X50;
TH1=0XFD;
for(z=0;z<=16; z++)
{
TR1=1;
SBUF=arr[i];
while(TI!=1);
TR1=0;
TI=0;
}
}
Ex 1(d):
Program the 8051 to receive bytes of data serially and put them on PORTB. Set the baud rate at 9600, 8-bit data, and 1 stop bit.
#include<reg51.h>
void dis();
void com();
int i;
sbit rs = P3^7 ;
sbit rw = P3^6 ;
sbit en = P3^5 ;
int c[]={0X01,0X0e,0X38,0X80,0X06} ;
void main()
{
for(i=0;i<5;i++)
{
P1=c[i];
com();
}
while(1)
{
TMOD=0X20;
SCON=0X50;
TH1=0XFD;
TR1=1;
while(RI!=1);
P1=SBUF;
dis();
TR1=0;
RI=0;
}
}
void delay(int u)
{
int l,o;
for(l=0;l<u;l++)
{
for(o=0;o<1250;o++)
{
}
}
}
void com()
{
rs=0;
rw=0;
en=1;
delay(10);
en=0;
}
void dis()
{
rs=1;
rw=0;
en=1;
delay(10);
en=0;
}
