Serial Communication Tutorial: Programming Microcontroller
►AVR Programming for UARTIn AVR, following set of registers are used to communicate over USART
- UCSRA
- UCSRB
- UCSRC
- UBRRH
- UBRRL
UCSRA: in this register there are flags for various errors that might occure during data transmission, e.g. parity error, frame error etc.
UCSRB: in this register we have a lot of enable bits. For example different interrupt enable bits but also the reciving and transmitting enable bits.
UCSRC: in this register we set the parity mode, stop bits and so on.
UBRRH & UBRRL: in UBRRH register, the higher byte and in UBRRL, lower byte is stored for generating a required Baud rate.
More information on the above registers can be found in the datasheet of the AVR you are using.
►Initialising USART in AVRCODE:
.include "m8515def.inc"
.def reg1 = r16
.def reg2 = r17
Serial_Init:
;Load UBRRH with 0 and UBRRL with 25
;to set a baud rate of 9600 at 4MHz
ldi reg1,00
out UBRRH,reg1
ldi reg2,25
out UBRRL,reg1
;Clear all error flags
ldi reg1,00
out UCSRA,reg1
;Enable Transmission and Reception
ldi reg1,(1<<RXEN)+(1<<TXEN)
out UCSRB,reg1
;Set Frame format
;8,N,1
ldi reg1,(1<<URSEL)|(3<<UCSZ0)
out UCSRC,reg1
ret
in C we can do this as..
CODE:
#include <avr/io.h>.
void serial_init(){
UCSRA=0x00;
UCSRB=0x18;
UCSRC=0x86;
UBRRH=0x00;
UBRRL=0x19;
}
To transmit data serial we need to put the data to send in 8-bit UDR (UART Data Register) and poll the empty transmit buffer to set. While receiving data we wait for the receive flag, when its set the data recieved can be read from the UDR register. This can be done as follows...
CODE:
Serial_Send:
;wait for empty transmit buffer flag
sbis UCSRA, UDRE
rjmp Serial_Send
;If the flag is set
;Then move the data to send in UDR
out UDR,reg2
ret
Serial_Read:
;Wait for Receive flag
sbis UCSRA,RXC
rjmp Serial_Read
;If falg is set
;Then read data from UDR
in reg2,UDR
ret
in C we can do this as..
CODE:
void serial_send(unsigned char dat){
while(!(UCSRA & (1<<UDRE)));
UDR = dat;
}
unsigned char serial_read(){
while(!(UCSRA & (1<<RXC)));
return UDR;
}
In 8051, we make use of Timer 1 to generate the required baud rate. Following are the registers that are need to be configured to commnunicate over UART.
TMOD: This register is used to set the mode of Timer0 and Timer1. It is also used to select whether the timers are used as Timer or Counter.
SCON: Serial Control register has various functions like.. it has flags for Framing error, Transmit interrup and receive interrupt. Its used to select the serial port mode, to enable or disable the reception etc.
TCON: This register has varios flag and control bits e.g. Timer overflow flags, interrupt edge flags, timer control bits to start/stop the timer.
TH1 & TL1: Timer registers for Timer 1 determines the baudrate of UART.
More information on the above registers can be found in the 8051 Hardware manual.
►Initializing USART in 8051CODE:
Serial_Init:
;Set timer 1 mode to 8-bit Auto-Reload
mov TMOD,#20H
;Enable reception
;Set Serial port mode to 8-bit UART
mov SCON,#50H
;Set baudrate to 9600 at 11.0592MHz
mov TH1,#0FDH
mov TL1,#0FDH
;Start Timer
setb TR1
ret
in C we can do this as..
CODE:
#include <reg51.h>.
void serial_init(){
TMOD = 0x20;
SCON = 0x50;
TH1 = 0xFD;
TL1 = 0xFD;
TR1 = 1;
}
To Send data to the serial port we just have to move the data in SBUF (serial buffer register) and wait for the Transmit Interrupt flag to be set. While receiving we wait for the Receive interrupt flag to be set and read the data from SBUF register. This can be done as shown below...
CODE:
Serial_Send:
;wait for last data to be
;sent completely
jnb TI,Serial_Send
;clear the transmit interrupt flag
clr TI
;Then move the data to send in SBUF
mov SBUF,A
ret
Serial_Read:
;Wait for Receive interrupt flag
jnb RI,Serial_Read
;If falg is set then clear it
clr RI
;Then read data from SBUF
mov A,SBUF
ret
in C we can do this as..
CODE:
void serial_send(unsigned char dat){
while(!TI);
TI = 0;
SBUF = dat;
}
unsigned char serial_read(){
while(!RI);
RI = 0;
return SBUF;
}
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