Objectives:
 To interface the serial port of PC with USART of AVR
 To  learn that  how  to  program  the  USART (Universal  Synchronous  Asynchronous
Receiver / Transmitter) of AVR to transmit & receive asynchronously
Introduction:
USART  of  AVR  has  normal  asynchronous, double-speed  asynchronous, master  synchronous
and  slave  synchronous  mode  features.  Synchronous  modes  can  be  used  to  transfer  data
between  AVR  and  external  peripherals  such  as  ADC  and  EEPROMs  etc. In  this  lab,  we  will
learn study and program the ATmega16 to transfer the data between AVR and PC using normal
asynchronous mode.
MAX232 – Level Converter IC
The  serial  port  of  computer  sends/receives  data  serially  at  logic  levels  between -12  to +12V
whereas, microcontroller works at logic levels between 0 to 5V (TTL). So we need a RS-232 to
TTL  and  TTL  to  RS-232  converter  and  this  is  done  by using RS-232  Level  Converter IC,
MAX232 between PC and ATmega16.

Important Registers and Flags Associated with USART:

Following five registers are associated with the USART of AVR:
UDR: USART Data Register: Holds a byte which is received or to be transmitted via USART.

UCSRA: USART Control Status Register A: For controlling serial communication in AVR
UCSRB: USART Control Status Register B: For controlling serial communication in AVR
UCSRC: USART Control Status Register C: For controlling serial communication in AVR
UBRR: USART Baud Rate Register: Value written in this register determines the baud rate

Baud Rate Calculation:

Relationship between desired baud rate and the Fosc (crystal frequency) is given by:

Desired Baud Rate =   Fosc
                                  16(X+1)

Where  X  is  the  value  loaded  in  the  UBRR  for  a  specific  desired  baud  rate. Above
calculation will be true for default setting upon reset.

UCSRA(USART CONTROL AND STATUS REGISTER A)

FE, PE and DOR are valid until UDR is read. Set these to zero for transmission
For  crystal frequency  of  1MHz  and  8MHz,  the  most  commonly  used  baud  rates  for
asynchronous  operation  can  be  generated  by  using  the  UBRR  settings shown  in  the
following table:

UCSRB(USART STATUS AND CONTROL REGISTER  B):

UCSRC(USART STATUS AND CONTROL REGISTER C):

Steps to Program the AVR to Transmit the Data Serially:
1. Enable the USART transmitter through UCSRB
2. Use 8 data bits, asynchronous mode, 1 stop bit and no parity through UCSRC
3. Calculate the value to be loaded in UBRR for a desired baud rate generation
4. Write the character into UDR which is to be transmitted serially
5. Monitor UDRE of TXC bits to check if character has been transmitted. You can
also enable  interrupts  associated  with above  flags  to  execute  their  respective
ISRs upon completion of transmission.
6. To transmit next character, go to step 4
Steps to Program the AVR to Receive the Data Serially:
1. Enable the USART receiver through UCSRB
2. Repeat steps 2 and 3 of transmit program
3. Monitor RXC bit to check if character has been received.  You can also enable
interrupts  associated  with above  flag  to  execute its  respective  ISR upon
reception of one complete character.
4. To transmit next character, go to step 3.

C Code for Serial Communication:

/*
This  program  receives  a  character  from  PC  and  transmit  again to  PC  it  after
incrementing.  When  transmission  is  complete,  PortA  is  incremented  to  show  that
transmission interrupt is executed
*/

#include<avr/io.h>
#include<avr/interrupt.h>
//Macros Definition
#define BitSet(p,m) ((p) |= (m))
#define BitClr(p, m) ((p) &= ~(m))
#define Bit(x) (0x01 << (x))
unsigned char x; //temporary variable, to hold the received character
int main(void)
{
DDRA = 0xFF; //PortA as output
UBRRL = 0x35; //0x35 = 53 ~ 53.0833 = (1000000/(16*1200))-1
BitSet(UCSRB, Bit(7)); //enable Receive Complete Interrupt
BitSet(UCSRB, Bit(6)); //enable Transmit Complete Interrupt
BitSet(UCSRB, Bit(4)); //enable Receiving
BitSet(UCSRB, Bit(3)); //enable Transmission
BitClr(UCSRB, Bit(2)); //enables 8 bit character size
BitSet(UCSRC, Bit(1)); //enables 8 bit character size
BitSet(UCSRC, Bit(2)); //enables 8 bit character size
BitSet(UCSRC, Bit(7)); //writes on UCSRC register if this bit is 1
//when clear, UBRRH value will be updated
sei(); //enable global interrupt
while(1); //stay here forever
}
//ISR for Receive interrupt
ISR (USART_RXC_vect)
{
x = UDR; //Store the received character into x
x++; //increment received character
UDR = x; //Transmit the incremented character
}
//ISR for Transmit interrupt
ISR (USART_TXC_vect) //This interrupt will trigger when transmission is complete
{
PORTA++; //increment PortA to show that transmit interrupt is executed
}