The attiny2313 has a built in UART for serial communication. You need a few outside parts to allow it to communicate with a PC over a serial connection. The attiny2313 uses TTL signals. You have to convert them to true RS232 signals using an external converter. For this example I will use a MAX232.

The AVR has an internal RC oscillator. The Atmel data sheet says this oscillator is calibrated to within +- 10%. You may be able to get a serial connection to work, but because of the margin of error it is best to have your chip use an external watch crystal instead of the internal RC oscillator. I have been successful in getting serial communication to work using only the internal RC oscillator but your mileage may vary.

Parts list:
1 MAX232IN ( mouser 595-MAX232IN)
4 16V 10uF capacitors ( mouser 140-MLRL16V10-RC)
1 3.6846 cyrstal
2 20pf capacitors
1 DB9 female serial connector

Below is the wireing schematic:

To get the AVR to use the external 3.6846 crystal instead of the internal RC oscillator I had to burn the fuses. I am using linux and avrdude, so I ran this command:
avrdude -p attiny2313 -P /dev/parport0 -c dt006 -u -U lfuse:w:0xed:m

Now the avr will only run with the external crystal.

First we look at the data sheet see what UBRRL needs to be set for our baud rate and crystal. We will be transmitting at 2400 baud.

On the PC side I will be using minicom on a linux PC. Set minicom (or hyperterminal on windows) to 2400 baud, 8N1. Minicom gave me a few problems when it would first connect to the AVR. To get around this, I would start minicom, then AFTER minicom was running, I would power up the AVR. I am not sure why this happened. If I started minicom after the AVR was powered up, I would get some garbage characters only.

This sample program will have the Avr listen for a character, then transmit back the next highest character. So if you type in the letter “B”, the avr will send be the letter “C”. (This program is written C and should would with the gcc compiler)

The code may be downloaded here.
Code:
#include <avr/io.h>
#include <avr/interrupt.h>

/* Prototypes */
void InitUART (unsigned char baudrate);
unsigned char ReceiveByte (void);
void TransmitByte (unsigned char data);


/* --------------------------------------------------------------
 Main - program that recieves a character then transmits back the next character.  
An example would be if you send in an A, the chip will return a B
---------------------------------------------------------------- */
int
main (void)
{
  unsigned char i;
  InitUART (95);     /* Set the baudrate to
2400 bps using a 3.6846MHz crystal */
  while (1)
    {
      TransmitByte (ReceiveByte () + 1);       
/* Echo the received character + 1.  Example send in A then send out B */
      for (i = 0; i < 200; i++);
    }
}

/* Initialize UART */
void
InitUART (unsigned char baudrate)
{
  /* Set the baud rate */
  UBRRL = baudrate;

  /* Enable UART receiver and transmitter */
  UCSRB = (1 << RXEN) | (1 << TXEN);

  /* set to 8 data bits, 1 stop bit */
  UCSRC = (1 << UCSZ1) | (1 << UCSZ0);

}

/* Read and write functions */
unsigned char
ReceiveByte (void)
{
  /* Wait for incomming data */
  while (!(UCSRA & (1 << RXC)));

  /* Return the data */
  return UDR;
}

void
TransmitByte (unsigned char data)
{
  /* Wait for empty transmit buffer */
  while (!(UCSRA & (1 << UDRE)));

  /* Start transmittion */
  UDR = data;
}

Resources used to make this howto:
http://www.booksbybibin.blogspot.com/ (has a great free ebook on AVR programming)
Avrfreaks design note #026 (also includes an alternate way to convert from ttl to rs232) http://avrfreaks.net/modules/FreaksFiles/files/405/DN_026.pdf