The AVR Architecture and I/O Registers

Topic: Introduction to Atmega 8 and its basic architectures to start with!!!

Refer to this data sheet, while I describe it to you.

Atmega8 Datasheet

Okay, Here you can see some of the properties of Atmega8 Ic that you need to know for now:

1. Operating Voltages: (2.7v to 5.5v) for atmega8L (Low power atmega) and (4.5v to 5.5v) for Atmega8.

** This should be very clear from the first, these digital ICs work like logical 0 for 0 v to 2.5v and logical 1 for 2.5v to 5.5v and above 5.5 the MCU won't recognize the i/p voltage.

2. (0-8)mhz Crystal for Atmega8L and (0-16)mhz Crystal for Atmega8.

** Crystal is the clock frequency needed for the MCU for its processing. These are basically the heart-beat of the system. Clock Frequency is  analogous to the processor speed of our PCs. 

3.   There are 32 Register Blocks of 8 bit each (as Atmega is 8bit mcu).

4.   6 channel, 10 bit ADC and 3 channel PWM are available.

I/O Registers of Atmega8

There are 3 Ports in Atmega 8 viz Port B, Port C and Port D. 

Each of the pins of the ports can be used as General Purpose Input/Output Pins.

They are used for sensing an input device like switches, temperature/humidity sensor, IR etc or running an output device like Leds, Motors etc.

N.B: A particular pin can be used as either input or output at a time.

To work with the I/O pins, three registers of 8 bits each have to be configured viz., DDRX, PORTX and PINX. (here 'X' indicates the name of the register B,C or D)

Data Direction Register (DDRX) : 

A MCU doesnot know what type of device is interfaced to its pins unless the programmer mentions it inside the code. Thus, we have to set its corresponding DDR bits as 0 or 1 in our code. Thus DDRX code is used to inform the MCU that it should give voltage to a certain pin or check the status of the pin.

For Eg. :  Let there is a Led (output device) connected to PB0, So the first bit of the PORTB register should be 1 and the rest are 0s. 

Note: 

• If a pin has nothing on it or it has an input device, that pin should be given 0 any way.

• The register bits are arranged from MSB to LSB as follows.

PORTX Register :

This register is used to give a logical 1 or 0 output at the output pins. Logical 1 represents voltages between 2.5 and 5.5. logical 0 repesents voltages between 0 and 2.5.

PINX Register :

This register is used to check the status of an input pin. It reads the analog or digital data from switches and sensors.

Sample Code:

1. Blinking Leds code.

#include<avr/io.h>     // Header File for AVR MCU.

#include <util/delay.h>    // Header File for delay functioning.

int main(void)

{

     DDRB=0b00001111;       // PB0 to PB3 are selected as the output pins, i.e., leds are connected to pins 14,15,16,17 of atmega8.  

while(1)                   //This is for the the infinity loop.

{

PORTB=0b00001111;       //Switch on all 4 LEDs

_delay_ms(2000);                       //Delay of 2sec=2000 ms

PORTB=0b00000000;       //Switch off all 4 LEDs

_delay_ms(2000);                       //Delay of 2sec=1000 ms

}  

}

Circuit Diagram: