AVR microcontrollers are the advanced microcontrollers. From 1996 onwards these are come into existence. In AVR family there are so many controllers are available. Mainly the AVR family is sub grouped as ATmega, ATtiny, Xmega, UC3, SAM3 and SAM4. In these form 8 to 32 bit controllers are available. All the controllers having some common and some different features, those are coming to common features all are having I/O ports, timer/counters, interrupts and etc, coming to special features those are A/D converters, PWM, D/A converter, on chip I2C, serial communication interfacings, on chip EEPROM and etc. we can use all these features according to our requirement by programming.
First we have to discuss about I/O ports. Generally AVR microcontrollers having four I/O ports named as PORTA, PORTB, PORTC, PORTD. Take example as ATMEGA8 or ATMEGA 16 or ATMEGA32 microcontrollers, these are any having four I/O ports and each port having 8 I/O lines and totally each controller having 32 I/O lines. These 32 I/O lines are bi-directional means we can use these I/O lines either input or output. In addition to each this pin has some of other functions like ADC, timers, interrupts, serial communication pins and other pins. To perform any operation with general purpose I/O (GPIO) pins there is a need to configure three registers. Those registers named as DDRx, PORTx, PINx (here ‘x’ indicates the name of the register A or B or C or D).
Each registers of these three registers are 8-bit registers means generally each port has 8 pins as like each register has 8-bits and each pin refers on bit of register. If we want to configure any pin of the port we can configure the corresponding bits of all three registers. In this article I am taking example as ATMEGA32 microcontroller. Now conceder pins 22-29 of the microcontroller, if we want to configure these 8-pins, there is a need of configure corresponding three registers of 8-bits. Here explaining clearly, in ATMEGA32 pin 22-29 refers PORTC. So if we want to configure PORTC, we need to configure DDRC, PORTC and PINC registers.
These 8-bits are divided into two 4-bit groups and named as lower nibbles upper nibbles. 0-3 bits are called as lower nibbles and 4-7 bits are called as upper nibbles.
Configuring the PORT:
We have discussed above that if want to configure one port; there is a need to configure corresponding three registers of the port. So now we are configuring the PORTc, so we need to configure DDRC, PORTC and PINC registers. Now we can discuss in detail about these three registers here.
The name of the register is Data Direction Register. The name only indicates complete use of the register that is data direction. There are only two directions. Those are controller to module and module to controller. If data direction is from module to controller that is input, if data is passing from controller to module that is output. So the two directions are input direction and output direction.
This register is used to assign the pin of the port as either input direction or output direction. In this register the port and pin indicated as “DDRxn”, here ‘x’ indicates the port name and ‘n’ indicates the pin number. For example pin 26 refers to 4th pin of PORTC is defined as DDRC4. Generally these GPIO pins are digital I/O pins that mean these are having only two logics as logic0 and logic1. Same like these DDR also. If DDRxn is written as logic one, the pin is configured as output pin. If DDRxn is written as logic zero, the pin is configured as input pin.