Interface LED with AVR Microcontroller

All the four ports can be configured to read an input from some external device or to give output to any external device as per the application. For e.g., a switch is connected to a particular pin, that pin should be configured as input to read the values from the switch (external Device in this case) and if you are connecting a LED to any pin of the port then that particular pin should be configured as output to transmit the signal to the LED (external device in this case). A single port can be configured such that some of the pins of the same port are input and some are output.

Configuring IO Ports:

Every port (PORTx, x = A or B or C or D) of AVR microcontrollers have three registers associated with it:

1.      DDRx: Data direction Register, to set the direction of each pin of PORTx and configuring it to be as input or output.

2.      PORTx: The values which are to be supplied at the output of the port are written in this register. These values acts as input to the device connected at output port of the microcontroller (through PORTx output configured pins).

3.      PINx: This register stores the input value from the external connected hardware, when the port is configured as input port. The input data is read from PINx register.

So the first step in configuring or initializing any of the IO port is to set its direction in data direction register (DDRx) to define the behavior of individual pins as input or output. A high (1) in any bit of the DDRx register means the corresponding pin is set as output and vice versa.

Example: Considering switch-LED scenario, suppose a switch is connected to Pin5 (PORTD.4) of PORTD and LED is connected to Pin8 (PORTD.7) of PORTD. Now we need to initialize the pins accordingly. The other pins of PORTD are not used in this case so they can be ignored as of now.

STEP-1: In order to configure PORTD.4 as input the value of Pin-5 (DDRD.5) in DDRD register is made 0.

DDRD.7	DDRD.6	DDRD.5	DDRD.4	DDRD.3	DDRD.2	DDRD.1	DDRD.0
-	-	0	-	-	-	-	-

Step-2: To initialize PORTD.7 as output the value of Pin-8 (DDRD.7) in DDRD register is made 1.

DDRD.7	DDRD.6	DDRD.5	DDRD.4	DDRD.3	DDRD.2	DDRD.1	DDRD.0
1	-	0	-	-	-	-	-

Step-3: Rest of the pins can have any value as they are not being used in this case. The default values of DDRx register is 0 for each pin i.e., all the ports of AVR microcontrollers are initialized as input.

DDRD.7	DDRD.6	DDRD.5	DDRD.4	DDRD.3	DDRD.2	DDRD.1	DDRD.0
1	0	0	0	0	0	0	0

So the value of DDRD will be initialized as 0x80.

The above command can also be written in this form:

Where, 0b is symbol used to represent binary numbers and following it is the actual 8-bit value.

Setting the pull up value:

All the four ports of Atmega16 are equipped with internal software controlled pull up resistors. Every pin of the ports can be pulled up by setting the values into the register PORTx. Following table illustrates the actual state of the pin with different combination of DDRx and PORTx values.

DDRx	PORTx	I/O State	Pull Up	Description
0	0	Input	No	PORTx pins set as input and pull-ups disabled.
0	1	Input	Yes	PORTx pins set as input with pull up enabled
1	0	Output	No	PORTx pins set as output and pull-ups disabled.
1	1	Output	Yes	PORTx pins set as output with pull ups enabled.

Example: Following commands can be used to disable and enable pull-ups on PORTD.

PORTD = 0xFF;         //PORTD pins pulled high.

PORTD = 0x00;          //Disable pull up registers.

To understand above commands a simple LED blinking experiment is explained below.

Circuit description:

Connect the circuit as shown in the circuit diagram. There is no crystal in the circuit. This project uses the inbuilt crystal of AVR microcontroller, thereby avoiding the need of external crystal. The value of the internal crystal varies from 1MHz to 8MHz, which can be set using the fuse bits. (Readers may connect the external crystal but modify your fuse bits accordingly. Fuse bits are explained in later articles).

Explanation of code: 

#include<avr/io.h>

To include all the input output files for avr microcontroller.

#include<util/delay.h>

WinAvr has in built function for providing delay. To use the delay functions include the above header file.

DDRA=0xFF;

To make port A as output port

PORTA=~PORTA;

Whatever was the value of PORTA compliment it and send back to PORTA.

_delay_ms(1000);

A delay function, gives a delay of 1000 millisecond.

To understand the compilation and execution of the above program, please refer the tutorial Working with AVRstudio.

What is to be observed?

Similarly, a code can be for blinking of LED on all four PORTS. It is observed that all pins are working properly except PC2, PC3, PC4 and PC5 pins. The reason is that the AVR microcontrollers has inbuilt JTAG, which needs to be disabled for using these pins as I/O pins.