AVR Programming:

Programming is the simplest part in robotics. We will use avr-gcc version of ‘C’, which is specifically designed for avr based microcontrollers.

Before you start any of your projects:

1. Plan your actions and visualize your expectations and outcome of the project
2. When writing a program, it is always good to write a pseudo code of what you expect the code to do. Note down all steps in simple English, or in any other language you prefer
3. You do not need to learn complex software to create a simple circuit. A rough sketch on a paper or even a paint program on your desktop would suffice. Once you are comfortable, get into those advanced simulators and circuit design software.
4. Working with electricity and these tiny components can be dangerous. Be cautious.
5. Last but not the least, do not try to overdo anything in a single day. If you are stressed, go out and enjoy. Love what you do and be proud of your mistakes too.

Let’s get started. Here is the code required to turn on a LED. You can download a copy of this program with necessary comments and the makefile, upload it to your microcontroller and see the results (To upload the code, you need a programmer which connects your computer and the microcontroller). Later I will explain what each line means, why and how are they used.

/*
File Name      : LEDON.c
Title          : Basic LEDON Program. 
Notes          : This is the first program in Basic LED series. 
All this program does is set DDR and Port registers to turn on a LED
*/
#ifndef F_CPU
#define F_CPU 1000000UL
#endif
#include <avr/io.h>    
int main(void)
{
DDRC = 0xff;
while (1)
{
PORTC = 0xff;
}
return (0);
}

Now we will understand what each line means.

/*
File … blah… blah…
*/

Anything between  /* and */ is considered as comments and is ignored by the complier. It is always a good practice to include brief description of your program. This practice helps you in better identification and documentation of what the program does. For inline comments, you can use //.

// This is an example of inline comment

#ifndef F_CPU
#define F_CPU 1000000UL
#endif

F_CPU is used to define the clock frequency in Hertz. It is surrounded by #ifndef (If not defined) and #endif which means this code should be executed if F_CPU is not already defined. This is necessarily not required as we will define the clock frequency in makefile.

Makefile??? Now what is a makefile? makefile is a set of instructions for any specific project which directs 'make'(utility that builds executable programs and libraries using your source code) on how to compile and link a program. It supports in finding the target file and dependencies if any. It also specifies what avr device is being used. Since we are using AVRStudio, makefile is automatically generated.

WinAVR already has a template for makefile where you need to make minimal changes for your project to get it up and running. If you have installed WinAVR, then you are most likely to find the file at <install>\sample directory, where <install> is the directory where you have WinAVR installed.

#include <avr/io.h>

Programmers generally separate certain elements of code from the program’s source code which can be reused in other programs when required. #include is used to include these separate chunks of code from an external file into this program. avr/io.h is one such file which contains I/O definitions for the particular device we use. If we use Atmega8, then respective file is included. But how does the compiler know which device we use? Very simple; we define the type of device in the "makefile” and appropriate device file ioxxxx.h (where xxxx is the device name) is included. It further includes additional header files:  #include <avr/sfr_defs.h>, #include <avr/portpins.h>, #include <avr/common.h>, #include <avr/version.h>. http://www.nongnu.org/avr-libc/user-manual/group__avr__io.html provides a complete description of io.h. If you have read the documentation, it clearly tells you not to include ioxxxx.h file directly and always get it called from io.h.

int main(void)

 The actual processing starts from main() which is common in most programming languages

DDRC  = 0xff;

To understand this line, you should have already referred to AVR basics. If not, take a break, read through the details in AVR Basics and return. Summarizing those details, all we do in this line is to set DDRC (Data-Direction-Register) as output. Be informed that 0xff (hexadecimal) is the same as 0b11111111 (binary), where "0x" refers to hexadecimal value, and “0b” refers to binary. You can as well write DDRC = 0b11111111;
If you are still not clear, refer to number-system tutorial for number conversion. In your program, you can either use binary or hexadecimal although I personally prefer hexadecimal values as it is easier to remember and write.

while(1)

Tutorial index:

1. Introduction: 
2. Electronics:
3. Breadboard-Programmer Setup:
4. Programming:
5. Blinking LED:
6. Bitwise Operators:
7. Assignment Operators:
8. Bitwise Shift Operators:

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