Nuts and Volts - September 2008

SMILEY’S WORKSHOP

CylonEyes.c into CylonEyes.hex. If you raise the hood on WinAVR, you would see a massively complex set of software that has been created over the years by folks involved in the open software movement.

When you have questions about WinAVR — and you will — check out the forums at www.AVRFreaks.net, especially the GCC forum, since WinAVR uses GCC to compile the C software. Try searching the forums before asking questions since someone has probably already asked your question and received good responses. Forum helpers tend to get annoyed with newbies who don’t do sufficient background research before asking questions.

■ FIGURE 11. Base board with CylonEyes.

Joe Pardue ( nv@smileymicros.com) has a BSEE and operates www.smileymicros.com from the shadows of the Great Smokey Mountains in Tennessee. He is author of Virtual Serial Port Cookbook and C Programming for Microcontrollers.

The Main() Thing

All C programs must have a ‘main’ function that contains the code that is first run when the program begins.

understand they do make working with microcontrollers easier, but for now, just humor me. The program tests the while(1) and finding it true, proceeds to the ‘for’ statement, which is also true and passes to the line:

PORTD = i;

int main (void)
{
// Do something
}

Listing 2 shows what CylonEyes has. In this function, we leave C for a moment and look at things that are specific to the AVR microcontroller. The line:

Listing 2

int main (void)
{

DDRD = 0xFF;

int i = 0;
// set PORTD for output
DDRD = 0xFF;

sets the microcontroller Data

Direction Register D to equal 0xFF.

This tells the microcontroller that port D pins, which are hooked up to our LEDs, are to be used to output voltage states (which we use to turn the LEDs on and off). We use the hexadecimal version, 0xFF, of 255 here because it is easier to understand what’s happening.

You disagree? Well, if you persist with these workshops, you’ll be using ^}hexadecimal numbers like a pro and

while(1)
{

This causes the microcontroller to set the port D pins to light up the LEDs with the pattern made by the value of i.

Say what? Okay, ‘i’ starts off equal to 1, which in binary is 00000001. This provides +3V on the rightmost

LED, and leaves the other LEDs unlit at 0V.

The first ‘for’ loop runs eight times, each time moving the lit LED to the left, then it exits. In the next ‘for’ loop, the -= operator subtracts i/2 from i and sets i equal to the results causing the LED to move to the right. When it is finished, the loop runs again ... for how long? Right! Forever! Or at least until either the universe ends or you unplug the Butterfly. We skimmed over a lot that you’ll see in detail in later workshops. You now know just enough to be dangerous and I hope the learning process hasn’t caused your forehead to do too much damage to your keyboard. NV