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Re: [avr-gcc-list] Using PORTB with variables, PORTB = var

From: Matthew MacClary
Subject: Re: [avr-gcc-list] Using PORTB with variables, PORTB = var
Date: Sat, 22 Apr 2006 09:20:45 -0700
User-agent: Mutt/1.4.1i

On Sat, Apr 22, 2006 at 07:29:10PM +1000, address@hidden wrote:
> This example doesn't, and I don't know why... I know it's specific
> to C, so that's why I'm asking here...

    When asking a question like this, it would be useful for you to
describe exactly what you are seeing and what you expected to
see. Here is my interpretation of what you are probably seeing: in
less than half a second, a lit LED shifts across your output port and
then nothing else ever happens again.

> I would like to keep the functions if possible, I'm thinking it has
> something to do with scope perhaps?

    You can do what you seem to try to be doing using functions. The
only scope problem I see is using a globally scoped variable

> I am new to C by the way, if it isn't evident already (:

    Great, you can learn the right way to do things from the start
without fighting bad habits!
    Here is how I would implement your program. I apologize in advance
since I don't have a board available to try the code on. I hope that
the main ideas are apparent, even if I didn't squash all the bugs.


void blink(volatile uint8_t * port, uint8_t mask) {

  *port = ~ mask;     // LEDs on
  sleep(500);         // milliseconds
  *port = 255;        // LEDs off

void change(uint8_t * var) {
  *var <<= 1;
  if (0 == *var) {
    *var = 1;

int main() {
  uint8_t led_mask = 1;

  DDRB = 255;  // PORTB as outputs
  PORTB= 255;  // LEDs off

  while (1) {
    blink(PORTB, led_mask);
  return 0;

/* Changes

1) Don't use global variables (a common exception to this rule is to
   communicate with an interrupt routine). [In my team you would lose
   a hand for doing this.]

2) Don't assign a 16 bit signed integer value to an 8 bit unsigned
   location (the port). [This would cost you your other hand ;-) ]

3) Don't write non-reentrant functions, that depend on and modify
   global data.

4) Don't blink the LEDs faster than you can see them.

5) Don't keep shifting a variable the same way forever, since it will
   end up all zeros.

6) Do keep programming AVRs in C!!


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