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

[Ned Konz]

On Apr 22, 2006, at 2:29 AM, address@hidden wrote:

> On Sat, 22 Apr 2006 08:30:33 +0200
> Lars Noschinski <address@hidden> wrote:
>
> > * address@hidden <address@hidden>  
> > [2006-04-22 08:09]:
> > >  How can I use the PORTB constant with variables? eg:
> > >
> > > while (1) {
> > >  var <<= 1;
> > >  PORTB = var
> > > }
> >
> > This will work. But after seven iterations (if var is (u)int8_t), var
> > will be 0.
> >
> > > That is what I'm trying to acheive.  I'd like to be able to do it  
> > > with variables
> > > like I do above, but it doesn't seem to work.  Once I replace  
> > > "var" with an
> > > actual number, it works as expected.
> >
> > The problem lies probably elsewhere.
>
>
> OK.  This is some brief code...  This works as expected.
>
> int main() {
>   DDRB = 255;
>   while (1) {
>     int var=1;
>     while (var != 0) {
>       PORTB = var;
>       var <<= 1;
>     }
>   }
>   return 0;
> }
>
>
> 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...
>
> int var 1;
// the problem is that "int" is both signed and 16-bits wide; you want
// an unsigned 8-bit value. And I assume you meant to have an = sign  
in there.
// Better to use:
unsigned short var = 1;
> void blink()
> {
>  PORTB ^= var; // LED on
// assuming you have an LED per pin and the low 8 bits of var are not  
zero, that'd work.
>  sleep(50);
>  PORTB = 255; // LEDs off
>  sleep(50);
> }
>
> void change() {
>   var <<= 1;
// the way you have it written, this would toggle each of the pins of  
PORTB once.
// is that what you're getting?
// The problem is that when var gets to 128, it next becomes 256,  
which is all
// 0s in the low 8 bits, so it won't affect your LEDs. The sequence  
would be:
// 1, 2, 4, 8, 16, 32, 64, 128,    as intended
// 256, 512, 1024, 2048, 4096, 8192, 16384, -32768,    // which won't  
work
// 0, 0, 0, ...   // which clearly won't work
// better to do this (assuming var is 8 bits):
        var <<= 1;
        if (var == 0) var = 1;
> }
>
> int main() {
>   DDRB = 255;
>   PORTB=255;
>   while (1) {
>     blink();
>     change();
>   }
>   return 0;
> }
>
>
> This doesn't work as expected...  not sure why.

It's always better to say *what it does do*, and *what you expected*,  
rather than "doesn't work as expected" if you want people to help you.

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

It's hard to tell why you would like to keep the functions.
You're probably in a better position to say why <g>.
--
Ned Konz
MetaMagix embedded consulting
address@hidden