Re: [avr-chat] Mega644 bit test problem

[tarmigan]

(Reading through some very old email, it looks like this one might not
have been resolved...)

On Fri, May 8, 2009 at 2:41 AM, Robert von Knobloch<address@hidden> wrote:
> Hallo,
>
> I am writing code for an ATMega 644P and have come across some very
> strange behaviour.
> A small part of the code is required to read an asynchronous serial
> signal (data & clock - low speed) over PORTB pins.
> The relevant code snippet is:
>
> snip
> #define CLOCK_TIMEOUT    2000
> #define ASYNCPIN    PORTB
> #define ASYNCCLK    0
> #define ASYNCDATA    3
>
>    for (i = 0 ; i < 16 ; ++i)
>    {
>        while ((ASYNCPIN & _BV(ASYNCCLK)) != 0)        // Wait for clock low
>        {
>        if (mSecdowncnt == 0)
>        {
>            noclock = 1;
>            break;                    // Timeout
>        }
>        }
>
>        result = result >> 1;
>        if ((ASYNCPIN & _BV(ASYNCDATA)) != 0)
>        {
>        result |= 0x8000;
>        }
>
>        while ((ASYNCPIN & _BV(ASYNCCLK)) != 1)        // Wait for clock

This line is probably the problem.

> This works fine. I want, however to use portb bits 1 & 3 for clock &
> data. I simply change to #define ASYNCCLK    1
> and re-compile
> Now, nothing works and the code generated is much smaller (0xe5 bytes
> instead of 0x193 bytes for the working variant).

_BV(0)==1 but _BV(1)==2, so when you do ((value & _BV(1)) != 1), it's
never false because the possible values of (ASYNCPIN & _BV(1)) are 0
or 2, never 1.

I would try to write these kinds of statements as
        while( !(ASYNCPIN & _BV(ASYNCCLK)) ) {
                  blah, blah;
        }

-Tarmigan