Re: [avr-chat] HELP! mega128 USART receive restarts execution

[David Kelly]

On Feb 12, 2006, at 11:35 AM, Rick Mann wrote:

> On Feb 12, 2006, at 7:10 AM, Larry Barello wrote:
>
> > You don't want to use '!' to negate a bit mask; That isn't doing  
> > what you
> > think it is.  You want to use '~'
>
> I must disagree with you here. '!' results in a non-zero byte  
> becoming 0. '~' (bitwise complement) results in all the 0 bits  
> becoming 1 and all the 1 bits becoming 0. If I used '~', then I'd  
> end up with all the bits in the result set ('1').

copied from earlier message:
>         UCSR0B =        _BV(RXCIE0)
>                 |       !_BV(TXCIE0)
>                 |       !_BV(UDRIE0)
>                 |       _BV(RXEN0)
>                 |       _BV(TXEN0)
>                 |       !_BV(UCSZ02);

in avr/sfr_defs.h:
#define _BV(bit) (1 << (bit))

in avr/iom128.h:
#define    TXCIE0       6

So !_BV(TXCIE0) is !(1<<6) which is usually nonsense and why Larry  
got tripped on it.

If I were writing the code, first off I would NOT use _BV(). Its a  
crutch which hides what is really being done and anyone worth their  
salt will instantly recognize (1<<TXCIE0).

The other thing is that I'd either list all the bits on separate  
lines as above, but comment out the 0's. Or I'd only list the bits  
I'm setting:

        UCSR0B = (1<<RXCIE0) | (1<<RXEN0) | (1<<TXEN0);

Another thing I sometimes do to make it positively obvious that I  
want a zero in a particular bit, while leaving it easy to toggle that  
bit in the future:

        UCSR0B = (1<<RXCIE0) | (0<<TXCIE0) | (0<<UDRIE0)
               | (1<<RXEN0)  | (1<<TXEN0)  | (0<<UCSZ02);

If starting from scratch I usually write something like this:

#define TXCIE0_b  (6)
#define TXCIE0_m  (1<<TXCIE0_b)

--
David Kelly N4HHE, address@hidden
========================================================================
Whom computers would destroy, they must first drive mad.