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

[Rick Mann]

On Feb 12, 2006, at 10:46 AM, David Kelly wrote:

> 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.

I don't think it's any more nonsensical than what you do below,  
shifting 0s. However, I'm beginning to lean toward that use.

> 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).

I only use it for compactness (I like having space around operators).

> 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)

Traditionally, Mac OS APIs were written like this, giving both a bit  
number and a mask value. However, the technique doesn't let you  
explicitly show which bits you want to be zero.

Perhaps better would be to define all the bit names as macros that  
generate a mask:

        #define RXCIE0m(v)              ((v) << RXCIE0)

Even better, when cplusplus is invoked:

        inline char RXCIE0m(char v)     { return v << RXCIE0; }

With that you could write:

        UCSR0B = RXCIE0m(1) | TXCIE0m(0) | UDRIE0m(0);

You could conceivable get fancier, and let multi-bit settings be set  
in a single call, but that doesn't work when the bits are  
(annoyingly) spread across multiple registers.

--
Rick

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