AVR-Chat

David Kelly wrote:
> On Thu, Mar 22, 2007 at 09:28:01AM -0700, larry barello wrote:
> 
>>One can define a struct for something like the USART peripheral and
>>use bit fields.  This has, in fact, been done for the GCC compiler.
> 
> 
> BTDT. An inline mask usually results in smaller and faster code. Also
> masks are much better when one is watching multiple bits.

   Me too, and it makes just as small of code as the bit shift code with 
GCC-AVR.  I compiled both ways and looked at the assembly generated.  I 
got spoiled by Microchip, which does this and I did it with the code 
that I was writing, but only for the registers that I wanted to use.

> 
>>Better yet to use the GCC shorthand _BV() to get the "bit value" of a
>>number.

   Personally I greatly dislike _BV since it is non-intuitive to read.

>>#define _BV(A) (1<<(A))
>>
>>While (UCSRA & _BV(UDRE))
>>	;
> 
> 
> Yuck. I find (1<<5) or (1<<UDRE) immediately says what is happening
> _BV(5) adds an extra level of indirection confusion and _BV(UDRE)
> doubles the indirection confusion. I don't find (1<<UDRE) to be
> confusing.
> 
> We're pretty much bound by the definitions in Atmel's datasheets. But
> when I don't have a historical mandate and am creating new definitions
> for a project I'll do it more like this:
> 
> #define UCSRA_UDRE_m	(1<<5)

yup.

> Which is obvious when reading that it only applies to UCSRA and _m is a
> reminder that its already in mask form.
> 

DLC
-- 
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Dennis Clark          TTT Enterprises
www.techtoystoday.com
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