Re: [avr-gcc-list] efficiency of assigning bits

[E. Weddington]

Royce & Sharal Pereira wrote:

> > So would it be a good idea (for saving uC time) to convert that 
> > above  line to this:
> >
> > PORTD = (PORTD & (unsigned char)~(_BV(0))) | (unsigned  
> > char)_BV(1)|_BV(2)|_BV(3));
>
>
> Whew! What a long winded way to do things! This takes less time/energy 
> to  enter:
>
> PORTD &= ~(1 | (1<<1) | (1<<2));
1. What you wrote above is different than what I wrote above. Follow the 
parentheses carefully to understand why.

> And is quite readable. 

2. In your opinion. Others may differ. In fact it is not always clear 
which bits are getting set or why. If you use symbolic names in place of 
the bit numbers, then it stands a better chance of being mnemonic. I 
prefer not having "magic numbers" in the source code.

For example, can you tell me offhand what this does on a mega128 without 
looking at the datasheet?

TIMSK |= (1<<5);

No?

However if you write it like this:

TIMSK |= _BV(TICIE1);

Then you can at least make out that this is setting the Timer 1 Input 
Capture Interrupt Enable bit which enables that interrupt.

When you are using symbolic names you have the opportunity to not be 
tied to a particular implementation. What this means is that if that 
particular bit changed position, then you wouldn't have to change all 
these hardcoded numbers all over your code. You just change the 
definition of the symbol. This method works very well in your own 
application where you can provide application specific symbols, such as:

#define STATUS_LED    3
//......
PORTA |= _BV(STATUS_LED);

What if the above code was for a prototype board, which then needed to 
be re-layed out? What if the status LED needed to move to a different 
pin? Then all you have to do is to change the definition, in one place 
only; the code stays the same.


> It also has the same effect as the (unsigned char)  attribute above.
3. IIRC, a constant defaults to an *integer*, which on the AVR is 16 
bits. It does not default to an unsigned char. So you still have the 
same problem and you will still have to typecast the constants.

Eric