Re: [avr-gcc-list] Bit-wise structure and unions

[David Kelly]

On Tue, Nov 21, 2006 at 01:31:13PM -0700, Eric Weddington wrote:
> You should really use the C language bitwise operators (|, &, ~, ^,
> <<, >>) and use bit masks to manipulate the bits that you want.
>
> The bit-wise operators have one advantage over using the bit-field
> structure. Using bit-wise operators you can operate on multiple,
> *non-contiguous* bits simultaneously within a variable. You can't do
> that with bit-field structures.

I agree with Eric. I'll typically define STATUS_LED_m and  
STATUS_LED_p and write STATUS_LED_p |= STATUS_LED_m or variations  
thereof. The generated code won't be any smaller with a bitfield.  
Eric makes a good point that one can smack multiple bits with one  
punch this way, not only for smaller code but because often one  
desires all outputs to change at the same time, not several clock  
cycles apart.

One other point is that I/O registers often return the value on the  
pin, not the value previously written. Doubt one wants a read-modify- 
write instruction to read an incorrect value as will happen if the  
pin is overloaded. In this case is best to have a copy of the port's  
intended value laying around in RAM, so one more define for  
STATUS_LED_r and write STATUS_LED_p = (STATUS_LED_r |= STATUS_LED_m);

Think that will evaluate correctly without parenthesis, but they help  
when reading.

Some MCU families (believe HC12 is one) have a separate register for  
reading the port's actual input/external value, and another for  
reading output latches containing the value last written. A bitfield  
works fine on the output latch register.

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