Re: [avr-gcc-list] Warning message

[Dave N6NZ]

Royce Pereira wrote:
> Hi all,
>
> What does the following warning mean?
>
> warning: comparison is always true due to limited range of data type
>
> Here is the context:
> I have a AVR I'm trying to connect to a DS1307 RTC chip via TWI.
>
> To check the integrity of the DS1307 internal ram, I plan to write 4 bytes, 
> 0x55,0xaa,0x55,0xaa in the 1st 4 loations, & read them back & verify, whenever 
> I power up.
>
> My code contains this:
>
> #define CHK_1307        0x55
>
> uint8 twi_read(void)
>     {
>         //..........
>         return TWDR;
>      }  
> //-----------------------
> uint8_t init_1307(void)
>    {
>        uint8_t ok = 1 ;
>
>       if(twi_read() != CHK_1307) ok= 0;
>       if(twi_read() != ~CHK_1307) ok= 0;
> //      ^^^^^^
> //        The offending line.
I believe what is going on is the C standard requires promotion to int, 
and that bites (bytes?) you :)

The result of twi_read() and also the ~CHK_1307 get promoted to ints 
before comparison. So... 0x00xx from twi_read() is compared against 
-xFFAA, which is clearly always true.

Check the generated code.  Casting to uint8_t in the comparison may get 
you what you want, but check the code, don't take my word for it.

-dave

>         
>         return ok;
>     }
> //--------------------------
> Thanks,
> --Royce.