Re: [avr-gcc-list] ADC problem

[Paulo Marques]

Andi wrote:
> Hi, Guys
>  
>  
> I have problem in my code for ATMega16. This is my code:
> ............
> [...]
> SIGNAL(SIG_ADC)
> {
>  // Get Data ADC
>  ADC_Data = GetADC(); 
>  //int temp;
>  //temp = (int)(GetADCH()<<8) | GetADCL();
>  //ADC_Data = temp;
> }
> [...]
> The program is stuck on SIGNAL(SIG_ADC), when I look to list file 
> (*.LST), looks like the compiler not compile the code in SIGNAL(SIG_ADC).
> Can you help me where is the problem ?

I don't know if this is the same problem as yours, but a while ago I hit 
a problem when compiling with "-combine -fwhole-program": variables that 
were only used inside interrupt functions but weren't declared volatile 
were being discarded by the compiler.

I assumed this had something to do with the compiler assuming that those 
functions were never called anywhere, so it only had to keep volatile 
accesses.

I didn't have time to pursue that problem further, and I eventually 
forgot about it, but I'll try to make a small test case to show it.

BTW, your code doesn't look pretty: all your functions are global (i.e., 
not "static") so the compiler is forced to emit them even if it decides 
to inline them. And this will affect the inlining decision: for 
instance, with -Os, the compiler will probably not inline them, but them 
it will create a huge prologue / epilogue on the interrupt function to 
preserve registers across function calls.

And it has a bug in the order of register accesses to read a 16 bit 
value, anyway. It would be better to just write it as:

SIGNAL(SIG_ADC)
{
  // Get Data ADC
  ADC_Data = ADCL;
  ADC_Data |= ADCH << 8;
}

This is just to make pretty obvious that we're reading the low byte first.

In my own code, I often use a union to simplify high byte / low byte 
accesses:

typedef struct {
  unsigned char l, h;
} BYTES;

typedef union {
  unsigned int w;
  BYTES b;
} WORD;

Then I can write code like this:

static int GetADC(void)
{
  WORD ret;
  ret.b.l = ADCL;
  ret.b.h = ADCH;
  return ret.w;
}

and gcc really handles this quite well.

--
Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
Web: www.grupopie.com

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