8-bit Microcontrollers中的主题 Simple problem with if-then statements, where am I going wrong?

Simple problem with if-then statements, where am I going wrong?

aaron800 — Thu, 17 Jan 2008 22:34:04 GMT

I am very confused why the code is doing the following in a simple if-then structure:

interrupt 12 void MTIM_ISR(void){

    MTIMSC_TOF = 0;

    if(i== 0)
    {
      PTBD_PTBD4 = 0;
      PTBD_PTBD5 = 1;
      PTBD_PTBD6 = 1;      // JUMPING FROM HERE
      i++;
    }
    else if(i==1)
    {
      PTBD_PTBD4 = 1;
      PTBD_PTBD5 = 0;
      PTBD_PTBD5 = 1;
      i++;           // TO HERE, AND THEN OUT
    }
    else
    {
      PTBD_PTBD4 = 1;
      PTBD_PTBD5 = 1;
      PTBD_PTBD6 = 0;
      i= 0;
    }


}

This is just a interrupt handler for a timer-based delay. Is the compiler optimizing the code in some weird way? This basically blinks three led's in order based on the i index, and then starts back over at the first led (i=0).

Thanks,
Aaron

Re: Simple problem with if-then statements, where am I going wrong?

Ake — Thu, 17 Jan 2008 22:48:15 GMT

Hi,

Assuming that i = 0 when the program starts.

Thís means that the first test will be true and PTB4 will turn low.

i is incremented.

The next time, i = 1, which means that PTB5 drops low.

i is incremented.

Next time i = 2, so neither of two earlier comparisons are OK, which makes PTB6 go low.

i is set to 0

Is there something hidden that I have not understood?

Regards,

Ake

Re: Simple problem with if-then statements, where am I going wrong?

fabio — Thu, 17 Jan 2008 22:52:14 GMT

Hi Aaron,

Yes, this is due to compiler optimization and I do not consider it weird.

The compiler is, in fact, reusing common code, instead of two increment instructions, it is using just one. Look at the disassembly listing and you will notice that.

Best regards,

Re: Simple problem with if-then statements, where am I going wrong?

tonyp — Thu, 17 Jan 2008 23:00:54 GMT

aaron800 wrote:
This basically blinks three led's in order based on the i index, and then starts back over at the first led (i=0).

Indeed, that's what it should do (if it didn't have a minor bug)!

    else if(i==1)
    {
      PTBD_PTBD4 = 1;
      PTBD_PTBD5 = 0;
      PTBD_PTBD5 = 1;

should be

    else if(i==1)
    {
      PTBD_PTBD4 = 1;
      PTBD_PTBD5 = 0;
      PTBD_PTBD6 = 1;

Re: Simple problem with if-then statements, where am I going wrong?

aaron800 — Thu, 17 Jan 2008 23:08:28 GMT

Tony, in the next case (i==2), the PTBD5 is turned back off, so I dont see the minor bug??

As far as debugging, yes I can step through the code in the debugger, I do a run from point at the beginning of the interrupt handler, I can step through and see it execute each time, the middle case (i==1) is never executed. Yes i is initialized as a global variable before the handler routine.

Thank you for helping a noobie! I am more familiar with C development on the Atmel AVR series, but these processors seem more powerful and are considerably cheaper too.

Aaron

Re: Simple problem with if-then statements, where am I going wrong?

aaron800 — Thu, 17 Jan 2008 23:10:32 GMT

tony, you're right, i am an idiot, I think i'm dyslexic sometimes :-P

I still don't see why it is jumping though, unless the optimizer says that is the same thing

Thanks

Re: Simple problem with if-then statements, where am I going wrong?

bigmac — Fri, 18 Jan 2008 02:35:48 GMT

Hello Aaron,

I tried compiling your sample code, and then single stepping through the ISR under full chip simulation. I found that the code does work as intended (apart from the typographical error previously observed).

The compiler seems to generate the following assembly code for the ISR (I have added the labels):

PSHH

LDA i

BNE B1

; Case: i = 0

BCLR 4,PTBD

BSET 5,PTBD

BSET 6,PTBD

BRA B2

B1: DBNZA B3

; Case: i = 1

BSET 4,PTBD

BCLR 5,PTBD

BSET 5,PTBD ; Typographical error

B2: LDHX #i

INC ,X

PULH

RTI

B3: ; Case: i > 1

BSET 4,PTBD

BSET 5,PTBD

BCLR 6,PTBD

CLRA

STA i

PULH

RTI

Because of the typograqhical error, the LED connected to PTBD5 will never flash.

Regards,

Mac

Re: Simple problem with if-then statements, where am I going wrong?

aaron800 — Fri, 18 Jan 2008 02:45:05 GMT

Hello, yes i corrected the mistake, and everything works now. Thanks again, sorry for my newbie question.