Problem With > Operator

[janne_aac 0]

Hi Folks,

 

I am seeing a very odd behaviour.

This is roughly my test program:

 

void testFunc(char test)

{

if(test > 3)

{

LED0_on(); //Turn on LED 0

}

else

{

LED1_on(); //Turn on LED 1

}

}

…..

testFunc(1);

…..

 

I almost cant believe what I am seeing but what happens is that LED 0 is switched on. How can 1 > 3 be true???

 

This seems to work correctly:

 

void testFunc(char test)

{

if(test < 3)

{

LED0_on(); //Turn on LED 0

}

else

{

LED1_on(); //Turn on LED 1

}

}

…..

testFunc(4);

…..

 

=> LED 1 turns on

 

When I look at the ASM file of the first program I see this which also looks odd:

 

MOVF test_Func_00000_arg_test, W

SUBLW 0x03

BTFSC STATUS,C

GOTO label1

.....

{if...}

 

 

label1

{else}

 

 

Shouldn't SUBWF be used instead of SUBLW?

[Reynard 0]

Can't see anything wrong here.

 

Assembler code is correct as you are comparing against a literal.

 

Your problem may lie somewhere else.

 

Cheers

 

Reynard

 

ps. Give us a clue to the chip type.

[ppulle 0]

Looks OK to me too.

 

Lets look at the asm

assume the argument is greater than 3, lets say 4;

 

MOVF test_Func_00000_arg_test, W

will move 4 into W

 

SUBLW 0x03

will subtract 3 from W, leaving 1 in W and no carry in the status register. No need to use SUBWF, we possibly save some code space by avoiding any bank switching if it were needed.

 

BTFSC STATUS,C

tests carry flag, and skips the next statement 'goto' if the flag is clear (Bit Test F, skip if Clear)

 

GOTO label1

goto will be skipped as expected and execution will begin in the main part of the if statement

.....

{if...}

 

 

label1

{else}

[davidb 0]

Since you haven't shown us the code for driving the LEDs or the target being used I suspect the problem may be there if you are manipulating the port pins directly.

 

In this case it is probably our old friend the read-modify-write problem. If you haven't come across this before then there are plenty of references available.

 

Try here for a good description: http://embeddedadventures.blogspot.com/200...s-and-sfrs.html

 

Regards

 

davidb

[trossin 0]

How does the debugger step through the code? I would guess that might help explain what is going on down in the LED helper functions.