Thread: Need help with simple math code please!

Hey guys,

I'm having some trouble with some math code used for my PIC16F684.

Code:

int16 PV, SP;
int16 E0;
uns8 gain;
int16 output;							
uns16 temp;

SP = 128;

PV = ADRESH;	// ADC value = ADRESH (8 MSbs)

E0 = PV - SP;

output = gain*E0;

if(output > 0){
	P1M1 = 0;
	temp = output;			
	temp /= 4;		// Shift bits 3 to 10 of temp twice to the right
	CCPR1L = temp;
	}

if(output < 0){
	P1M1 = 1;			
	temp = -output;
	temp /= 4;		// Shift bits 3 to 10 of temp twice to the right
	CCPR1L = temp;	
	}

Don't worry about the A/D conversion, I just deleted the other code. The problem I'm having is that I don't believe the second if statement is working properly. I want to take the absolute value of "gain" but my compiler doesn't have the absolute function. Instead I just put "-gain" which compiles fine and I think works.

When output > 0, the PIC outputs a PWM square wave which shows up fine on the oscilloscope. It's when output should be < 0, the output on the oscilloscope is a very strange square wave, that looks noisy and has weird superimposing square waves. I think the problem is with the math though.

Can someone verify if the way I initialized the variables will properly store data? I get confused when I'm doing math with mixes of signed and unsigned numbers. Keep in mind CCPR1L and ADRESH can only store 8 bit numbers.

I'd appreciate the help a lot. Thanks!

Hey,

Unfortunately the compiler I use doesn't have that shift command for some reason .

Otherwise looks okay? My concern is with the temp. It's okay that it takes the -output value? That value is technically a signed integer, so does temp just throw away the sign bit?

yea that makes sense. but my point is that "temp" is unsigned and -output is signed. When -output gets placed inside "temp", does it just drop the sign bit?

I wouldn't count on that.

if(number < 0) number *= -1;

Would be my preference.

My concern is that because i'm using a mix of signed and unsigned integers, the sign bit might be getting lost somewhere, or even worse used as a bit that is placed inside a register such as CCPR1L.

Code:

int16 temp;
int16 output;

if(output < 0){
	P1M1 = 1;			
	temp = -output;
	CCPR1L = (unsigned char)temp/4;	
	}

Would the code above guarantee that CCPR1L only gets the MSBs without the sign bit? For example, if temp is 10 bits of data + 1 sign bit (11 bits), would the CCPR1L register be given bits 2-9 (disregarding the 10th bit)? Hope this ain't too confusing lol

Well, I need to keep track of negatives so they must be signed integers. The only reason I'm using UNSIGNED is because "CCPR1L" is a register in a microcontroller. I do not want a sign bit being placed in a register as it will just consider it part of the numerical value. I want to make the number unsigned THEN put it into the register to ensure this.

I definitely am not an expert programmer so if someone knows a better way I'd really appreciate it. I simulated the code and the CCPR1L values are not correct in the (output < 0) part of the code. There is definitely something wrong with them but I can't figure it out