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signed int mult => unsigned result =>?

 
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Felix Althaus



Joined: 09 Sep 2003
Posts: 67
Location: Winterthur, Switzerland

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signed int mult => unsigned result =>?
PostPosted: Sun Sep 04, 2005 12:04 pm     Reply with quote
Hello

I have some problems with signed integer multiplication:
I multiply two signed ints together, to get a signed long, but the result looks like it's treatend unsigned internally:

Code:

#include "bootloader.h" // fuse settings, delay, etc

#use fast_IO(B)

signed int clicks;
signed int Kp = 3;


// clicks is counted in an isr

void main(void)
{
signed long P;
signed int error;
signed int setpoint = 15;
signed long duty = 0;

     setup_timer_2(T2_DIV_BY_4,255,1);
     setup_ccp1(CCP_PWM);
     set_tris_B(0B10000000);

while(TRUE)
{

clicks = 0;
enable_interrupts(INT_RB);
enable_interrupts(GLOBAL);
delay_ms(200);
disable_interrupts(GLOBAL);

error = setpoint - clicks;

P = (setpoint - clicks) * Kp; // This calc is wrong

duty = duty + P; // looks ok

printf("%d\t%ld\t%ld\n\r", error, P, duty);

if(duty > 255L) duty = 255L;
if(duty < 0L)   duty = 0L;

set_pwm1_duty((unsigned int)duty);
}

}

Yes, it is a P controller, perhaps a little bit too complicated, but im experimenting...

Some debugging output:
Code:

error    P        duty

-6      238     493
-7      235     490
-7      235     490
-6      238     493

P should be something like -21. duty looks fine, so I don't think it's just a display error.
Did I screw this up or does the compiler?

Compiler: 2.732
PIC: PIC16F876


Many thanks
Felix
mbge5amw



Joined: 13 Dec 2004
Posts: 15
Location: Yorkshire, UK

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PostPosted: Sun Sep 04, 2005 12:28 pm     Reply with quote
I think this might be that the compiler looks at the variables in your equation and sees that they are all ints so reckons it can get away with 8 bit arithmetic.

This would be in keeping with -21 coming out as 235 (256-21 = 235)

There are 2 options,

1. Cast. precede your variables with (unsigned long) to tell the compiler what sort of arithmetic todo with these numbers.
2. spread the equation over a few lines of C code:

Code:


P = setpoint; // store the value of setpoint in a 16 bit variable so compiler uses 16 bit arithmetic
P -= clicks;
P *= Kp;



The

HTH

Andy
Mark



Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA

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PostPosted: Sun Sep 04, 2005 8:58 pm     Reply with quote
The compiler is pretty old. Probably just an error with the printf function.
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