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Felix Althaus
Joined: 09 Sep 2003 Posts: 67 Location: Winterthur, Switzerland
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signed int mult => unsigned result =>? |
Posted: Sun Sep 04, 2005 12:04 pm |
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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);
}
}
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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
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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 |
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mbge5amw
Joined: 13 Dec 2004 Posts: 15 Location: Yorkshire, UK
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Posted: Sun Sep 04, 2005 12:28 pm |
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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;
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The
HTH
Andy |
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Mark
Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA
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Posted: Sun Sep 04, 2005 8:58 pm |
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The compiler is pretty old. Probably just an error with the printf function. |
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