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Math problems

 
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leandroguida



Joined: 09 Dec 2008
Posts: 4

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Math problems
PostPosted: Tue Dec 09, 2008 10:00 pm     Reply with quote
Hello there.

I'm doing a test code to calculate a period of time. Since I'm using a 4MHz crystal, my internal clock is 1Mhz giving me 1uS per cycle.

So here's my idea, if I can count the total cycles, I can get how many uS the process took.

So for this, I created the code below:
Code:
#include <16f877a.h>
#include <stdio.h>

#use delay(clock=4000000)
#fuses HS, NOWDT, NOPROTECT, NOLVP
#use rs232(baud=9600, BITS=8, PARITY=N, STOP=1,x mit=PIN_C6, rcv=PIN_C7)

unsigned int16 tCcp = 0, tCcpAtual = 0;
unsigned int over = 0;
unsigned int32 tTotal, test;
volatile unsigned boolean ccpFlag = false;

#int_ccp1
void ccpH(){
   tCcp = CCP_1;
   ccpFlag = true;
}

#int_timer1
void timerHandler(){
   over++;
}   
   
void main(){
   setup_ccp1(CCP_CAPTURE_RE);
   setup_timer_1(T1_INTERNAL|T1_DIV_BY_1);
   enable_interrupts(INT_CCP1);
   enable_interrupts(INT_TIMER1);
      
   enable_interrupts(GLOBAL);

   while(true){
      if(ccpFlag){
         setup_ccp1(CCP_OFF);
         setup_timer_1(T1_DISABLED);   
         tCcpAtual = tCcp;
         test = (over*65535);
         tTotal = test+tCcpAtual;
         printf("Tempo Captura: \%ld\n\r",tTotal);
         ccpFlag = false;
      }
      delay_us(100);         
      
   }
}   



I'm using the PIC16F877A. I'm thinking like that: Since my timer is a 16bit one, when I have a overflow interrupt I'll have counted 65535 uS.

So I increment a 'over' variable to count how many overflow interrupts I had.

Then I get the CCP_1 which is the timer value when the CCP_1 event occurred.

My Total time would be:

Code:
tTotal = (over*65535)+tCcpAtual;


Where tCcpAtual, as showed in the code, is the CCP_1.

I can't get the right 'over' total overflow time. When I use a normal number, it works. (Like, if I do 10*65535 I'll have 655350). But using the variable 'over', don't work.

What is the problem, is it my variable type ? Is the math instructions wrong?

serial exit is just as a feedback for me.
FvM



Joined: 27 Aug 2008
Posts: 2337
Location: Germany

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PostPosted: Wed Dec 10, 2008 1:10 am     Reply with quote
The product of two int16 numbers is also int16. over*65535 is calculated as an int16 quantity, discarding the overflowing bits and then assigned to the int32 variable.

I think, that the multiply should be with 0x10000 or 65536 anyway, which reduces the multiply to a shift respectively assignment to high word, achievable e. g. by make32() CCS private function.

Using general C-syntax, a type cast would be the means to clarify your intentions, e. g. (int32)over*65535.
leandroguida



Joined: 09 Dec 2008
Posts: 4

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Thanks...
PostPosted: Wed Dec 10, 2008 5:15 am     Reply with quote
Thanks a lot. It worked!
Ttelmah
Guest
PostPosted: Wed Dec 10, 2008 9:24 am     Reply with quote
As a comment, FvM, is 'dead right', that you would handle this more correctly, and much quicker, using shifts, or make32.
One 'route', that is 'non obvious', if you are not used to C unions, and is very efficient indeed, is to use a union for this. So something like:
Code:

struct words {
    low_word;
    high_word;
}

union {
    struct words w;
    int32 whole;
} timer32;

Then in your overflow routine, just increment timer32.w.high_word. Copy your tCcp value into timer32.w.low_word, and the required result should be in timer32.whole.
No arithmetic involved.
Very much faster, and more compact.

Best Wishes
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