topic Re: Timing to do multiplication and divide in 8-bit Microcontrollers

Timing to do multiplication and divide

New_to_HC08 — Tue, 15 Jun 2010 22:16:18 GMT

Hardware: MS9SO8GT60 with 32.768 kHZ xtal

programmer: Cyclone Pro HC08 - HCS08

IDE: Freescale Codewarrior 5.9.0

I seem to be having a timing issue with how long it takes to perform the calculation, and then assigning the computed value.

I assumed it wouldnt move to the next step, before the calculation had finished, but when I step through the code, it looks like it assigns 'tempunit' to activeArray before the value is finished calculating. and therefore, assigns '0'.

so all this is doing is a 6 digit number (date) and putting it into a 3 byte array (activeArray)

so for example

divisor = 10/06/02

then activeArray should be [ 10, 6, 2 ];

but it ends up being [0, 6, 2]

  for (i=0;i<=2;i++){        tempunit = (BYTE)(divisor%10);    divisor /= 10;    //new value to mod by 10        temptens =  (BYTE)(divisor%10);    temptens *= 10;    temptens += tempunit;    activeArray[j] = tempunit;    j--;    divisor /= 10;    //new value to mod by 10      }

is this possible?

Re: Timing to do multiplication and divide

bigmac — Wed, 16 Jun 2010 01:19:47 GMT

Hello,

Assuming the initial value of the variable divisor is the unsigned 16-bit value 100602 (not entirely clear from your post), the initial value of variable j is 2, and that you complete the loop three times, rather than twice, I would expect to get the result 0, 6, 2.

I think your problem is with the line activeArray[j] = tempunit;

It would seem that the expression should be activeArray[j] = temptens;

Perhaps the code might be simplified, as follows

for (j = 2; j; j--) {  // Loop twice only  activeArray[j] = (byte)(divisor % 100);  divisor /= 100;      // new value to mod by 100}activeArray[0] = (byte)divisor;

Regards,

Mac

Re: Timing to do multiplication and divide

New_to_HC08 — Thu, 17 Jun 2010 18:39:11 GMT

Wow. I am a retard.
I ended up just coding a new loop anyway, coz I often find, when I cant see the bug, it helps to start from scratch...is it just me that has this problem?
it is very similar to your solution

   for (i=0;i<=2;i++){     activeArray[j] =  (BYTE)(divisor%100);      divisor /= 100;      if (j>0) j--;         }

Thanks anyway!