calculation error in msp430??

karthik raja

Other Parts Discussed in Thread: MSP430F2618, CC430F5137

hi all am using msp430f2618 controller .. in my coding some arithmetic calculations is there that is

lSignal2 = lSig*1410/wSa1+1024;

in this code lSig = -2;
wSa1 =500;

as per the manual calculation the answer should be lSignal2 = 1018 but am getting lSignal2 =-183787 while i debug my code by means of step by step
but if the lsig value is positive means its working properly,

since i declared lSignal2,lSig variables as a "signed long int" and wSa1 as a "unsigned int" variable...

what should i do to rectify this calculation errors???

over 13 years ago

0 Josef Irlinger over 13 years ago

Prodigy 175 points

Hi,
maybe the problem is due to your declaration of lSignal2,lSig as a "signed long int", as there is no long on 16bit architectures.
Try to to go with an "int" declaration.
Maybe it works then, but I am just guessing.

0 old_cow_yellow over 13 years ago

Guru 58965 points

I do not think MSP430F2618 made the mistake in calculation. Your problem is between you and the c-compiler you are using.

0 karthik raja over 13 years ago in reply to old_cow_yellow

Intellectual 840 points

c compiler in the sense what are the possibilities for this error ? am using code composer studio v4..

0 Norman Wong over 13 years ago in reply to karthik raja

Guru 26430 points

I'd hazard to guess it is something to do with integer promotion causing the wrong conversion from signed to unsigned. Or an outright bug in the compiler. Maybe try breaking up the calculation to see which stage it goes wrong.

signed long int lSignal2;
signed long int lSig;
unsigned int wSa1;

lSig = -2;
wSa1 =500;

//lSignal2 = lSig*1410/wSa1+1024;
lSignal2 = lSig;
lSignal2 *= 1410;
lSignal2 /= wSa1;
lSignal2 += 1024;

0 karthik raja over 13 years ago in reply to Norman Wong

Intellectual 840 points

ya i fond that i got error during lsignal2=lsig*1410;

0 Norman Wong over 13 years ago in reply to karthik raja

Guru 26430 points

Maybe try forcing the type by adding a 'L' to the constant.

lsignal2=lsig*1410L;

0 Chester Gillon over 13 years ago in reply to karthik raja

Guru 92251 points

karthik raja said:
as per the manual calculation the answer should be lSignal2 = 1018

Er, I think the answer should be 1019. Running the code on a CC430F5137 produced the correct answer, so I can't see any problems caused by a last of casting.

karthik raja said:
ya i fond that i got error during lsignal2=lsig*1410;

Can you check the setting for "Link in hardware version of RTS mpy routine (--use_hw_mpy)" found under Project Properties -> C/C++ Build -> Tool Settings -> MSP430 Linker -> Basic Settings.

For a MSP430F2618 it should be set to "16". I found that if the setting for the RTS mpy routine doesn't match the available hardware multiplier in the MSP430 then incorrect results occur.

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calculation error in msp430??