problem in changing spwm amplitude

[notbad]

i’m trying to generate a sinusoidal PWM from frequency 1hz to 100hz.
sinusoidal points are updated in int_ccp4 working with timer1
and its overflow rate is determined by pre-calculated ccp4vals selected by adc.
and for frequencies less than 50 hz, amplitude must be weakened wich function vf_calc does that correctly.
the problem is that if freq is less than 50 and vf_calf comes to work, the output frequency is corrupted.
why do you think?

[Ttelmah]

/50 is the killer. At 16MHz, this takes nearly 300uSec to execute. The * takes 132uSec. Both unacceptable times. Total over 400uSec....

Now, this is effectively multiplication by 5.1. Speed is always under 100. So if you stay with int16, instead of using int32, and multiply speed by 326, then >>6 (/64), you get effectively *5.09375, taking about 60uSec instead. May be low enough to let it work.

Best Wishes

[asmboy]

TTelmah is pointing you in the direction of much reduced #int time
and very small error
B U T
if you can stand a 2% error -
really cut to the chase and use :

[notbad]

wow thanks
i didn't think those instructions will take that long to execute.
another question is what happens in the processor? we are in the isr routine and another interrupt occurs. does it get serviced? if yes, shouldn't it cause stack overflow and hence device reset?

[Ttelmah]

No.
You are only using one hardware interrupt level.
Second interrupt will be serviced when the first handler finishes.

The manual has a section 'How much time do maths operations take'.
Now there are extensions and caveats. For instance /64, can be done using simple rotations, so this only takes perhaps 5uSec at your clock rate. The compiler is actually 'smart enough', that if you ask for a fixed division by a binary value (2,4,8 etc.), it will substitute a rotation.
Then other things can be much slower than you think, vara=varb, takes just a couple of instructions for an int8 (depends on whether bank switching is involved). However vara=varc[varb];, will typically take nearly twenty instructions (has to do a table lookup. This is where playing with very small programs, and the stopwatch in MPLAB SIM, can be really informative.

Best Wishes

[notbad]

[Ttelmah]

On the interrupt, neither.
If an interrupt triggers a second time inside itself, the second trigger is lost. For each interrupt, there is only one interrupt flag. This is cleared when the interrupt exits. It doesn't matter if it has triggered ten times, or one....
This is why interrupt handlers should always be as short as possible.

One other way of doing what you want, is to just have a second array. Have the code switch which array it uses based on the number. No maths at all.

Best Wishes

[notbad]

thank you Ttelmah
and you asmboy.
it's good to have you guys here.

[FvM]

Besides the good things that have been said about effective arithmetic: It seems completely useless to calculate the magnitude factor A in the ISR...