There are two potential solutions:
Solution 1 is to change the lookup table to signed 16-bit and fudge the last entry to 32767 so it doesn't overflow, and add some casts to do part of the interpolation calculation in 32-bit:
return (1.0f/32768.0f) (t1 + (int)((((int32_t)t2 - t1) frac) >> 8));
Solution 2 is to change t1 and t2 to int32_t. This will be a few cycles slower on 16-bit platforms, but retain the behavior of returning exactly 1.0 at pi/2 and -1.0 at 3pi/2.
I would be fine with either. I think 1 was my original intent, but after all the iterations testing the speed and accuracy of the calculations, I forgot to make the final edits.
EDIT: No measured speed difference when testing, so I went with solution 2. Pull request #416
Thanks to ChrisMicro on the forum for finding this bug in the fast sin/cos added last year https://github.com/simplefoc/Arduino-FOC/pull/285
Forum thread https://community.simplefoc.com/t/simplefoc-sin-calculation/5131
There are two potential solutions: Solution 1 is to change the lookup table to signed 16-bit and fudge the last entry to 32767 so it doesn't overflow, and add some casts to do part of the interpolation calculation in 32-bit: return (1.0f/32768.0f) (t1 + (int)((((int32_t)t2 - t1) frac) >> 8));
Solution 2 is to change t1 and t2 to int32_t. This will be a few cycles slower on 16-bit platforms, but retain the behavior of returning exactly 1.0 at pi/2 and -1.0 at 3pi/2.
I would be fine with either. I think 1 was my original intent, but after all the iterations testing the speed and accuracy of the calculations, I forgot to make the final edits.
EDIT: No measured speed difference when testing, so I went with solution 2. Pull request #416