Fix for simplefoc#415 sin/cos integer overflow on 16-bit CPUs

Terrible bug in the fast sin/cos added last year, resulting in rough sine waves and a spike from +1 to -1 at pi/2

src/common/foc_utils.cpp

@@ -9,20 +9,21 @@ __attribute__((weak)) float _sin(float a){
   // 16 bit precision on sine value, 8 bit fractional value for interpolation, 6bit LUT size
   // resulting precision compared to stdlib sine is 0.00006480 (RMS difference in range -PI,PI for 3217 steps)
   static uint16_t sine_array[65] = {0,804,1608,2411,3212,4011,4808,5602,6393,7180,7962,8740,9512,10279,11039,11793,12540,13279,14010,14733,15447,16151,16846,17531,18205,18868,19520,20160,20788,21403,22006,22595,23170,23732,24279,24812,25330,25833,26320,26791,27246,27684,28106,28511,28899,29269,29622,29957,30274,30572,30853,31114,31357,31581,31786,31972,32138,32286,32413,32522,32610,32679,32729,32758,32768};
-  unsigned int i = (unsigned int)(a * (64*4*256/_2PI));
-  int t1, t2, frac = i & 0xff;
+  int32_t t1, t2;
+  unsigned int i = (unsigned int)(a * (64*4*256.0f/_2PI));
+  int frac = i & 0xff;
   i = (i >> 8) & 0xff;
   if (i < 64) {
-    t1 = sine_array[i]; t2 = sine_array[i+1];
+    t1 = (int32_t)sine_array[i]; t2 = (int32_t)sine_array[i+1];
   }
   else if(i < 128) {
-    t1 = sine_array[128 - i]; t2 = sine_array[127 - i];
+    t1 = (int32_t)sine_array[128 - i]; t2 = (int32_t)sine_array[127 - i];
   }
   else if(i < 192) {
-    t1 = -sine_array[-128 + i]; t2 = -sine_array[-127 + i];
+    t1 = -(int32_t)sine_array[-128 + i]; t2 = -(int32_t)sine_array[-127 + i];
   }
   else {
-    t1 = -sine_array[256 - i]; t2 = -sine_array[255 - i];
+    t1 = -(int32_t)sine_array[256 - i]; t2 = -(int32_t)sine_array[255 - i];
   }
   return (1.0f/32768.0f) * (t1 + (((t2 - t1) * frac) >> 8));
 }