--- a/asin.py	Wed Dec 21 23:59:36 2016 +0100
+++ b/asin.py	Thu Dec 22 13:26:17 2016 +0100
@@ -17,8 +17,8 @@
    ANGLE( 0.448), # atan(1/128)
    ANGLE( 0.224), # atan(1/256)
    ANGLE( 0.112), # atan(1/512)
-   ANGLE( 0.056), # atan(1/1024)
-   ANGLE( 0.028), # atan(1/2048)
+#   ANGLE( 0.056), # atan(1/1024)
+#   ANGLE( 0.028), # atan(1/2048)
 ]
 
 scale = 1 << 14
@@ -35,23 +35,22 @@
     x = scale
     y = 0
     for i, atan in enumerate(ATAN):
-        if y < value:
-            for j in range(2):
-                tx = x
-                x -= y>>i
-                y += tx>>i
-            result += atan << 1
-        else:
-            for j in range(2):
-                tx = x
-                x += y>>i
-                y -= tx>>i
-            result -= atan << 1
+        for j in range(2):
+            dx = y >> i
+            dy = x >> i
+            dr = atan
+            if y < value:
+                dx = -dx
+                dy = -dy
+                dr = -dr
+            x += dx
+            y -= dy
+            result -= dr
+        value += value >> (i << 1)
         mx = max(x, mx)
         my = max(y, my)
         nx = min(x, nx)
         ny = min(y, ny)
-        value += value >> (i << 1)
         mv = max(value, mv)
         nv = min(value, nv)
     return result

--- a/imath.c	Wed Dec 21 23:59:36 2016 +0100
+++ b/imath.c	Thu Dec 22 13:26:17 2016 +0100
@@ -66,32 +66,32 @@
 }
 
 /*
- * Takes a number between -1<<14 and 1<<14 and returns an angle between
- * -1800 (for -180°) and 1800 (for 180°).
+ * Takes a number between -1<<13 and 1<<13 and returns an angle between
+ * -900 (for -90°) and 900 (for 90°).
  */
 int16_t iasin(int16_t value) {
     int16_t result = 0;
-    int16_t x = SCALE;
+    int16_t x = 1<<13;
     int16_t y = 0;
     int16_t dx, dy, dr;
 
     for (uint8_t i = 0; i < ITERATIONS; ++i) {
         for (uint8_t j = 0; j < 2; ++j) {
             dx = y >> i;
-            dy = -(x >> i);
-            dr = -pgm_read_word(&ATAN[i]);
+            dy = x >> i;
+            dr = pgm_read_word(&ATAN[i]);
             if (y < value) {
                 dx = -dx;
                 dy = -dy;
                 dr = -dr;
             }
             x += dx;
-            y += dy;
-            result += dr;
+            y -= dy;
+            result -= dr;
         }
         value += value >> (i << 1);
     }
-    return value;
+    return result;
 }
 
 /*