mixer_interrupt branch merged to trunk

timers on stock board should be more accurate

src/board_gruvin9x.cpp

@@ -119,6 +119,11 @@ inline void board_init()
   OCR4A = 0x7d;
   TCCR4B = (1 << WGM42) | (3<<CS40); // CTC OCR1A, 16MHz / 64 (4us ticks)
 
+  /* Mixer interrupt */
+  OCR5A = 0x7d * 40; /* 20ms */
+  TCCR5B = (1 << WGM52) | (3<<CS50); // CTC OCR1A, 16MHz / 64 (4us ticks)
+  TIMSK5 |= (1<<OCIE5A);
+
 #ifdef EXTRA_ROTARY_ENCODERS
   //configure uart1 here
   DDRD &= ~(1 << 2);