pwm: channel D is inverted

This is to avoid power spikes when all three channels switch on.
We make channel D to switch on on OCR1D match, while other two
channels (A and B) switch off on OCR1A and OCR1B match, respectively.

diff --git a/firmware/pwm.c b/firmware/pwm.c
index 0c4f3d081a5b78ddbc9f2493255a17b0da6496bf..2df2e6bb8b9abe762c1bb6208a13e6a4dde4cd7c 100644 (file)
--- a/firmware/pwm.c
+++ b/firmware/pwm.c
@@ -4,6 +4,8 @@
 
 #include "lights.h"
 
+#define PWM_MAX 0x1FF
+
 void init_pwm()
 {
        /* Async clock */
@@ -15,16 +17,20 @@ void init_pwm()
                ;
        PLLCSR |= _BV(PCKE);
 
-       TCCR1C = _BV(COM1D0) | _BV(COM1D1) | _BV(PWM1D);
-       TCCR1A = _BV(COM1A0) | _BV(COM1A1) | _BV(COM1B0) | _BV(COM1B1) | _BV(PWM1A) | _BV(PWM1B);
-       TCCR1B = _BV(7)                         // PWM1X: PWM inversion mode
-               | _BV(CS10)                     // no clock prescaling
-               ;
-       TC1H = 0x01;
-       OCR1C = 0xFF;                           // TOP value
+       // PWM channel D is inverted, ...
+       TCCR1C = _BV(COM1D1) | _BV(COM1D0) | _BV(PWM1D);
+       // PWM channels A and B are not
+       TCCR1A = _BV(COM1A1) | _BV(COM1B1) | _BV(PWM1A) | _BV(PWM1B);
+       TCCR1D = 0;
+       TCCR1B = _BV(CS10);                     // no clock prescaling
+       TC1H = PWM_MAX >> 8;
+       OCR1C = PWM_MAX & 0xFF;                         // TOP value
+
+       TC1H = PWM_MAX >> 8;
+       OCR1D = PWM_MAX & 0xFF;
 
        TC1H = 0x00;
-       OCR1D = OCR1B = OCR1A = 0;              // initial stride is 0
+       OCR1B = OCR1A = 0;              // initial stride is 0
 
        DDRB  &= ~(_BV( PB1 ) | _BV( PB3 ) | _BV( PB5 )); // tristate it
        PORTB &= ~(_BV( PB1 ) | _BV( PB3 ) | _BV( PB5 )); // set to zero
@@ -54,7 +60,15 @@ void pwm_set(unsigned char n, unsigned char stride)
        switch (n) {
        case 0: OCR1A = stride; break;
        case 1: OCR1B = stride; break;
-       case 2: OCR1D = stride; break;
+       case 2: {
+                       uint16_t s16 = PWM_MAX - (uint16_t)stride;
+                       volatile unsigned char hi, lo;
+                       hi = s16 >> 8;
+                       lo = s16 & 0xFF;
+                       TC1H = hi;
+                       OCR1D = lo;
+               }
+               break;
        }
 }