Option to use 32768Hz watch crystal for timing. Code suggested by Georg von Zengen.

cpu/avr/dev/clock-avr.h

@@ -37,7 +37,44 @@
   TIMSK = _BV (OCIE0);
 
 #elif defined (__AVR_ATmega1284P__) || (__AVR_AT90USB1287__) || (__AVR_ATmega1281__)
-
+/*
+  The Raven has a 32768Hz watch crystal that can be used to clock the timer
+  while the 1284p is sleeping. The Jackdaw has pads for a crystal. The crystal
+  can be used to clock the 8 bit timer2.
+*/
+#if AVR_CONF_USE32KCRYSTAL
+#define OCRSetup() \
+  /* Clock from crystal on TOSC0-1 */ \
+  ASSR = _BV(AS2);		      \
+\
+  /* Set counter to zero */   \
+  TCNT2 = 0;				  \
+\
+  /*						  \
+   * Set comparison register: \
+   * Crystal freq. is 32768,\
+   * pre-scale factor is 8, we want 125 ticks / sec: \
+   * 32768 = 8 * 124.1 * 33, less 1 for CTC mode\
+   */ \
+  OCR2A = 32; \
+\
+  /* 								\
+   * Set timer control register: 	\
+   *  - prescale: 8 (CS21) \
+   *  - counter reset via comparison register (WGM21) \
+   */ 								\
+  TCCR2A = _BV(WGM21); \
+  TCCR2B = _BV(CS21);  \
+\
+  /* Clear interrupt flag register */ \
+  TIFR2 = TIFR2; \
+\
+  /* \
+   * Raise interrupt when value in OCR2 is reached. Note that the \
+   * counter value in TCNT2 is cleared automatically. \
+   */ \
+  TIMSK2 = _BV (OCIE2A);
+#else
 #define OCRSetup() \
   /* Select internal clock */ \
   ASSR = 0x00; 				  \
@@ -48,10 +85,10 @@
   /*						  \
    * Set comparison register: \
    * Crystal freq. is 8000000,\
-   * pre-scale factor is 1024, i.e. we have 125 "ticks" / sec: \
-   * 8000000 = 1024 * 125 * 62.5 \
+   * pre-scale factor is 1024, we want 125 ticks / sec: \
+   * 8000000 = 1024 * 126.01 * 62, less 1 for CTC mode \
    */ \
-  OCR0A = 62; \
+  OCR0A = 61; \
 \
   /* 								\
    * Set timer control register: 	\
@@ -69,6 +106,7 @@
    * counter value in TCNT0 is cleared automatically. \
    */ \
   TIMSK0 = _BV (OCIE0A);
+#endif /* AVR_CONF_USE32KCRYSTAL */
 
 #define AVR_OUTPUT_COMPARE_INT TIMER0_COMPA_vect