Calculate best prescaler based on F_CPU and CLOCK_CONF_SECONDS. Inspired by patch from Ivan Delamer.

cpu/avr/dev/clock-avr.h

@@ -37,6 +37,7 @@
   TIMSK = _BV (OCIE0);
 
 #elif defined (__AVR_ATmega128RFA1__) && 0
+/* Uses the general routine below at present */
 
 #define AVR_OUTPUT_COMPARE_INT TIMER0_COMPA_vect
 #define OCRSetup() \
@@ -113,8 +114,49 @@
    * counter value in TCNT2 is cleared automatically. \
    */ \
   TIMSK2 = _BV (OCIE2A);
+#else /* !AVR_CONF_USE32KCRYSTAL */
+
+/* Determine the largest value that can be used with 8 bit timer0 */
+#ifndef F_CPU
+#error "Please define CPU clock speed for your platform. #define F_CPU 8000000UL is typical."
+#endif
+#if CLOCK_CONF_SECOND == 0
+#error "Please define timer ticks per second for your platform. #define CLOCK_CONF_SECOND 128 is typical."
+#endif
+
+#ifdef AVR_CONF_TMR0_PRESCALE
+#elif F_CPU/CLOCK_CONF_SECOND < 256
+  #define AVR_CONF_TMR0_PRESCALE 1
+#elif F_CPU/CLOCK_CONF_SECOND < 256 * 8
+  #define AVR_CONF_TMR0_PRESCALE 8
+#elif F_CPU/CLOCK_CONF_SECOND < 256 * 64
+  #define AVR_CONF_TMR0_PRESCALE 8
+#elif F_CPU/CLOCK_CONF_SECOND < 256 * 256
+  #define AVR_CONF_TMR0_PRESCALE 256
 #else
+  #define AVR_CONF_TMR0_PRESCALE 1024
+#endif
+
+#if F_CPU/CLOCK_CONF_SECOND/AVR_CONF_TMR0_PRESCALE > 255
+#error "Can not prescale CPU clock to get specified ticks per second. F_CPU/CLOCK_CONF_SECOND/1024 must be less than 256."
+#endif
+
+#if AVR_CONF_TMR0_PRESCALE == 1
+  #define AVR_TCCR0B_CONF _BV(CS00)
+#elif AVR_CONF_TMR0_PRESCALE == 8
+  #define AVR_TCCR0B_CONF _BV(CS01)
+#elif AVR_CONF_TMR0_PRESCALE == 64
+  #define AVR_TCCR0B_CONF _BV(CS01) | _BV(CS00)
+#elif AVR_CONF_TMR0_PRESCALE == 256
+  #define AVR_TCCR0B_CONF _BV(CS02)
+#elif AVR_CONF_TMR0_PRESCALE == 1024
+  #define AVR_TCCR0B_CONF _BV(CS02) | _BV(CS00)
+#else
+#error "Prescale factor not supported. Allowed values are 1,8,64,256,1024."
+#endif
+
 #define AVR_OUTPUT_COMPARE_INT TIMER0_COMPA_vect
+
 #define OCRSetup() \
   /* Select internal clock */ \
   ASSR = 0x00; 				  \
@@ -124,19 +166,19 @@
 \
   /*						  \
    * Set comparison register: \
-   * Crystal freq. is F_CPU,\
+   * Crystal freq. is F_CPU, prescale is given, \
-   * pre-scale factor is 1024, we want CLOCK_CONF_SECOND ticks / sec: \
+   * We want CLOCK_CONF_SECOND ticks / sec: \
-   * F_CPU = 1024 * CLOCK_CONF_SECOND * OCR2A, less 1 for CTC mode \
+   * F_CPU = AVR_CONF_TMR0_PRESCALE * CLOCK_CONF_SECOND * OCR2A, less 1 for CTC mode \
    */ \
-  OCR0A = F_CPU/1024UL/CLOCK_CONF_SECOND - 1; \
+  OCR0A = F_CPU/AVR_CONF_TMR0_PRESCALE/CLOCK_CONF_SECOND - 1; \
 \
   /* 								\
    * Set timer control register: 	\
-   *  - prescale: 1024 (CS00 - CS02) \
+   *  - prescale according to AVR_CONF_TMR0_PRESCALE \
    *  - counter reset via comparison register (WGM01) \
    */ 								\
   TCCR0A = _BV(WGM01); \
-  TCCR0B =  _BV(CS00) | _BV(CS02); \
+  TCCR0B =  AVR_TCCR0B_CONF; \
 \
   /* Clear interrupt flag register */ \
   TIFR0 = TIFR0; \