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

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This commit is contained in:
David Kopf 2011-08-17 13:15:26 -04:00
parent d4eeeeb8c9
commit 36fa735323
1 changed files with 48 additions and 6 deletions
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54
cpu/avr/dev/clock-avr.h
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@ -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,\
* pre-scale factor is 1024, we want CLOCK_CONF_SECOND ticks / sec: \
* F_CPU = 1024 * CLOCK_CONF_SECOND * OCR2A, less 1 for CTC mode \
* Crystal freq. is F_CPU, prescale is given, \
* We want CLOCK_CONF_SECOND ticks / sec: \
* 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; \