diff options
Diffstat (limited to 'firmware/libs-device/osccal.c')
| -rw-r--r-- | firmware/libs-device/osccal.c | 114 |
1 files changed, 85 insertions, 29 deletions
diff --git a/firmware/libs-device/osccal.c b/firmware/libs-device/osccal.c index 939d5c3..b8d1bb5 100644 --- a/firmware/libs-device/osccal.c +++ b/firmware/libs-device/osccal.c @@ -8,49 +8,105 @@ */ #include <avr/io.h> - #ifndef uchar #define uchar unsigned char #endif +int usbMeasureFrameLengthDecreasing(int); + /* ------------------------------------------------------------------------- */ /* ------------------------ Oscillator Calibration ------------------------- */ /* ------------------------------------------------------------------------- */ /* Calibrate the RC oscillator. Our timing reference is the Start Of Frame * signal (a single SE0 bit) repeating every millisecond immediately after - * a USB RESET. We first do a binary search for the OSCCAL value and then - * optimize this value with a neighboorhod search. + * a USB RESET. + * + * + * Optimized version by cpldcpu@gmail.com, Nov 3rd 2013. + * + * Benefits: + * - Codesize reduced by 54 bytes. + * - Improved robustness due to removing timeout from frame length measurement and + * inserted NOP after OSCCAL writes. + * + * Changes: + * - The new routine performs a combined binary and neighborhood search + * in a single loop. + * Note that the neighborhood search is necessary due to the quasi-monotonic + * nature of OSCCAL. (See Atmel application note AVR054). + * - Inserted NOP after writes to OSCCAL to avoid CPU errors during oscillator + * stabilization. + * - Implemented new routine to measure frame time "usbMeasureFrameLengthDecreasing". + * This routine takes the target time as a parameter and returns the deviation. + * - usbMeasureFrameLengthDecreasing measures in multiples of 5 cycles and is thus + * slighly more accurate. + * - usbMeasureFrameLengthDecreasing does not support time out anymore. The original + * implementation returned zero in case of time out, which would have caused the old + * calibrateOscillator() implementation to increase OSSCAL to 255, effictively + * overclocking and most likely crashing the CPU. The new implementation will enter + * an infinite loop when no USB activity is encountered. The user program should + * use the watchdog to escape from situations like this. + * + * This routine will work both on controllers with and without split OSCCAL range. + * The first trial value is 128 which is the lowest value of the upper OSCCAL range + * on Attiny85 and will effectively limit the search to the upper range, unless the + * RC oscillator frequency is unusually high. Under normal operation, the highest + * tested frequency setting is 192. This corresponds to ~20 Mhz core frequency and + * is still within spec for a 5V device. */ + void calibrateOscillator(void) { -uchar step = 128; -uchar trialValue = 0, optimumValue; -int x, optimumDev, targetValue = (unsigned)(1499 * (double)F_CPU / 10.5e6 + 0.5); - - /* do a binary search: */ - do{ - OSCCAL = trialValue + step; - x = usbMeasureFrameLength(); /* proportional to current real frequency */ - if(x < targetValue) /* frequency still too low */ - trialValue += step; - step >>= 1; - }while(step > 0); - /* We have a precision of +/- 1 for optimum OSCCAL here */ - /* now do a neighborhood search for optimum value */ - optimumValue = trialValue; - optimumDev = x; /* this is certainly far away from optimum */ - for(OSCCAL = trialValue - 1; OSCCAL <= trialValue + 1; OSCCAL++){ - x = usbMeasureFrameLength() - targetValue; - if(x < 0) - x = -x; - if(x < optimumDev){ - optimumDev = x; - optimumValue = OSCCAL; - } - } - OSCCAL = optimumValue; + uchar step, trialValue, optimumValue; + int x, optimumDev, targetValue; + uchar i; + + targetValue = (unsigned)((double)F_CPU * 999e-6 / 5.0 + 0.5); /* Time is measured in multiples of 5 cycles. Target is 0.999µs */ + optimumDev = 0x7f00; // set to high positive value + optimumValue = OSCCAL; + step=64; + trialValue = 128; + + /* + Performs seven iterations of a binary search (stepwidth decreasing9 + with three additional steps of a neighborhood search (step=1, trialvalue will oscillate around target value to find optimum) + */ + + for(i=0; i<10; i++){ + OSCCAL = trialValue; + asm volatile(" NOP"); + + x = usbMeasureFrameLengthDecreasing(targetValue); + + if(x < 0) /* frequency too high */ + { + trialValue -= step; + x = -x; + } + else /* frequency too low */ + { + trialValue += step; + } + + /* + Halve stepwidth to perform binary search. Logical oring with 1 to ensure step is never equal to zero. + This results in a neighborhood search with stepwidth 1 after binary search is finished. + */ + + step >>= 1; + step |=1; + + if(x < optimumDev){ + optimumDev = x; + optimumValue = OSCCAL; + } + } + + OSCCAL = optimumValue; + asm volatile(" NOP"); } + /* Note: This calibration algorithm may try OSCCAL values of up to 192 even if the optimum value is far below 192. It may therefore exceed the allowed clock |