69 lines
2.6 KiB
C
69 lines
2.6 KiB
C
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/* Name: osccal.c
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* Author: Christian Starkjohann
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* Creation Date: 2008-04-10
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* Tabsize: 4
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* Copyright: (c) 2008 by OBJECTIVE DEVELOPMENT Software GmbH
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* License: GNU GPL v2 (see License.txt), GNU GPL v3 or proprietary (CommercialLicense.txt)
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*/
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#include <avr/io.h>
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#ifndef uchar
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#define uchar unsigned char
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#endif
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/* ------------------------------------------------------------------------- */
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/* ------------------------ Oscillator Calibration ------------------------- */
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/* ------------------------------------------------------------------------- */
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/* This is a "C" implementation. You can customize it to your needs easily.
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* If you want smaller code size, there is an improved version in the
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* micronucleous project. See
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* https://github.com/micronucleus/micronucleus/blob/master/firmware/osccalASM.S
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*/
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/* Calibrate the RC oscillator. Our timing reference is the Start Of Frame
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* signal (a single SE0 bit) repeating every millisecond immediately after
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* a USB RESET. We first do a binary search for the OSCCAL value and then
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* optimize this value with a neighboorhod search.
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*/
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void calibrateOscillator(void)
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{
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uchar step = 128;
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uchar trialValue = 0, optimumValue;
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int x, optimumDev, targetValue = (unsigned)(1499 * (double)F_CPU / 10.5e6 + 0.5);
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/* do a binary search: */
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do{
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OSCCAL = trialValue + step;
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x = usbMeasureFrameLength(); /* proportional to current real frequency */
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if(x < targetValue) /* frequency still too low */
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trialValue += step;
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step >>= 1;
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}while(step > 0);
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/* We have a precision of +/- 1 for optimum OSCCAL here */
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/* now do a neighborhood search for optimum value */
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optimumValue = trialValue;
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optimumDev = x; /* this is certainly far away from optimum */
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for(OSCCAL = trialValue - 1; OSCCAL <= trialValue + 1; OSCCAL++){
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x = usbMeasureFrameLength() - targetValue;
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if(x < 0)
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x = -x;
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if(x < optimumDev){
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optimumDev = x;
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optimumValue = OSCCAL;
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}
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}
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OSCCAL = optimumValue;
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}
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/*
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Note: This calibration algorithm may try OSCCAL values of up to 192 even if
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the optimum value is far below 192. It may therefore exceed the allowed clock
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frequency of the CPU in low voltage designs!
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You may replace this search algorithm with any other algorithm you like if
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you have additional constraints such as a maximum CPU clock.
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For version 5.x RC oscillators (those with a split range of 2x128 steps, e.g.
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ATTiny25, ATTiny45, ATTiny85), it may be useful to search for the optimum in
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both regions.
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*/
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