osccal.c

/* Name: osccal.c
 * Author: Christian Starkjohann
 * Creation Date: 2008-04-10
 * Tabsize: 4
 * Copyright: (c) 2008 by OBJECTIVE DEVELOPMENT Software GmbH
 * License: GNU GPL v2 or v3 (see License.txt)
*/

/*
	This file has been modified for GemmaBoot, by Adafruit Industries

	Copyright (c) 2013 Adafruit Industries
	All rights reserved.

	GemmaBoot is free software: you can redistribute it and/or modify
	it under the terms of the GNU Lesser General Public License as
	published by the Free Software Foundation, either version 3 of
	the License, or (at your option) any later version.

	GemmaBoot is distributed in the hope that it will be useful,
	but WITHOUT ANY WARRANTY; without even the implied warranty of
	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
	GNU Lesser General Public License for more details.

	You should have received a copy of the GNU Lesser General Public
	License along with GemmaBoot. If not, see
	<http://www.gnu.org/licenses/>.
*/

#include <avr/io.h>
#include <avr/power.h>

#ifndef uchar
#define uchar   unsigned char
#endif

/* ------------------------------------------------------------------------- */
/* ------------------------ 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.
 */
void    calibrateOscillator(void)
{
    #ifdef LOW_VOLTAGE
    clock_prescale_set(clock_div_1); // previously the clock was divided by 2 because it might have been 3.3V
    // but since the USB reset triggered this function, we know we have 5V and can run at a faster clock
    #endif

    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;
}
/*
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
frequency of the CPU in low voltage designs!
You may replace this search algorithm with any other algorithm you like if
you have additional constraints such as a maximum CPU clock.
For version 5.x RC oscillators (those with a split range of 2x128 steps, e.g.
ATTiny25, ATTiny45, ATTiny85), it may be useful to search for the optimum in
both regions.
*/