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author | Christian Starkjohann <cs+github@obdev.at> | 2008-04-17 19:00:20 +0000 |
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committer | Christian Starkjohann <cs+github@obdev.at> | 2008-04-17 19:00:20 +0000 |
commit | 95ca3f5bd696b5450820929e1b1a444d02f0bd1e (patch) | |
tree | 70ac0161a0e4548acaf2f89d2ecb77132579ff7a /libs-device/osccal.c | |
parent | 2f465daef9e6c5a6b7bda3074c050cd7e8515770 (diff) | |
download | v-usb-95ca3f5bd696b5450820929e1b1a444d02f0bd1e.tar.gz v-usb-95ca3f5bd696b5450820929e1b1a444d02f0bd1e.tar.bz2 v-usb-95ca3f5bd696b5450820929e1b1a444d02f0bd1e.zip |
- imported new files into project
Diffstat (limited to 'libs-device/osccal.c')
-rw-r--r-- | libs-device/osccal.c | 64 |
1 files changed, 64 insertions, 0 deletions
diff --git a/libs-device/osccal.c b/libs-device/osccal.c new file mode 100644 index 0000000..5734a84 --- /dev/null +++ b/libs-device/osccal.c @@ -0,0 +1,64 @@ +/* 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 (see License.txt) or proprietary (CommercialLicense.txt) + * This Revision: $Id$ + */ + +#include <avr/io.h> + +/* ------------------------------------------------------------------------- */ +/* ------------------------ 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. + * This algorithm may also be used to calibrate the RC oscillator directly to + * 12 MHz (no PLL involved, can therefore be used on almost ALL AVRs), but this + * is wide outside the spec for the OSCCAL value and the required precision for + * the 12 MHz clock! Use the RC oscillator calibrated to 12 MHz for + * experimental purposes only! + */ +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; +} +/* +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. +*/ |