keyboard/onekey/vusb_osccal.c

   1 // V-USB with ATtiny45 / ATtiny85 without a crystal
   2 // http://codeandlife.com/2012/02/22/v-usb-with-attiny45-attiny85-without-a-crystal/
   3 #include <avr/io.h>
   4 #include <usbdrv.h>
   5
   6
   7 #define abs(x) ((x) > 0 ? (x) : (-x))
   8
   9 // Called by V-USB after device reset
  10 void hadUsbReset() {
  11     int frameLength, targetLength = (unsigned)(1499 * (double)F_CPU / 10.5e6 + 0.5);
  12     int bestDeviation = 9999;
  13     uint8_t trialCal, bestCal, step, region;
  14
  15     // do a binary search in regions 0-127 and 128-255 to get optimum OSCCAL
  16     for(region = 0; region <= 1; region++) {
  17         frameLength = 0;
  18         trialCal = (region == 0) ? 0 : 128;
  19
  20         for(step = 64; step > 0; step >>= 1) {
  21             if(frameLength < targetLength) // true for initial iteration
  22                 trialCal += step; // frequency too low
  23             else
  24                 trialCal -= step; // frequency too high
  25
  26             OSCCAL = trialCal;
  27             frameLength = usbMeasureFrameLength();
  28
  29             if(abs(frameLength-targetLength) < bestDeviation) {
  30                 bestCal = trialCal; // new optimum found
  31                 bestDeviation = abs(frameLength -targetLength);
  32             }
  33         }
  34     }
  35
  36     OSCCAL = bestCal;
  37 }