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| author | Christian Starkjohann <cs+github@obdev.at> | 2008-10-18 16:03:37 +0000 |
|---|---|---|
| committer | Christian Starkjohann <cs+github@obdev.at> | 2008-10-18 16:03:37 +0000 |
| commit | 9411e36b44895d39a9f7be4a801c922aa7a4ed26 (patch) | |
| tree | a0609682a892706ce8ea91675cb3603af0f67593 /libs-device | |
| parent | 81d3988ca8af8a763a9fddda0efa1b6f3ddfb251 (diff) | |
| download | v-usb-9411e36b44895d39a9f7be4a801c922aa7a4ed26.tar.gz v-usb-9411e36b44895d39a9f7be4a801c922aa7a4ed26.tar.bz2 v-usb-9411e36b44895d39a9f7be4a801c922aa7a4ed26.zip | |
- Added a code example for keeping the RC oscillator in sync with the USB frame
clock
Diffstat (limited to 'libs-device')
| -rw-r--r-- | libs-device/osctune.h | 49 |
1 files changed, 49 insertions, 0 deletions
diff --git a/libs-device/osctune.h b/libs-device/osctune.h new file mode 100644 index 0000000..3248152 --- /dev/null +++ b/libs-device/osctune.h @@ -0,0 +1,49 @@ +/* Name: osctune.h + * Author: Christian Starkjohann + * Creation Date: 2008-10-18 + * Tabsize: 4 + * Copyright: (c) 2008 by OBJECTIVE DEVELOPMENT Software GmbH + * License: GNU GPL v2 (see License.txt) or proprietary (CommercialLicense.txt) + * This Revision: $Id$ + */ + +/* +General Description: +This file is declared as C-header file although it is mostly documentation +how the RC oscillator can be kept in sync to the USB frame rate. The code +shown here must be added to usbconfig.h or this header file is included from +there. This code works only if D- is wired to the interrupt, not D+!!! + +You may want to store a good calibration value in EEPROM. You know that the +calibration value is good when the first USB message is received. Do not store +the value on every received message because the EEPROM has a limited endurance. +*/ + +#define TIMER0_PRESCALING 8 /* must match the configuration for TIMER0 in main */ +#define TOLERATED_DEVIATION_PPT 5 /* max clock deviation before we tune in 1/10 % */ +/* derived constants: */ +#define EXPECTED_TIMER0_INCREMENT ((F_CPU / TIMER0_PRESCALING) & 0xff) +#define TOLERATED_DEVIATION (TOLERATED_DEVIATION_PPT * F_CPU / (1000000 * TIMER0_PRESCALING)) + +#ifdef __ASSEMBLER__ +macro tuneOsccal + in YL, TCNT0 + lds YH, lastTimer0Value + sts lastTimer0Value, YL + sub YL, YH ; time passed since last frame + subi YL, EXPECTED_TIMER0_INCREMENT + in YH, OSCCAL + cpi YL, (TOLERATED_DEVIATION) + 1 + brlt notTooHigh + dec YH ; clock rate was too high + rjmp osctuneDone +notTooHigh: + cpi YL, -TOLERATED_DEVIATION + brge osctuneDone ; not too low + inc YH ; clock rate was too low +osctuneDone: + out OSCCAL, YH ; store tuned value + endm +#endif + +#define USB_SOF_HOOK tuneOsccal |