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22
tools/avrusb/libs-device/Readme.txt Normal file
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This is the Readme file for the libs-device directory. This directory contains
code snippets which may be useful for USB device firmware.
WHAT IS INCLUDED IN THIS DIRECTORY?
===================================
osccal.c and osccal.h
This module contains a function which calibrates the AVR's built-in RC
oscillator based on the USB frame clock. See osccal.h for a documentation
of the API.
osctune.h
This header file contains a code snippet for usbconfig.h. With this code,
you can keep the AVR's internal RC oscillator in sync with the USB frame
clock. This is a continuous synchronization, not a single calibration at
USB reset as with osccal.c above. Please note that this code works only
if D- is wired to the interrupt, not D+.
----------------------------------------------------------------------------
(c) 2008 by OBJECTIVE DEVELOPMENT Software GmbH.
http://www.obdev.at/

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tools/avrusb/libs-device/osccal.c Normal file
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/* 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), GNU GPL v3 or proprietary (CommercialLicense.txt)
* This Revision: $Id: osccal.c 692 2008-11-07 15:07:40Z cs $
*/
#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.
*/
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.
*/

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tools/avrusb/libs-device/osccal.h Normal file
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/* Name: osccal.h
* 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), GNU GPL v3 or proprietary (CommercialLicense.txt)
* This Revision: $Id: osccal.h 692 2008-11-07 15:07:40Z cs $
*/
/*
General Description:
This module contains a function which calibrates the AVR's internal RC
oscillator so that the CPU runs at F_CPU (F_CPU is a macro which must be
defined when the module is compiled, best passed in the compiler command
line). The time reference is the USB frame clock of 1 kHz available
immediately after a USB RESET condition. Timing is done by counting CPU
cycles, so all interrupts must be disabled while the calibration runs. For
low level timing measurements, usbMeasureFrameLength() is called. This
function must be enabled in usbconfig.h by defining
USB_CFG_HAVE_MEASURE_FRAME_LENGTH to 1. It is also recommended to call
calibrateOscillator() from the reset hook in usbconfig.h:
#ifndef __ASSEMBLER__
#include <avr/interrupt.h> /* for sei() */
extern void calibrateOscillator(void);
#endif
#define USB_RESET_HOOK(resetStarts) if(!resetStarts){cli(); calibrateOscillator(); sei();}
This routine is an alternative to the continuous synchronization described
in osctune.h.
Algorithm used:
calibrateOscillator() first does a binary search in the OSCCAL register for
the best matching oscillator frequency. Then it does a next neighbor search
to find the value with the lowest clock rate deviation. It is guaranteed to
find the best match among neighboring values, but for version 5 oscillators
(which have a discontinuous relationship between OSCCAL and frequency) a
better match might be available in another OSCCAL region.
Limitations:
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!
Precision depends on the OSCCAL vs. frequency dependency of the oscillator.
Typical precision for an ATMega168 (derived from the OSCCAL vs. F_RC diagram
in the data sheet) should be in the range of 0.4%. Only the 12.8 MHz and
16.5 MHz versions of AVR-USB (with built-in receiver PLL) can tolerate this
deviation! All other frequency modules require at least 0.2% precision.
*/
#ifndef __OSCCAL_H_INCLUDED__
#define __OSCCAL_H_INCLUDED__
void calibrateOscillator(void);
/* This function calibrates the RC oscillator so that the CPU runs at F_CPU.
* It MUST be called immediately after the end of a USB RESET condition!
* Disable all interrupts during the call!
* It is recommended that you store the resulting value in EEPROM so that a
* good guess value is available after the next reset.
*/
#endif /* __OSCCAL_H_INCLUDED__ */

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tools/avrusb/libs-device/osctune.h Normal file
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/* 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), GNU GPL v3 or proprietary (CommercialLicense.txt)
* This Revision: $Id: osctune.h 692 2008-11-07 15:07:40Z cs $
*/
/*
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+!!!
This is an alternative to the osccal routine in osccal.c. It has the advantage
that the synchronization is done continuously and that it has more compact
code size. The disadvantages are slow synchronization (it may take a while
until the driver works), that messages immediately after the SOF pulse may be
lost (and need to be retried by the host) and that the interrupt is on D-
contrary to most examples.
You may want to store a good calibration value in EEPROM for the next startup.
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.
Notes:
(*) You must declare the global character variable "lastTimer0Value" in your
main code.
(*) Timer 0 must be free running (not written by your code) and the prescaling
must be consistent with the TIMER0_PRESCALING define.
(*) Good values for Timer 0 prescaling depend on how precise the clock must
be tuned and how far away from the default clock rate the target clock is.
For precise tuning, choose a low prescaler factor, for a broad range of tuning
choose a high one. A prescaler factor of 64 is good for the entire OSCCAL
range and allows a precision of better than +/-1%. A prescaler factor of 8
allows tuning to slightly more than +/-6% of the default frequency and is
more precise than one step of OSCCAL. It is therefore not suitable to tune an
8 MHz oscillator to 12.5 MHz.
Thanks to Henrik Haftmann for the idea to this routine!
*/
#define TIMER0_PRESCALING 64 /* 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 / (1000 * TIMER0_PRESCALING)) & 0xff)
#define TOLERATED_DEVIATION (TOLERATED_DEVIATION_PPT * F_CPU / (1000000 * TIMER0_PRESCALING))
#ifdef __ASSEMBLER__
macro tuneOsccal
push YH ;[0]
in YL, TCNT0 ;[2]
lds YH, lastTimer0Value ;[3]
sts lastTimer0Value, YL ;[5]
sub YL, YH ;[7] time passed since last frame
subi YL, EXPECTED_TIMER0_INCREMENT ;[8]
#if OSCCAL > 0x3f /* outside I/O addressable range */
lds YH, OSCCAL ;[6]
#else
in YH, OSCCAL ;[6] assembler modle uses __SFR_OFFSET == 0
#endif
cpi YL, TOLERATED_DEVIATION + 1 ;[10]
brmi notTooHigh ;[11]
subi YH, 1 ;[12] clock rate was too high
; brcs tuningOverflow ; optionally check for overflow
rjmp osctuneDone ;[13]
notTooHigh:
cpi YL, -TOLERATED_DEVIATION ;[13]
brpl osctuneDone ;[14] not too low
inc YH ;[15] clock rate was too low
; breq tuningOverflow ; optionally check for overflow
osctuneDone:
#if OSCCAL > 0x3f /* outside I/O addressable range */
sts OSCCAL, YH ;[12-13] store tuned value
#else
out OSCCAL, YH ;[12-13] store tuned value
#endif
tuningOverflow:
pop YH ;[17]
endm ;[19] max number of cycles
#endif
#define USB_SOF_HOOK tuneOsccal