Microchip Technology XMEGA Manuale utente
AN2644
XMEGA Internal RC Oscillator Calibration
Features
• Calibration of Internal RC Oscillator Frequency with +/-1% Accuracy
• Support for all XMEGA® Devices with Tunable Internal RC Oscillators via JTAG Interface
• Calibration Using AVR® Programming Tools
• Calibration at any Operating Voltage and Temperature
• Firmware Description and Example for use During Production Programming
Introduction
This application note describes a fast and accurate way of calibrating the internal RC oscillators on
XMEGA® devices. An easily adaptable calibration firmware example is available through Atmel® START ,
which can be used with any XMEGA with one or more tunable internal RC oscillators and a JTAG
interface. The routine is based on using an AVR programming tool for generating a reference clock signal
at a known frequency and comparing this to the frequency of the internal RC oscillator.
The internal RC oscillator frequency can be calibrated to within +/-1% of the frequency specified in the
device data sheet. This feature offers great flexibility and significant cost savings compared to using an
external oscillator.
The factory calibration is performed at a fixed operating voltage and temperature, typically at 3V and
85°C. As the frequency of internal RC oscillators are affected by both operating voltage and temperature,
it may be desired to perform a secondary calibration in conditions matching the specific application
environment. This secondary calibration can be performed to gain higher accuracy than the standard
calibration offers, to match a specific operating voltage and/or temperature.
The calibration method described in this application note only takes a fraction of a second longer than
reading the factory calibration byte from the signature row and writing it back to the to the device memory.
Thus, the overall programming time is almost unaffected when performing calibration during the
programming step in production.
In some systems it may be more beneficial to perform run-time oscillator calibration. This may be
desirable in applications that need an accurate system clock over the entire temperature range and
independent of operating voltage. In that case a 32.768 kHz watch crystal may offer a reliable and cost
efficient solution.
© 2018 Microchip Technology Inc. Application Note DS00002644A-page 1
Table of Contents
Features.......................................................................................................................... 1
Introduction......................................................................................................................1
1. Theory of Operation – Internal RC Oscillators...........................................................3
1.1. Clock Selection.............................................................................................................................3
1.2. Internal Calibrated RC Oscillator Overview..................................................................................3
1.3. Runtime Calibration Using a 32.768 kHz Reference Clock..........................................................4
1.4. Oscillator Characteristics..............................................................................................................4
2. Calibration Firmware Implementation........................................................................6
2.1. Calibration Clock Accuracy.......................................................................................................... 6
2.2. Calibration Protocol...................................................................................................................... 6
2.3. Algorithm for Determining the Oscillator Calibration Value ......................................................... 7
3. Measuring the Calibration Clock Frequency..............................................................9
4. Performing Calibration.............................................................................................10
5. Get Source Code from Atmel | START.................................................................... 11
6. Revision History.......................................................................................................12
The Microchip Web Site................................................................................................ 13
Customer Change Notification Service..........................................................................13
Customer Support......................................................................................................... 13
Microchip Devices Code Protection Feature................................................................. 13
Legal Notice...................................................................................................................14
Trademarks................................................................................................................... 14
Quality Management System Certified by DNV.............................................................15
Worldwide Sales and Service........................................................................................16
AN2644
© 2018 Microchip Technology Inc. Application Note DS00002644A-page 2
1. Theory of Operation – Internal RC Oscillators
In production the internal RC oscillators are mostly calibrated at 3V/85°C. Refer to oscillator
characteristics in the data sheet for the individual devices for information about the temperature and
operating voltage used during calibration. If a design requires accuracy of +/-1% at operating voltages
and temperatures other than what is offered by the standard factory calibration, it is possible to perform a
secondary calibration of the RC oscillator. By doing this it is possible to obtain frequency accuracy within
+/-1% at any operating voltage and temperature. A secondary calibration can thus be performed to
improve or tailor the accuracy or frequency of the oscillator.
1.1 Clock Selection
The XMEGA System Clock source is selectable from software and can be changed during normal
operation. Each oscillator option has a status flag that can be read from software to check that the
oscillator is ready. After Reset the XMEGA starts running from the internal 2 MHz calibrated RC oscillator.
An overview of the available clock selection options is presented in the data sheets.
1.2 Internal Calibrated RC Oscillator Overview
There are three factory calibrated internal RC oscillators on XMEGA, with nominal frequencies of 32.768
kHz, 2 MHz and 32 MHz, respectively. The 2 MHz and 32 MHz oscillators feature automatic run-time
calibration. All factory calibrated oscillators can be used as the main system clock.
The following sections provide an overview of the internal calibrated RC oscillators available in the
XMEGA microcontrollers.
1.2.1 Calibrated 32.768 kHz RC Oscillator
This RC oscillator provides an approximate 32.768 kHz clock. A factory-calibrated value is written to the
32.768 kHz oscillator Calibration register during Reset to ensure that the oscillator is running within its
specification. The Calibration register can also be written from software for runtime calibration of the
oscillator frequency. The oscillator employs a built-in prescaler providing both a 32.768 kHz output and a
1.024 kHz output.
1.2.2 Calibrated 2 MHz RC Oscillator
This RC oscillator provides an approximate 2 MHz clock. The oscillator employs a Digital Frequency
Locked Loop (DFLL) that can be enabled for automatic run-time calibration of the oscillator. A factory-
calibrated value is written to the 2 MHz DFLL Calibration register during Reset to ensure that the
oscillator is running within its specification. The Calibration register can also be written from software for
manual run-time calibration of the oscillator.
1.2.3 Calibrated 32 MHz RC Oscillator
This RC oscillator provides an approximate 32 MHz clock. The oscillator employs a DFLL that can be
enabled for automatic run-time calibration of the oscillator. A factory-calibrated value is written to the 32
MHz DFLL Calibration register during reset to ensure that the oscillator is running within its specification.
The Calibration register can also be written from software for manual run-time calibration of the oscillator.
AN2644
Theory of Operation – Internal RC Oscillators
© 2018 Microchip Technology Inc. Application Note DS00002644A-page 3
1.3 Runtime Calibration Using a 32.768 kHz Reference Clock
The XMEGA Clock System provides two DFLLs, one for the 2 MHz RC oscillator and one for the 32 MHz
RC oscillator. The DFLLs can be configured individually to use either the internal 32.768 kHz RC
oscillator or an external 32.768 kHz watch crystal as a reference for the calibration process.
Once enabled, a DFLL provides continuous calibration of its oscillator based on the clock reference.
When entering Sleep mode, the current state is frozen and the calibration loop continues from where it
stopped when exiting from Sleep mode again.
If a DFLL is disabled, the current calibration value for the oscillator will remain in effect until the DFLL is
enabled again and the calibration process continues.
For more information please refer to the device data sheet and application note AVR1003 Using the
XMEGA Clock System.
1.4 Oscillator Characteristics
The specific frequency of the internal 32.768 kHz RC oscillator depends on operating temperature and
voltage. An example of this dependency is seen in Figure 1-1, which shows the output frequency of the
internal 32.768 kHz RC oscillator on an ATxmega128A1U microcontroller. As seen from the figure, the
frequency increases with increasing temperature, and decreases slightly with increasing operating
voltage. These characteristics will vary from device to device. For details on a specific device refer to its
data sheet.
Figure 1-1. Internal 32.768kHz Oscillator Output Frequency vs. Temperature
All XMEGA devices with tunable 32.768 kHz RC oscillators have an RC32KCAL register for tuning the
oscillator frequency. An increasing value in RC32KCAL will result in an increase in frequency. This
information is very relevant when searching for the best calibration value to fit a given frequency.
The two built-in DFLLs in all XMEGA devices can be used to improve the accuracy of the 2 MHz and 32
MHz internal oscillators. The reference clock sources can be selected to be the internal 32.768 kHz RC
oscillator or an external 32.768 kHz watch crystal. That means the precision of the 2 MHz and 32 MHz
internal oscillators will be decided by the reference clock accuracy. When the DFLL is enabled it will count
each oscillator clock cycle, and for each reference clock edge, the counter value is compared to the fixed
ideal relationship between the reference clock and the oscillator frequency. If the internal oscillator runs
AN2644
Theory of Operation – Internal RC Oscillators
© 2018 Microchip Technology Inc. Application Note DS00002644A-page 4
too fast or too slow, the DFLL will decrement or increment the corresponding DFLL Calibration register
value by one to adjust the oscillator frequency slightly. For details refer to the XMEGA manual.
Knowing the fundamental characteristics of the RC oscillators, it is possible to make an efficient
calibration routine that calibrates the RC oscillator to a given frequency at any operating voltage and at
any temperature with an accuracy of +/-1%.
AN2644
Theory of Operation – Internal RC Oscillators
© 2018 Microchip Technology Inc. Application Note DS00002644A-page 5
2. Calibration Firmware Implementation
This section is describes the calibration protocol in general, as well as the overall flow of the calibration
algorithm. The protocol utilizes the TDI and TDO pins of the JTAG interface, and can be adapted for most
test or programming tools. This facilitates execution of the calibration routine on a PCB-mounted device
and in the production environment for a final product
The Atmel ICE programming and debugging tool supports the described calibration routine, and an
example application that combines the Atmel ICE and Atmel Studio is available through Atmel START.
2.1 Calibration Clock Accuracy
The accuracy of the calibration is highly dependent on the accuracy of the external calibration clock. The
calibration clock frequency generated by the AVR tools may vary. It is therefore important to measure the
exact frequency of the signal on the JTAG TDI pin, and update the corresponding reference clock
frequency value in the main.c file. Since resonators are dependent on both operating voltage and
temperature, the frequency of the calibration clock should be measured when these parameters equal the
conditions during calibration.
2.2 Calibration Protocol
The basic concept is that the programming tool generates the calibration clock (C-clock) and the device
uses this as a reference to calibrate its internal RC oscillator. When the device has completed the
calibration it signals “OK” to the tool on the TDO line.
Prior to initiating the calibration, the exact frequency of the calibration clock signal must be measured and
provided to the calibration firmware. The signal should have a nominal frequency of approximately 32
kHz.
The XMEGA device should enable its internal pull-up resistor on the TDI line, and the programming tool
should enable a pull-up resistor on the TDO line. Unfortunately, the programming tool is in many cases
behind level converters, so the device should set the TDO line high to ensure that noise is unlikely to
corrupt the calibration.
The calibration procedure consists of the following steps:
1. The tool writes the calibration firmware into the device and releases the Reset line.
2. The JTAG disable bit in the MCUCSR register on the device is written to one.
3. The calibration clock is applied on the TDI line by the programming tool.
4. When the device detects the calibration clock, a binary search is used to find an RC32KCAL value
that meets the accuracy criterion. If calibration fails, the TDO line is set low and program flow goes
to step 7.
5. The oscillator calibration bytes are written to EEPROM.
6. TDO line is toggled 8 times/4 cycles by the device. The toggling of the TDO line is performed on the
falling edge of the clock on the TDI line (C-clock), but 5 to 10 CPU cycles delayed.
7. JTAG interface is re-enabled and the device goes into an infinite loop.
8. If the device does not have an EESAVE fuse, the tool must read back the calibration bytes from
EEPROM, for later restoring when the calibration firmware has been erased from the Flash. If the
device have an EESAVE fuse, this fuse can be set so that erasing the Flash does not also erase
the EEPROM.
AN2644
Calibration Firmware Implementation
© 2018 Microchip Technology Inc. Application Note DS00002644A-page 6
Note: Please note that it is necessary to copy the calibration byte from EEPROM or FLASH to the
calibration register at run-time. A routine for this must therefore be implemented in the final firmware.
2.3 Algorithm for Determining the Oscillator Calibration Value
A timer/counter can be used to compare the frequencies of the calibration clock (C-clock) and the internal
RC oscillator. The 16-bit Timer/Counter C0 (TCC0) is recommended since it is present on most XMEGA
devices with tunable RC oscillators. The idea is to capture the frequency of the C-clock using the XMEGA
Event System and compare the frequency to predefined limits. The exact frequency of the C-clock is
assumed to be known by the application, and by adjusting the RC32KCAL calibration value the frequency
of the internal RC oscillator can be tuned according to the C-clock.
A suggested algorithm for determining the most suitable oscillator calibration value is described in the
flowchart in Figure 2-1
AN2644
Calibration Firmware Implementation
© 2018 Microchip Technology Inc. Application Note DS00002644A-page 7
Figure 2-1. Flowchart of algorithm determining relationship between the C-clock and the internal
oscillator frequency
Start
Initialize Timer,
Event System
and Port
Wait for
frequency
stabilization
Step size
== 1?
Captured
frequency <
nom. value
Y
Divide
step size
by two
NY
Save value in
EEPROM
Send
Handshake
signal
Return
Y
N
N
Step size =
0x80
Precision within
+/- 1%?
Enable
Oscillator
DFLLs
Increase
RC32KCAL
with step size
Decrease
RC32KCAL
with step size
AN2644
Calibration Firmware Implementation
© 2018 Microchip Technology Inc. Application Note DS00002644A-page 8
3. Measuring the Calibration Clock Frequency
In order for the calibration routine to provide an accurate result, the exact frequency of the calibration
clock signal should be measured externally and provided to the calibration firmware. A basic way to
accomplish this would be to connect an oscilloscope to the calibration clock line and simply include the
measured value in the firmware source code.
The calibration clock signal should be transmitted on the JTAG TDI pin during calibration. To measure its
frequency, simply initiate a calibration session with the tool to be used after connecting the JTAG interface
to the target device. The calibration firmware is not required to run on the target device. The clock signal
should then be available on the TDI line sufficiently long enough to be captured by an oscilloscope or
other measurement device. The frequency of the calibration clock signal should be in the 32 kHz range.
If the calibration firmware is not running on the target device, the calibration command should return an
error message, which can be ignored at this time.
A calibration sequence can be initiated in the following way:
1. Connect the selected programming tool to the target device using the JTAG interface.
2. Connect the programming tool to the computer running Atmel Studio 7.0 and power up the target
device.
3. In Atmel Studio, open the command line by selecting Tools → Command Prompt.
4. In the command prompt, run the following command to initiate the calibration:
atprogram -t [tool] -d [device] calibrate
Example: atprogram -t atmelice -d atxmega128a1u calibrate
Note: Run atprogram without arguments to display documentation and available options. The atprogram
tool can be accessed directly from the installation path of Atmel® Studio 7.0 in the folder atbackend.
AN2644
Measuring the Calibration Clock Frequency
© 2018 Microchip Technology Inc. Application Note DS00002644A-page 9
4. Performing Calibration
The command line tool atprogram is used for initiating the calibration sequence. This application is
included in the Atmel Studio 7.0 installation, and it connects to a supported AVR programming tool to
generate the required calibration clock signal. Upon successful calibration, the atprogram tool can also be
used to read the resulting oscillator calibration bytes from EEPROM.
The following programming tools support the calibration protocol:
• Atmel ICE
• Power Debugger
• JTAGICE3
Prerequisites for the calibration procedure:
• The exact calibration clock frequency has been measured.
• The target has been programmed with the calibration firmware and has been provided with the
exact calibration clock frequency.
A calibration sequence can be initiated in the following way:
1. Connect the selected programming tool to the target device using the JTAG interface.
2. Connect the programming tool to the computer running Atmel Studio 7.0 and power-up the target
device.
3. In Atmel Studio 7.0, open the command line by selecting Tools → Command Prompt.
4. In the command prompt, run the following command to initiate the calibration:
atprogram -t [tool] -d [device] calibrate
Example: atprogram -t atmelice -d atxmega128a1u calibrate
Upon successful calibration the command returns with the message:
Oscillator calibration sequence succeeded
5. If needed, the resulting calibration bytes can be read from the EEPROM with the atprogram "read"
command. If the five oscillator calibration bytes on an XMEGA128A1U are stored in bytes 0-4 on
page 0 in EEPROM, this should be specified in the command along with the desired output format.
Example: atprogram -t atmelice -i jtag -xr -d atxmega128a1u read -ee -o 0 -s 5 --format hex
The expected output and format from this command is:
Firmware check OK
400A400D89
The five bytes will be printed in hexadecimal format starting with the byte stored at the lowest
address. If the bytes are stored in the following order: [DFLLRC32M.CALA][DFLLRC32M.CALB]
[DFLLRC2M.CALA][DFLLRC2M.CALB][OSC.RC32KCAL], the first 0x40 value would represent the
value of DFLLRC32M.CALA. The values illustrated here are examples only.
Note: The atprogram tool also supports writing the read-command result directly to file.
Note: Run atprogram without arguments to display documentation and available options. The atprogram
tool can also be accessed directly from the installation path of Atmel Studio 7.0 in the folder atbackend.
AN2644
Performing Calibration
© 2018 Microchip Technology Inc. Application Note DS00002644A-page 10
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