Microchip Technology XMEGA Manual de usuario

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.

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

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

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

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

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

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

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

Divide

step size

by two

Save value in

EEPROM

Send

Handshake

signal

Return

Step size =

0x80

Precision within

+/- 1%?

Enable

Oscillator

DFLLs

Increase

RC32KCAL

with step size

Decrease

RC32KCAL

with step size

AN2644

Calibration Firmware Implementation

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

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

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