Atmel AVR053 Instrucciones de instalación y funcionamiento
AVR 8-bit Microcontrollers
AVR053: Internal RC Oscillator Calibration for
tinyAVR and megaAVR Devices
APPLICATION NOTE
Introduction
This application note describes a fast and accurate method to calibrate the
internal RC oscillator of the Atmel® tinyAVR® and megaAVR® devices that
have ISP or JTAG interfaces. It offers firmware source code that allows
calibration using the AVRISP mkII, JTAGICE mkII, JTAGICE3, or Atmel-ICE
programming tools. It could also be adapted to production programmers.
These are covered in the “AT06015: Production Programming of Atmel
Microcontrollers” application note.
Many tinyAVR and megaAVR devices offer the possibility to run from an
internal RC oscillator. Typically, this oscillator can be user calibrated to within
±1% of the frequency specified in the datasheet. This feature offers
significant cost savings compared to using an external oscillator.
Factory calibration is performed at a fixed operating voltage and
temperature. The calibration technique in this application note can be
performed by the user to achieve higher accuracy than the standard
calibration offers, to match a specific operating voltage or temperature, or
even to tune the oscillator to a different frequency.
In some systems it may be necessary to perform run-time calibration of the
oscillator using an external crystal. This is covered in the “AVR055: Using a
32kHz XTAL for run-time calibration of the internal RC” application note.
Features
•Calibration using the following programming tools: AVRISP mkII,
JTAGICE mkII, JTAGICE3, and Atmel-ICE
•Support for tinyAVR and megaAVR devices with tunable RC oscillator
and ISP, or JTAG interface
•Adjustable internal RC oscillator frequency with ±1% accuracy typical
•Tune RC oscillator to any frequency at any operating voltage and
temperature within specification
•No external components required for calibration
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Table of Contents
Introduction......................................................................................................................1
Features.......................................................................................................................... 1
1. Internal RC Oscillator.................................................................................................3
1.1. User Calibration of the Internal RC Oscillator...............................................................................3
1.2. Base Frequency........................................................................................................................... 3
1.3. Clock Selection.............................................................................................................................3
1.4. Tunable RC Oscillator.................................................................................................................. 3
1.5. Oscillator Characteristics..............................................................................................................4
1.6. RC Oscillator Revision History..................................................................................................... 5
1.6.1. Version 1.x Oscillators................................................................................................... 6
1.6.2. Version 2.x Oscillators................................................................................................... 6
1.6.3. Version 3.x Oscillators................................................................................................... 6
1.6.4. Version 4.x Oscillators................................................................................................... 6
1.6.5. Version 5.x Oscillators................................................................................................... 6
2. Calibration..................................................................................................................7
2.1. Calibration Protocol...................................................................................................................... 7
2.2. Steps Involved in the Calibration Procedure................................................................................ 7
2.3. Calibration Firmware.................................................................................................................... 8
2.3.1. File Organization............................................................................................................8
2.4. Binary Search...............................................................................................................................8
2.4.1. Binary Search of OSCCAL............................................................................................ 9
2.4.2. Determining Oscillator Frequency................................................................................. 9
2.4.3. Correcting Timing Inaccuracies....................................................................................11
3. Steps to Calibrate Device Using AVR Tools............................................................ 12
3.1. Assembling the Calibration Firmware.........................................................................................12
3.2. Command Line Tool....................................................................................................................12
3.3. Batch Files..................................................................................................................................12
3.4. Performance of the Calibration Firmware...................................................................................12
3.5. Calibration Clock Accuracy.........................................................................................................13
3.6. Quick Start guide........................................................................................................................13
3.7. Adding Support for New Devices................................................................................................15
4. References.............................................................................................................. 16
5. Revision History.......................................................................................................17
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1. Internal RC Oscillator
Most tinyAVR and megaAVR devices include a factory-calibrated internal RC oscillator. This is usually the
default clock source for the CPU and does not require any external components.
1.1. User Calibration of the Internal RC Oscillator
In production the internal RC oscillator is calibrated at a specific supply voltage, typically 5V or 3.3V.
Refer to the datasheet of the individual device for information about the operating voltage used during
calibration. The accuracy of the factory calibration is typically within ±3% or ±10%, depending on the
device (refer to the datasheet). If a design needs accuracy beyond what can be 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 a frequency accuracy typically within ±1% (±2% for devices that have a ±10% accuracy
from factory calibration). It is also possible to shift the frequency of the oscillator as part of the user
calibration procedure.
1.2. Base Frequency
The base frequency of an oscillator is defined as the oscillator frequency prior to any prescaling. Some
AVR® devices feature a system clock prescaler. The prescaler register (CLKPR) can be used to scale the
system clock with predefined factors. Also, the prescaler can typically be preset through the AVR device
fuses. For example, programming the CKDIV8 fuse on devices that offer this option will set up the CLKPR
to divide the system clock by eight. This can be done to make sure that the device is operated below a
maximum frequency specification. The CLKPR can be modified at run-time to change the frequency of
the system clock internally.
1.3. Clock Selection
The AVR device fuse settings control the system clock source being used. To calibrate the internal RC
oscillator, it is required to use the internal RC oscillator as the CPU clock source by setting the
appropriate fuse. An overview of the fuses is available in the datasheet of the specific device.
1.4. Tunable RC Oscillator
An RC oscillator whose frequency can be trimmed by software is called a tunable RC oscillator. In AVR
devices, the Oscillator Calibration Register (OSCCAL) is used for this purpose. It can be used to trim the
calibrated internal RC oscillator to remove process variations from the oscillator frequency, as shown in
the figure below. The OSCCAL register is one byte wide.
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Figure 1-1. Calibrated 8MHz RC Oscillator Frequency vs. OSCCAL Value (ATmega8)
When an RC oscillator in a device is factory calibrated, the calibration byte is stored in the Signature Row
of the device. The calibration byte can vary from one device to the other, as the RC oscillator frequency is
process dependent. If a device has more than one oscillator, a calibration byte for each of the RC
oscillators is stored in the Signature Row.
In most devices, the default RC oscillator calibration byte is automatically loaded from the Signature Row
and copied into the OSCCAL register at start-up. For example, the default ATmega8 clock setting is the
internal 1MHz RC oscillator; for this device the calibration byte corresponding to the 1MHz RC oscillator is
automatically loaded at start-up. If the fuses are altered so that the 4MHz oscillator is used instead of the
default setting, the calibration byte must be loaded into the OSCCAL register manually. A programming
tool can be used to read the 4MHz calibration byte from the Signature Row and hence store it in a Flash
or EEPROM location, which is read by the main program and copied into OSCCAL at run-time.
1.5. Oscillator Characteristics
The frequency of the internal RC oscillator depends on the temperature and operating voltage. An
example of this dependency is in the figure below, which shows the frequency of the 8MHz RC oscillator
of the ATmega8. The frequency increases with increasing voltage, and decreases with increasing
operating temperature. These characteristics will vary from device to device. For details on a specific
device, refer to its datasheet.
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Figure 1-2. Calibrated 8MHz RC Oscillator Frequency vs. VCC (ATmega8)
1.6. RC Oscillator Revision History
Different RC oscillators have been utilized in the AVR microcontroller throughout its history. An overview
of the RC oscillators with device examples is provided in the table below. The list is sorted by oscillator
type, which is also roughly equivalent to sorting by release date. Only tunable oscillators are listed in the
table. For oscillator details on a specific device, refer to the device datasheet.
Table 1-1. Oscillator Versions
Oscillator version Example device RC oscillator frequency [MHz] CKDIV PRSCK
1.1 ATtiny12 1.2 - -
1.2 ATtiny15 1.6 - -
2.0 ATmega163 1.0 - -
3.0 ATmega16 1.0, 2.0, 4.0, and 8.0 - -
3.1 ATmega128 1.0, 2.0, 4.0, and 8.0 XDIV (1)-
4.0 ATmega169 (2)8.0 Yes Yes
4.1 ATtiny13 4.8 and 9.6 Yes Yes
4.2 ATtiny2313 4.8 and 9.6 Yes Yes
5.0 ATmega169P (2)8.0 Yes Yes
1. The prescaler register is in these devices is named XDIV.
2. ATmega169 revision A-E uses oscillator version 4.0, while ATmega169P uses oscillator version 5.0.
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1.6.1. Version 1.x Oscillators
This version is the earliest internal RC oscillator for AVR that can be calibrated. It is offered with
frequencies ranging from 1.2 to 1.6MHz. The calibration byte is stored in the Signature Row, but is not
automatically loaded at start-up. The loading of the OSCCAL register must be handled at run-time by the
firmware. The oscillator frequency is highly dependent on operating voltage and temperature in this
version.
1.6.2. Version 2.x Oscillators
This oscillator version has a nominal frequency of 1MHz. The dependency of oscillator frequency on
operating voltage and temperature is reduced significantly compared to version 1.x.
1.6.3. Version 3.x Oscillators
This version of the oscillator system offers multiple oscillator frequencies. Four different RC oscillators
with nominal frequencies 1, 2, 4, and 8MHz are present in the device. This version features automatic
loading of the 1MHz calibration byte from the Signature Row. Because four different RC oscillators are
present, four different calibration bytes are stored in the Signature Row. If frequencies other than the
default 1MHz are desired, the OSCCAL register should be loaded with the corresponding calibration byte
at run-time.
1.6.4. Version 4.x Oscillators
A single oscillator frequency of 8MHz is offered in version 4.0. For later 4.x versions, two frequencies are
offered: 4 and 8MHz for ATtiny2313, and 4.8 and 9.6MHz for the ATtiny13. The OSCCAL register is
changed so that only seven bits are used to tune the frequency for the selected oscillator. The MSB is not
used. Auto-loading of the default calibration value and system clock prescaler is present.
1.6.5. Version 5.x Oscillators
A single oscillator frequency of 8MHz is offered in version 5.0. Auto loading of the default calibration value
and system clock prescaler is present. All eight bits in the OSCCAL register are used to tune the oscillator
frequency. The OSCCAL register is split in two parts. The MSB of OSCCAL selects one of two
overlapping frequency ranges, while the seven least significant bits are used to tune the frequency within
this range. The ATmega406 has a frequency of 4MHz but is otherwise the same.
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2. Calibration
This chapter is divided into calibration protocol and calibration firmware. The protocol can be adapted into
any test or programming tool to support calibration. The AVRISP mkII, JTAGICE mkII, JTAGICE3, and
Atmel-ICE programming tools support the implemented calibration protocol. The usage of these tools to
calibrate a device is described in later sections.
2.1. Calibration Protocol
The protocol for calibration is simple and fast to ensure that it can be used in production environments.
The pins used for programming the device, either the ISP interface or the JTAG interface, are also used
for calibration since they are most likely to be available in a final product (or on the PCB).
Two pins are used for calibration: MOSI and MISO on the ISP interface, or TDI and TDO on the JTAG
interface. To simplify the following explanations, only MOSI and MISO are mentioned, although these can
be replaced with TDI and TDO for the case of the JTAG interface.
The overall 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 programming tool on the MISO line.
The device is responsible for enabling a pull-up on the MOSI line, and the programming tool is
responsible for enabling a pull-up on the MISO line. Unfortunately the programming tool is in many cases
behind level converters, so the device sets also the MOSI line high. This is done to make sure that noise
is unlikely to corrupt the calibration.
The programming tool can use 1024 C-cycles (cycles on the C-clock) as a time-out period, as the
calibration routine is guaranteed to be completed within this number of C-cycles.
2.2. Steps Involved in the Calibration Procedure
1. The programming tool writes the calibration firmware into the device and possibly enables the
MISO pull-up, and releases the reset line. If the JTAG interface is used, the JTAG disable bit in
MCUCSR is set by the calibration firmware in the device. The calibration clock (approximately
32kHz) is applied on the MOSI line by the programming tool.
2. The device enables the internal pull-up on the MOSI line, sets the MISO line high, and starts
listening for the calibration clock on MOSI.
3. When the device detects the calibration clock a binary search is performed to find an OSCCAL
value that meets the criteria of 1% accuracy. If the binary search does not reveal a value that meets
this requirement, the neighboring values of the outcome of the binary search are tested to identify
one that does. If calibration fails the MISO line is set low and program flow goes to step 6.
4. The calibration value is stored in EEPROM (skipped if calibration fails).
5. The MISO line is toggled 8 times/4 cycles by the device. The toggling of the MISO line is performed
on the falling edge of the clock on the MOSI line (C-clock), but 5 to 10 CPU cycles delayed. In the
case of failing calibration the MISO line is not toggled.
6. If necessary, the JTAG interface is re-enabled and the device goes into an infinite loop.
7. If the device does not have an EESAVE fuse, the programmer must read back the calibration byte
from EEPROM, for later restoring when the calibration firmware has been erased from the flash
memory. If the device has an EESAVE fuse, this fuse can be set so that erasing the Flash does not
also erase the EEPROM. It is necessary to copy the calibration byte from EEPROM or FLASH to
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the OSCCAL register at run-time. A routine for this must therefore be implemented in the final
firmware.
2.3. Calibration Firmware
The calibration code is written in AVRASM2 assembly, for Atmel Studio 7.0 (or later). The calibration
firmware is structured in a way so that it can easily be updated to support new tinyAVR or megaAVR
devices that have an ISP or JTAG interface. Also, the interface for calibration can be configured.
2.3.1. File Organization
File type File path and name(s) Description
Atmel Studio
solution file
rc_calib.atsln Solution file that should be opened in Atmel
Studio to begin working with the project
Root code file rc_calib/RC_Calibration.asm Root assembly file that includes all the other
files
Device specific
file
rc_calib/Device specific/all.asm Uses the device setting in Atmel Studio to
determine additional parameters for the
device, such as the oscillator version
Interface
specific files
rc_calib/Interface specific/
isp_atmelice_interface.inc,
jtag_jtagice3_interface.inc, etc.
Specify additional parameters that are specific
to a tool. Only one should be included in
RC_Calibration.asm and the others should
be commented out.
Common files rc_calib/Common/macros.inc,
main.asm, and memoryMap.inc
The root file, RC_Calibration.asm, refers to (includes) the following files:
1. A device specific file that defines additional parameters depending on which device is selected in
Atmel Studio. This file further includes the following:
1.1. A memory map file that defines where the code is located.
1.2. An oscillator version number. This parameter determines the initial step size used in the
binary search to account for the fact that some OSCCAL registers are 7 and some are 8
bits wide.
1.3. Redefinitions of bit and register names may also be present in the device specific file.
2. A calibration interface specific file. This file assigns the ISP or JTAG port, and pins with names
(labels) used in the main code. The calibration clock frequency is specified in this file.
3. The file defining the macros used, macros.inc.
4. The common calibration code file, main.asm.
The calibration code is structured to make it easy to change, in order to match a desired target device
and programming tool. Furthermore, the extensive use of macros ensures that the code has the smallest
possible footprint. Finally, the way devices and calibration interfaces are designed ensures that support
for new devices or interfaces can be implemented with minimal effort.
2.4. Binary Search
A binary search algorithm finds the position of a specified input value within an array sorted by values.
The sorted array space is repeatedly cut in half according to how the required value compares with the
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middle element. The figure below explains this algorithm with an example to find value t = 11 from a
sorted array A.
Figure 2-1. Binary Search
2.4.1. Binary Search of OSCCAL
The search is based on a binary search method, a divide-and-conquer method:
1. The OSCCAL register is loaded with the initial value, which is half the maximum value of OSCCAL.
The initial value of OSCCAL is defined as the initial step size.
2. The frequency of the system clock is then compared to an external reference, the calibration clock.
2.1. If the frequency is within 1% accuracy limit, go to step 5.
2.2. If the system clock is found to be too fast the OSCCAL value is reduced, and if the clock is
too slow OSCCAL is increased, go to step 3.
3. The step size is assigned the value of half the previous step size.
3.1. If the step size is zero, the binary search has been unsuccessful. Go to step 4.
3.2. If the step size is not zero, the step size is added to or subtracted from the current value in
the OSCCAL register to increase or decrease the oscillator frequency, then go to step 2.
4. Test the four nearest neighbor-values of OSCCAL. This is done to compensate for the lack of a
strictly monotonic relationship between OSCCAL and oscillator frequency.
5. If a tested OSCCAL value is within the accuracy limits, continue to store value in EEPROM, signal
success and stop calibration.
6. If none of the tested OSCCAL values are within the limits (not expected), signal on MISO that the
calibration has failed by driving the line low and go to stop calibration.
2.4.2. Determining Oscillator Frequency
The comparison between the calibration clock (C-clock) and the internal RC oscillator is performed using
the 8-bit Timer/Counter0 (TC0). The 8-bit timer is used since it is present in all devices that have a
tunable RC oscillator. The idea is to time the duration of 40 C-clock cycles and compare the number of
timer ticks to predefined limits. The C-frequency is specified in the interface specific include file. The
method for determining the oscillator frequency is described in the flowchart below.
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Figure 2-2. Flowchart of Algorithm Determining Relationship Between the C-clock and Internal RC Frequency
To cover the full range of oscillator frequencies from 1MHz to 9.6MHz, inspection of the TC0 overflow
(OVF) flag is used to expand the timer range by eight bits, effectively providing a 16-bit timer. The OVF
flag is inspected once every half-cycle (of the C-clock), which is sufficiently often to ensure that all TC0
OVF events are detected. In relation to the range of the 16-bit timer implemented, the worst-case for
overflow is at 9.6MHz where the OSCCAL register is loaded with 0xFF. In this case, the oscillator can be
100% above the specified frequency. The timer will in this case count to 23,541, which is still within the
range of the 16-bit timer.
Going in the other direction, the lowest oscillator frequency must also be considered. The lowest
obtainable frequency occurs when writing 0x00 to OSCCAL. In that case the frequency may be 50%
lower than the specified one. Since the TC0 OVF flag is inspected every half-cycle, there is potentially no
more than seven CPU-cycles to handle the OVF flag and detect the next C-clock edge - at a specified
frequency of 1MHz. This timing constraint can be met when the OVF flag is not set, but when the flag is
set eight cycles are required. This will cause a small error in the detection of the timing, but will not affect
the overall outcome: the oscillator will correctly be determined as too slow. However, these extremes are
very unlikely to be encountered due to the binary search method used.
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