ATMEL AVR1506 User Manual

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AVR1506: Xplain training - XMEGA clock system

Prerequisites

• Required knowledge
Basic knowledge of microcontrollers and the C programming language
Completed AVR1500 Xplain Training – XMEGA™ Basics

• Software prerequisites
Atmel
WinAVR/GCC 20100110 or later

• Hardware prerequisities
XPLAIN evaluation board
JTAGICE mkII

• Estimated completion time:
2 hours

1 Introduction

Atmel XMEGA has an advanced clock system, supporting a large number of clock
sources. It incorporates both integrated oscillators, and external crystal oscillators
and resonators. A high frequency Phase Locked Loop (PLL) and clock prescalers
can be used to generate a wide range of clock frequencies. A calibration feature
(DFLL) is available, and can be used for automatic run-time calibration of the
internal oscillators. A Crystal Oscillator Failure Monitor can be enabled to issue a
Non-Maskable Interrupt and switch to internal oscillator if the external oscillator
fails. This training will cover the basics, but you will find more details of the XMEGA
clock system in Application Note AVR1003.

®

AVR® Studio® 4.18 SP2 or later

8-bit
Microcontrollers

Application Note

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2 Introduction to the XMEGA clock system

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The Atmel XMEGA Clock System provides a large portfolio of clock sources, both
internal and external. In addition, an internal PLL can be used to multiply selected
clock sources with a factor ranging from 1x to 31x.

The internal 2 MHz and 32 MHz oscillators available have hardware support for
automatic calibration against a 32 kHz clock source with the help of the built-in Digital
Frequency Locked Loop (DFLL).

The Atmel XMEGA also has a dedicated Real Time Counter (RTC) which is typically
used to keep track of time in more human friendly units like milliseconds, seconds,
etc.

In the following chapters we will take a more detailed look into the different parts of
the clock system.

2.1 Clock sources

In order to ease implementations, the default clock setting for the XMEGA is to start
up running from an internal 2 MHz factory-calibrated source. In this way, if the default
settings are sufficient, no external components or software configuration is required to
start executing code.

There are five internal clock sources (including the internal PLL), ranging from an
ultra low-power (ULP) 32 kHz RC oscillator to a 32 MHz factory-calibrated ring
oscillator with auto-calibration features. All sources except the ULP 32 kHz RC
oscillator can be used for the main system clock.

Any number of the internal sources can be enabled at any given time, even if none
are used for the main system clock. Also, some clock sources might even be used for
multiple purposes, such as the 32 kHz RC oscillator that can be used as a main
system clock and as a clock source for the Real Time Counter module at the same
time.

Some of the internal clock sources can be used as a reference to the internal PLL in
order to generate even higher frequencies.

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The following illustration shows the XMEGA clock system in more detail:

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2.2 System clock prescaler

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The system clock prescalers as shown in the illustration above can be illustrated in
more details as follows:

The A, B and C prescalers gives us even more flexibility in the input clock selection.
By using any of the available clock sources together with the prescalers, we are able
to run the peripherals and CPU at various frequencies just by reconfiguring the A, B
and C prescalers.

2.3 Runtime calibration

2.4 Real-time counter

The Atmel XMEGA has two built-in Digital Frequency Locked Loops (DFLLs) which
can be used to improve the accuracy of the 2 MHz and 32 MHz internal oscillators.
The DFLL compares the oscillator frequency with a more accurate reference clock to
do automatic run-time calibration of the oscillator. The choices for the reference clock
sources are:

• 32 kHz Calibrated Internal Oscillator

• 32 kHz Crystal Oscillator connected to the TOSC pins

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 two clocks. If the internal oscillator runs 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.

The XMEGA has a dedicated counter for keeping track of time in a more human
friendly way than regular timers, the Real Time Counter or RTC.

The reference clock for this timer is typically an external high accuracy 32 kHz watch
crystal. This timer/counter is clocked asynchronously and is active in several of the
low power sleep modes. This allows the microcontroller to keep track of time even
when in sleep.

If less accuracy is needed, it is possible to use the internal 32 kHz RC oscillator to
clock the RTC instead of an external crystal.

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The input clock for the RTC can be the 32 kHz clock directly or a 1 kHz prescaled
version of the 32 kHz clock. In addition the RTC has a separate prescaler block in
order to choose accuracy over long time-out periods. With a maximum resolution of

30.5 μs, time-out periods range up to 2000 seconds. With a resolution of 1 second,
the maximum time-out period is over 18 hours (65536 seconds).

The RTC includes both a period and compare register that can be used to generate
interrupts and/or events.

2.5 External clock source failure monitor

The Atmel XMEGA has functionality for handling external clock source failures. We
will not show this functionality in this training. For more information please refer to the
device datasheet and manual.

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3 Overview

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Here is a short overview of the tasks in this training:

Task 1: Changing system clock

In this task we will show how to change between the different oscillators as the main
clock.

Task 2: Using the internal PLL

In this task we will use the internal PLL and show how to change the settings for the
PLL. We also look at the internal prescalers that are available.

Task 3: Runtime calibration

In this task we will show you how to enable and disable the automatic runtime
calibration of the internal oscillators.

Task 4: Real time counter

In this task we will show you how to configure the RTC and how to keep track of time
in milliseconds, seconds, minutes, hours and days.

Good luck!

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4 Task 1: Clock switching

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Atmel XMEGA has a lot of clock sources. In this task we will show how to dynamically
switch between the internal oscillators while the MCU is running.

The goal for this task is that you know how to:

• Use the available clock system drivers

• Enable the available clock sources

• Dynamically switch between the different clock sources

TASK:

1. Locate the XMEGA-ClockSystem folder, find the Task 1 folder and open the
ClockSwitching.aps project file in AVR Studio

2. Look through the code and ensure you understand how things are set up

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3. Build the project; ensure there are no errors

4. Start the debugging session

5. Use the switches to change the oscillator. Do you observe a difference in the rate
of blinks on the LEDs?

6. Reset the debugging and restart the code, and notice the rate in which the LEDs
are blinking

Which oscillator is the default after a reset?

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5 Task 2: Using the internal PLL

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Atmel XMEGA also have a built-in Phase Locked Loop (PLL) that can be used to
generate runtime frequency dynamically. In this task we show you how to configure
the XMEGA PLL and how to reconfigure it.

The goal for this task is that you know how to:

• Change the system prescalers

• Enable the PLL and use it as the main clock source

• Modify the PLL parameters

TASK:

1. Locate the XMEGA-ClockSystem folder, find the Task 2 folder and open the
PLL.aps project file in Atmel AVR Studio

2. Look through the code and ensure you understand how things are set up

3. Build the project; ensure there are no errors

4. Start the debugging session

5. Use the switches to change the oscillator. Do you observe a difference in the rate
of blinks on the LEDs?

Why are we able to switch to the 2 MHz internal oscillator as main clock without
enabling it and waiting for it to be stable as we did in Task 1?

What is the procedure for reconfiguring the running speed of the internal PLL?

Can the system clock prescalers be changed at any time?

6. If you have time: Add code to allow you to generate 62 MHz to the clk
MHz to the CPU

and 15.5

per4

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6 Task 3: Runtime calibration

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The Atmel XMEGA has a built-in Digital Frequency Locked Loop (DFLL) which can be
used to calibrate the internal 32 MHz and/or the 2 MHz RC oscillator. The DFLL can
use either an external 32 kHz crystal connected to the TOSCx pins, or the internal 32
KHz oscillator, as input. When the DFLL is enabled the frequency of the 2 MHz/32
MHz oscillator is constantly being tuned to be in sync with the 32 kHz source. The
best accuracy of the runtime calibration is achieved by using an external 32 kHz
crystal.

The goal for this task is that you know how to:

• Enable and disable the runtime calibration for the 2 MHz and 32 MHz internal

oscillator

TASK:

1. Locate the XMEGA-ClockSystem folder, find the Task 3 folder and open the
RuntimeCalibration.aps project file in AVR Studio

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2. Look through the code and ensure you understand how things are set up

3. Build the project; ensure there are no errors

4. Start the debugging session

5. Use the switches to enable and disable the oscillator

Do you observe any difference in the rate of blinks on the LEDs?

Why not?

Is it possible to use an external 32 kHz crystal as reference for the 2 MHz and the
internal 32 kHz oscillator as a reference for the 32 MHz?

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7 Task 4: Real time counter

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The Atmel XMEGA A1 includes a 16-bit Real Time Counter (RTC). The RTC can be
clocked from an accurate 32.768 kHz Crystal Oscillator, the 32.768 kHz Calibrated
Internal Oscillator, or from the 32 kHz Ultra Low Power Internal Oscillator.

I this task we will show you how to use the real time counter to keep track of time
similar to a real time clock.

The goal for this task is that you know how to:

• Enable the Real Time Counter (RTC)

• Use the RTC to keep track of time

TASK:

1. Locate the XMEGA-ClockSystem folder, find the Task 4 folder and open the
RealtimeCounter.aps project file in AVR Studio

8 Summary

2. Look through the code and ensure you understand how things are set up

3. Build the project; ensure there are no errors

4. Start the debugging session

5. Holding the switches you can show the current value of the RTC_ticks,
RTC_seconds, RTC_minutes and RTC_hours

What is the purpose of setting the RTC.PER and what value should be set in order to
correspond to one second?

In this training we have looked at different aspects of the Atmel XMEGA Clock
System. We have shown how to dynamically change the clock, how to use the
internal PLL, how to runtime calibrate the internal oscillators and how to use the real
time counter. We have also learned how to use the drivers for the clock system from
application note AVR1003 to more efficiently start using the XMEGA clock system
functionality.

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9 Resources

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• Atmel XMEGA Manual and Datasheets

o http://www.atmel.com/xmega

• Atmel AVR Studio with help files

o http://www.atmel.com/products/AVR/

• WINAVR GCC compiler

o http://winavr.sourceforge.net/

• Atmel IAR Embedded Workbench

o http://www.iar.com/

10 Atmel Technical Support Center

Atmel has several support channels available:

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