Microchip Technology XMEGA Manual

XMEGA Internal RC Oscillator Calibration

Features

• Calibration of Internal RC Oscillator Frequency with +/-1% Accuracy
• Support for all XMEGA
• Calibration Using AVR
• 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
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.
®
Devices with Tunable Internal RC Oscillators via JTAG Interface
®
Programming Tools
Application Note
AN2644
®
START ,
DS00002644A-page 1