AN-949| Application Note
AN-949 APPLICATION NOTE
One Technology Way P.O. Box 9106 Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 Fax: 781.461.3113 www.analog.com
Compensating the ADE71xx/ADE75xx Family RTC for Accurate Timekeeping
by Meghan Baker and Aileen Ritchie
INTRODUCTION
This application note describes how to calibrate and keep accurate time with the ADE71xx/ADE75xx RTC at 25 C and over temperature. It details the hardware compensation provided in the real time clock (RTC) peripheral and provides an algorithm to adjust the compensation over temperature. The ADE75xx/ADE71xx integrates the analog front end and fixed function DSP of the Analog Devices, Inc., energy (ADE) metering ICs with an enhanced 8052 MCU core. The integrated RTC, LCD driver, and peripherals comprise a fully integrated metering solution.
Once calibrated the RTC is accurate to within 0.5 sec/day at a constant temperature and within 0.15 sec/day C over temperature. This results in an RTC that should not gain or lose more than 30 seconds per month over the temperature range of -40 C to +85 C.
THEORY
Crystals, like other common electrical components, such as resistors and capacitors, have certain tolerances associated with them. A crystal that is specified to have a nominal frequency of 32.768 kHz at 25 C may actually have a frequency 20 ppm from the nominal. The crystal frequency also changes over temperature. For these reasons, it is important to be able to compensate the crystal frequency variation to keep accurate time. The ADE71xx/ADE75xx makes it easy to achieve this with automatic periodic hardware compensation of the RTC and an on-board temperature ADC.
DESIGN GOALS
Many energy meters offer time-of-use billing where customers are charged a different rate for peak usage and off-peak usage. Because these meters are installed for 10 to 20 years, having an accurate clock to determine the time-of-use bins is essential.
RTCCOMP 32.768kHz CRYSTAL CALIBRATION CALIBRATED RTCEN 32.768kHz TEMPCAL
ITS1 8-BIT PRESCALER
ITS0
HUNDREDTHS COUNTER HTHSEC INTERVAL TIMEBASE SELECTION MUX
SECOND COUNTER SEC
ITEN
MINUTE COUNTER MIN
HOUR COUNTER HOUR
MIDNIGHT EVENT
8-BIT INTERVAL COUNTER INTERVAL SFR
EQUAL?
ALARM EVENT
07159-001
Figure 1. RTC Implementation
Rev. 0 | Page 1 of 8
AN-949 Application Note analog Download PDF
Add this permalink to your bookmarks for future download of AN-949 ApplicationNote
Permalink: http://application.emcelettronica.com/analog/AN-949
