AN2971 Datasheet, PDF (1/12 Pages) STMicroelectronics – Using the typical temperature characteristics of 32 KHz crystal

AN2971
Application note
Using the typical temperature characteristics of 32 KHz crystal
to compensate the M41T83 and the M41T93 serial real-time clocks
Introduction
Typical real-time clocks employ 32 KHz tuning fork crystals. While being well suited to the
low-power needs of battery-backed applications, they can drift significantly over the
industrial temperature range –40 to +85°C. At the extreme temperatures, drifts are
somewhere between –108 and –177 parts per million (ppm), a loss of about 5 to 8 minutes
per month. Full temperature compensation is possible, but costly. However, a simplified
technique can be employed which can greatly improve the drift, and hence improve the
timekeeping accuracy of the RTC without adding significant cost.
Knowing the general shape of the tuning fork crystal's temperature curve, such a
compensation scheme is possible in applications where a temperature sensor is in use.
As shown in Figure 1 below, in the shaded portion nearer the bottom, the accuracy of the
32 KHz crystal falls off appreciably as the temperature approaches the limits of operation.
The pink line in the middle of that region is considered the typical behavior. If the oscillator
is adjusted for the expected or typical curve, the accuracy approaches the shaded region
depicted at the top of the figure, to something nearer the range ±25 ppm, about 1 minute per
month, a much more tolerable range of accuracy. Essentially, this technique takes the
bottom shaded region and straightens it out along the temperature axis.
In many applications that level of accuracy is all that is required. This document describes
how to implement such a compensation scheme.
Figure 1. 32 KHz crystal drift versus temperature
30
20
10
0
-10 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100
-20
-30
-40
-50
-60
-70
-80
-90
Uncompensated
range of drift versus
temperature
-100
-110
MAX
-120
-130
TYP
-140
MIN
-150
post-max
-160
post-min
-170
-180
TEMP (°C)
AM03096v1
July 2009
Doc ID 15627 Rev 1
1/12
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