July 2009

Doc ID 15627 Rev 1

1/12

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

-50 -40 -30 -20 -10

0

10

20

30

40

50

60

70

80

90 100

TEMP (°C)

MIN

TYP

post-max

post-min

Uncompensated

range of drift versus

temperature

MAX

-180

-130

-80

-30

20

-170

-120

-70

-20

30

-160

-110

-60

-10

-150

-100

-50

0

-140

-90

-40

10

AM03096v1

www.st.com