April 2006

Rev 1

1/20

AN2326

Application note

Calibrating the RC oscillator of the

ST7ULTRALITE MCU using the mains

Introduction

The ST7ULTRALITE microcontroller contains an internal RC oscillator which can be

trimmed to a specific frequency with the required accuracy. The oscillator frequency has to

be calibrated by software using the RCCR register (RC Control Register) and the SICSR

register (System Integrity Control/Status Register). The value entered in the RCCR/SICSR

registers will switch ON a corresponding number of resistors that will modify the oscillator

frequency. Whenever the ST7ULTRALITE microcontroller is reset, the 10-bit value

contained in the RCCR/SICSR registers is restored to its default value (3FFh) i.e. the lower

possible frequency, so each time the device is reset, you have to load the calibration value in

the RCCR/SICSR registers. There are predefined calibration values stored in memory (refer

to the” Internal RC Oscillator Adjustment” section in the ST7ULTRALITE datasheet). You

can load one of these values in the RCCR/SICSR registers if one of the operating conditions

matches that in your application. Otherwise, you can define your own value, store it in non

volatile memory and load it in the RCCR/SICSR registers after each reset. However, if any

of the external conditions (temperature or voltage, for instance) changes too drastically, the

stored value may no longer produce the required accuracy. One solution is to recalculate the

RCCR/SICSR register values after each reset, based on an external reference.

The purpose of this application note is to present a software solution using the frequency of

the European standard mains (220V/50Hz) as a timebase to adjust the internal RC oscillator

of the ST7ULTRALITE to 8 MHz. The same approach can also be used for the US mains

standard (110V/60Hz).

The basic software takes less than 200 ms to calibrate the oscillator and uses less than128

bytes of program memory and five bytes of RAM for its simplest version. These RAM bytes

can be freed for other purposes when the calibration is done. Another example using

averages is given in this application note. This can be useful with noisy mains.

This application note also contains the diagram of a low cost circuit which converts the

mains into a 5 volt power supply and protects the microcontroller from overcurrent on the

input connected to the mains.

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