ST7FLITE29: Oscillator calibration

Posted on October 27, 2004 at 08:34

Hello,

We use the LITE TIMER 2 of the ST7FLITE29 as timebase for a real time clock. It works perfectly with 5V Vdd. But with 3V Vdd the timer slows down about 5%. That happens although we recalibrate the oscillator with the OCCR1 value. Indeed that recalibration does not seem to affect the oscillator frequency at all.

According to the datasheet, p.23, the oscillator frequency is

mailto:700kHz@3Vdd

instead of 1MHz. Does that mean that the 2ms timebase is wrong too?

Are there any other reasons why the oscillator frequency may be to slow?

Thank you!

Al

Posted on October 27, 2004 at 11:11

Hi Al,

using a 3V supply, you can not be shure to reach f_rc = 1MHz. Best illustrated by fig. 56 at page 97 of the datasheet.

But as f_osc depends on f_rc (see fig. 13 at p. 25) and the timebase depends on f_osc, the 2ms will vary relative to it. Means with f_rc = 700kHz you'll get a timebase of 2.86ms.

The times in the datasheet normaly are given relative to f_osc @ 8MHz, which you can't reach with the internal RC oscillator @3V....

Hope it helps, WoRo