MC9S08QG8 RTI frquency trim

[paul]

I need the MC9S08QG8 RTI to keep time (count seconds) when it is running and also in stop3 mode. I have no problems doing this, however, I cant seem to trim the frequency. It appears the 1KHz RTI oscilator is independant of the internal reference oscilator. Am I right?

If this is the case, I can RTI every 8ms and count down 125 to increment seconds. By changing the counter preset (125) up or down I will able to trim for accuracy. My question is: How stable is this oscilator when it comes to temperature or PSU changes? I see no information in the data sheet for this oscilator, only for the ICS clock reference.

[paul]

The specs also state that the DCO could be between 25Khz and 41KHz (32.7KHz typ), but once trimmed, will remain +/- 0.5% from 0 to 70 degrees C. I consider that quite stable for no crystal.

Would the accuracy be the same for RTI oscilator?

Has anyone used this MCU's internal clock source for real time clock purposes?

[Alban]

Hi Paul,

I wouldn't expect the RTI clock to be very precise.

Principal reason is that the RTI would be used to wake the MCU up every so often for it to analyze and see if there is something to do.

As the power consumption is important there, that will be an RC oscillator just for the RTI module.

Actually it will be precise on one MCU and won't vary a lot at a given temperature. But the next MCU will be different because of the fab process and no characterization done on it.

Cheers,

Alban.

[peg]

The accuracy of this clock is quoted as +/- 30% so I couldn't imagine its stability would be too brilliant.

Peg

[Alban]

Yep Peg,

I would see a typical 25% for this kind of really simple RC-oscillators.
The 30% is probably quoted with some guard band to guarantee it across production.

Alban.