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Clock source for RTC in MC9S08SG8

Question asked by Bhupender Singh on Oct 5, 2016
Latest reply on Oct 7, 2016 by vicentegomez

rtchcs08 low power mode stop3 mc9s08sg8

RTC clock source can be low power oscillator which is 1KHz oscillator. Also Internal reference clock which is a 32KHz source can be opted.

If we go ahead with low power oscillator that is 1KHz as clock source for RTC timer then what would be the impact on the RTC interrupt timing for the following case.

Suppose RTC clock source is divided further with 1000 and RTCMOD = 0; which means there will be a RTC interrupt every second. 

As per device datasheet(MC9S08SG8.pdf, rev 8) Appendix A and Table A.12.1 low power oscillator period can be 600 micro second minimum and 1500 micro second max. It means the the performance of low power oscillator could get worsen in extreme temperature conditions(because a 1000us period timer is varying from 600 to 1500us). If this is true for low power oscillator then it is applicable to RTC which is using low power oscillator as clock source. As per calculation an RTC interrupt which is configured for every second, could vary from 0.598 seconds to 1.49 seconds. Please help me in understanding this impact. 

2. What is best option between Internal reference clock and low power oscillator as clock source for RTC if someone needs the RTC in low power mode-Stop3 mode.

Please answer.


Bhupender Singh