Hi everyone,
I have an MSP430F5438A capturing a test tone of 1kHz using the ADC12 module. The ADC12 module is clocked from SMCLK/2. The trigger from the sample timer is sourced from Timer B. Timer B is sourced from ACLK. SMCLK is from the DCO/FLL referenced to XTAL1/ACLK.
If I use low power mode is 1, the stored samples are noise free - almost perfect 1kHz tone. If I then use low power mode is 3, to save more power, the stored samples are noisy. If I periodically enter lpm1, I see the noise go away.
My understanding is that the difference between lpm1 and lpm3 is that the DCO Generator is also off and that SMCLK is disabled. But why does it work at all? There is no frequency drift, just noise. To further prove this, if I increase the sample time the noise goes away. So it definitely appears to be an interaction between the SMCLK and the Sample&Hold.
Additionally, I am thinking of testing lpm1+SMCLK vs lpm3+ADC12OSC, to see which is lowest power - if indeed lpm3+SMCLK is not a valid configuration.
Your thoughts would be greatly appreciated.