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MSP430F5435A: REFOCLK changes, when entering LPM3

Stefan Kanev
Prodigy 190 points
Part Number: MSP430F5435A

I'm using RTC_A, clocked by REFO . In active mode CPU is clocked by XT2(11Mhz).

In Active mode, REFO (measured with scope on ACLK pin)   is 32900Hz,

   entering LPM3 causes ACLK to rise up to 33000Hz.

I tryed to keep settled SCG0 and SCG1(in Active mode) , but there was no change,

What causes REFOCLK change and is possible to eliminate this change?

 

  • 0
    TI__Guru 56685 points
    Hello Stefan,

    Not much you could do in this instance I believe. The slight change (<1%) is most likely due to the decreased load on the internal core LDO and thusly a slight voltage change on the LDO output (or possibly the main supply as well). The most relatable spec would be the REFO Frequency supply voltage drift specification in which the change you describe is within the typical limits.

    If your application can't tolerate this slight change, then I would suggest moving to an external 32768 Xtal that would stay stable across the LPM modes.
  • 0 in reply to Jace H
    Prodigy 190 points
    Hello, Jace,
    I noticed - when FET (2wires) is connected - consumption is 60uA(LPM3 with RTC ISR per second,MCLK sourced by REFO), and REFOCLK does not change. Disconecting FET causes consumption to drop to 9uA and REFOCLK to increase.
    So Your suggestion is probably best bet.
    I still have doubts about load differencies for REFO as reason for change in frequency .
    32kHz is not option for now.
    I have compensation of REFO - based on XT2 - in active mode, probably will try same in LPM, with some uA in cash. :-)
  • 0 in reply to Stefan Kanev
    TI__Guru 56685 points
    Hello Stefan,

    Just as an FYI, when the FET is connected, extra consumption happens on the part as well as engaging parts of the EEM on the chip even when not actively debugging.

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