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Battery backup of RTC only (not MCU!) with AUXVCC on MSP430F6720?

Johan Bodin
Prodigy 220 points

My current project needs an RTC which must be battery backed to survive unintentional AC power losses. There is no reason to keep the MCU alive at all during blackouts; no need for any battery powered LPMx mode.

I want the RTC to be powered by the main DVCC in normal operation and switch to the battery, a small Li coin cell, only when AC supply fails.

MSP430F672x looks like the ideal candidate at first sight but the AUX power system seems to be designed to switch the MCU power, not the RTC power, and none of the examples of AUX operation in the UG (SLAU208M) covers this simple case. It looks like I have to use an external switch for the RTC power...

Will this work:
  AVCC = DVCC = normally +3.3V / 0V during blackouts
  AUXVCC1 = AUXVCC2 = 0V (grounded)
  AUXVCC3 = normally 3.3V, externally switched to battery voltage during blackouts

or do I have to involve the MCU in battery operation and go through the trouble of writing code for all this?

Kind Regards
Johan

  • 0
    Guru 227245 points

    The RTC as well as the crystal are always powered by AUXVCC3. So if AUXVCC2 and AUXVCC1 are grounded, in case of a power fail, the MSP will be without power but the RTC should continue to run.
    If AUXVCC3 is not powered, neither RTC nor the crystal will run, even if VCC is there.

    CPU intervention is only needed if the AUX system shall be used for charging the backup battery or shutting down operation in a controlled manner (e.g. retaining some base operations while shutting down everything that isn't required)

  • 0 in reply to Jens-Michael Gross
    Guru 12050 points

    A msp430 when powered down and waiting for pin IRQ would use like 1uA
    The MCU could be on for years from a 100mAh battery when A/C is off.
    So why bother of physically shutting the Vcc off?, just go with a basic 430 that will do the job.

  • 0 in reply to Tony Philipsson
    Guru 227245 points

    Peter, you're right that on some documentation, logic might be positive or negative. Just like 'programming' a flash cell means changing it from '1' to '0'. Same for the fuses of an Atmel. Programming a fuse means setting it to '0' (perhaps for the same reason: flash cells)

    However, AFAIK 'set' in MSP world always means '1'. If polarity could be an issue, 'active' and 'passive' or 'idle' are used. Mostly in conjunction with UART signal levels and SPI clock. And in I2C, 'active' always means pulling the line low.

    To answer your other question: LPMx.5 shuts down the complete MSP core except for the RTC (3.5) and I/O circuitry (3.5 and 4.5). This includes the JTAG interface. The MSP is "gone" in LPM3.5. And JTAG connection must be re-established after it is back again as if the supply was off and has been switched on again. So no debugging (and no breakpoints!) across an LPM x.5 entry.

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