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MSP430I2040: Is watchdog timer still active when MCU is LPM4 or LPM4.5?

Zhiyong Li (FT)
Expert 2460 points
Part Number: MSP430I2040

Hi,

I am a bit surprised that this MCU draws 210uA current when in LPM3, while draws much less to almost nothing when in LPM4 and LPM4.5. Datasheet states watchdog timer is still active in LPM3, but doesn't say one way or another about LPM4/4.5. Does that mean watchdog timer is disabled in LPM4/4.5?

Thanks.

  • 0
    Guru 88580 points

    The difference between LPM3 and LPM4 is ACLK, which is driven by the DCO, i.e. the DCO is (implicitly) active in LPM3, which is a bit different from other MSP430s.

    Based on data sheet (SLAS887) Sec 5.5 and Table 5-2, the DCO adds 85uA to the LPM4's 70uA = 155uA. That doesn't quite meet Sec 5.5's 210uA, but gets it closer. I don't know what the difference is.

    User Guide (SLAU335) Fig 7-1 implies that if the WDT is active (using ACLK) in LPM4, ACLK doesn't turn off, so you're really in LPM3 anyway.

  • 0 in reply to Bruce McKenney47378
    Expert 2460 points

    My take from your answer is that if WDT is enabled, LPM4 is basically the same as LPM3.

    How about LPM4.5? Will enabled WDT prevent MCU from entering LPM4.5? My understanding is, LPM4.5 is basically shutdown but with shorter power up time, 0.45ms vs, 0.75ms, and WDT is either disabled or if enabled MCU won't enter into LPM4.5.

  • 0 in reply to Zhiyong Li (FT)
    Guru 88580 points

    Per User Guide (SLAU335) Sec 7.2.5, running the WDT (with ACLK) prevents LPM4. Since you get to LPM4.5 by entering LPM4 (with REGOFF=1), I think that means you can't enter LPM4.5 in this case either.

  • 0 in reply to Bruce McKenney47378
    Expert 2460 points

    I think this is the most likely result. I will measure the actual current draw and report back when my customized PCB is back. Thanks.

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