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PMIC (TPS65381-Q1) and MCU (RM48L950)

R11
Expert 1485 points
Other Parts Discussed in Thread: RM48L950, TPS65381-Q1

I am a bit confused on what to do in terms of an external pull resistor on the ERROR/WDI and nERROR pins of the PMIC (TPS65381-Q1) and MCU (RM48L950). The TPS65381-Q1 datasheet describes two modes of operation: TMS570 mode and PWM mode. If using TMS570 mode, it describes an error condition being detected when the ERROR/WDI pin remains low for a programmed amount of time. This almost makes it sound like a pull-up should be used.

The RM48L950 datasheet shows the default pin state for nERROR is a 20uA pull-down, which contradicts my assumption above. Lastly, the RM48 HDK schematic on Page 7 shows a 1K pull-up on nERROR through an LED, which seems like it would support my initial assumption but contradict the default pull-down state for the nERROR pin. So, if adding an external pull device, should it be pull-up or pull-down…or could it be either and why?

  • 0
    TI__Intellectual 1480 points

    Rustin,

    The reason why we have an pull down on the nERROR pin for the MCU  is to indicate to the TPS than an error occured if the the MCU gets unintentionally  powerdown.

     The reason we have an 1K pull-up on nERROR through an LED is to switch it on when the  nError is pulled down .

     

    -Hercules forum supoort

  • 0 in reply to Hari Pendurty
    Expert 1485 points

     We do not have an LED, so what would be appropriate for our application?  I would expect we should either leave no connect or parallel an external pull-down to support the condition mentioned regarding the MCU getting unintentionally powered-down.  Is this correct?

    On the HDK, doesn’t the 1K pull-up through LED effectively counteract the reason for the internal nERROR pull-down?

  • 0 in reply to R11
    Expert 1485 points

    Any update on the question above?

  • 0 in reply to R11
    TI__Guru 59540 points

    Hello Rustin,

    I apologize for the lag in response to your question. The nERROR pin is driven high as long as no error has been identified in the ESM. This means that the pin will by high after power on reset as long as there were no error conditions detected during power up. When an error condition is detected, the nERROR pin is driven low for the time specified by the ESMLTCPR register. More information on the LTCP can be found in the ESM chapter of the TRM.

    So, to answer your question, if you would like to make use of the ERROR pin to signal the TPS device in case of an error, then it is not recommended to place anything other than a pull down on the pin since a loss of MCU power or pin failure would result in the pull down asserting nERROR. If a pull up is used, then there would be notification to the TPS in the event of a power/pin failure on the MCU.

  • 0 in reply to Chuck Davenport
    Expert 1485 points

    I have a question about the last paragraph you wrote... should the last sentence say “If a pull-up is used, then there would be no notification to the TPS in the event of a power/pin failure on the MCU.”?  Just want to verify…

  • 0 in reply to R11
    TI__Guru 59540 points

    Hello Rustin,

    Yes, you are correct. It seems I dropped a key word there. If a pullup is placed on nERROR the output would be pulled high and the TPS65381 would not be able to recognize the failure in the MCU. My apologies for the mix up.