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MSP430FR5043: Recommended configuration for XPBx External Bias Circuitry

Boone
Prodigy 185 points
Part Number: MSP430FR5043

Hello,

I'd like to use the external bias pin to set the bias on an external op amp, as in the gas evaluation kit.  In the reference design, there is a no pop option for using the external bias pin.  Could you please confirm the recommended circuit here.  See schematic snapshot below from the TI reference design:

I'm assuming to use the XPB0 option, we would populate R47 and depopulate R65.  Is that correct?  Is there anything else that should be done here?

Thanks in advance for the help!

  • +1
    TI__Mastermind 49692 points

    Hi Boone,

    Thanks for your post. For other readers, the schematic snippet above comes from page 3 of the schematic for the TIDM-02003 Ultrasonic sensing subsystem reference design for gas flow measurement reference design.

    Boone said:
    I'm assuming to use the XPB0 option, we would populate R47 and depopulate R65.  Is that correct?  Is there anything else that should be done here?

    You're on the right track. Looking at this circuit, R65 and R67 seems to be a very high impedance voltage divider to apply a VCC/2 bias to pin 3 of the op-amp. Thus, if you want to use another source for the bias voltage, you probably should remove R65 and R67.

  • 0 in reply to James Evans
    Prodigy 185 points

    Hi James,

    Thanks for the reply!

    What you've described makes sense.  A couple follow ons:

    1. I'm assuming the XPB0 pin would then be driven low when unused...  Just want to make sure this isn't floating.  Is there anything from a firmware perspective to change here?  Sorry for my ignorance on the fw functionality Slight smile

    2. Can you provide any guidance on the firmware change required to ensure that the XPB0 pin is fired up at the right time during the measurement sequence?

  • 0 in reply to Boone
    TI__Mastermind 49692 points
    Boone said:
    1. I'm assuming the XPB0 pin would then be driven low when unused...  Just want to make sure this isn't floating.  Is there anything from a firmware perspective to change here?  Sorry for my ignorance on the fw functionality

    You could keep the pull-down resistor to help with that or you could configure that pin as a GPIO and drive low when not using the XPB0 functionality.

    Based on Section 21.3.4 in the user's guide, SAPH_AICTL0.XPB0FEN must be set to enable the bias on XPB0. Setting SAPH_AICTL0.XPBSW0 turns it on. I also see a note that says HSPLL clock must be running for the control logic to work.

    Boone said:
    2. Can you provide any guidance on the firmware change required to ensure that the XPB0 pin is fired up at the right time during the measurement sequence?

    I don't have specific guidance on that but I would expect it should be done early to ensure everything stabilizes before any measurements are taken.

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