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How do I wake up on proximity with the FD2x1x parts?

John Peck
Prodigy 130 points
Other Parts Discussed in Thread: FDC2112, FDC2114, FDC2214

I'd like for the FDC2112 to wake up my system when a hand approaches it.  The application note SNOA940 suggests that this is possible, though the datasheet only talks about asserting the interrupt pin when the status register changes -- when new data is ready.  Is there a way to set a threshold in the FDC2112 so that it only asserts its interrupt pin when there's a minimum capacitance level detected?  

  • 0
    TI__Expert 8050 points

    Hello John,

    There is a trick to achieve this feature. You can specify an "offset" to be subtracted from the output data. If the net value is negative, it is an error and can trigger the interrupt. By tuning the offset, you can essentially set the threshold.

    Regards,

    Yibo

  • 0 in reply to Yibo Yu
    Prodigy 130 points

    Interesting.  It sounds like this would only apply to the FDC2112 and FDC2114 parts, since these are the parts with offset registers.  Is this correct?

    Once you generate negative data, does this trigger the CH0_WD_ERR flag (for channel 0)?  If it does, it looks like I could fire the interrupt by setting the WD_ERR2INT bit.  Does this make sense?

  • 0 in reply to John Peck
    TI__Expert 8050 points
    Sorry I only gave you half of the answer. You actually need to use some reserved bits.
  • 0 in reply to Yibo Yu
    TI__Expert 8050 points
    There is an under-range error flag in 0x00, DATA_CH0 [15], similarly for CH1 in DATA_CH1 [15]. In addition, register 0x18, STATUS [13] also flags an under-range error for an active channel. Register 0x19, ERROR_CONFIG [7] sets the under-range error trigger to INTB.

    You need to experiment with your sensor setup such that it gives you enough dynamic range to make use of this threshold feature. Sensor design is also important to prevent false triggering due to various environmental factors.

    Regards,
    Yibo
  • 0 in reply to Yibo Yu
    Prodigy 130 points

    Yibo -- thanks for the detailed response.  This will not work for the FDC221x parts right?  Without an offset setting, I don't see how they'd generate an under-range error.  The eval board I have has a FDC2214 on it though, and I'd have to come up with another way to talk to a FDC2112 in order to test this.

  • 0 in reply to John Peck
    TI__Expert 8050 points
    Hi John,

    That's right. This method only works for FDC211x.

    Regards,
    Yibo