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MSP-CAPT-FR2633: CAPTIVATE Sensor Conversion Time

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Replies: 6

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Part Number: MSP-CAPT-FR2633

Hi there!

I'm trying to understand the total sensor conversion time calculation.

When I click help in the Design Center GUI under Scan Time Estimate tab of the controller properties, it says "As shown in the formula below" but there is no formula.

When I try to apply a formula from the Design Guide section Optimizing for the Shortest Measurement Time, it gives me wrong values (I have a feeling that there is an error in EQN2 and it should be ChargeTransferPeriond[f]*(ConversionCount+STABLETIME)).

Could you, please, suggest what is the correct way to calculate Scan Time for the sensor?

  • Hi Andrei!

    Thanks for posting.  The formula contained in the Design Guide that you reference is correct.  I compared results from the formulas with the CapTIvate Design Center implementation, and it looks like there is a bug in the built-in time estimator tool.  From what I can deduce by manipulating values, it is not correctly incorporating the stabilization time.  It's using the divided CAPOSC frequency to compute the stabilization time, when the undivided CAPOSC frequency is what is actually applied in the device.  This is why EQN2 adds the quotient of the stabilization time (in CAPOSC cycles) over the undivided CAPOSC frequency.

    Here is an example:

    CAPOSC Frequency: 16 MHz (default)
    CAPOSC Output Frequency Divider: /8
    Charge Phase Length: 1 divided CAPOSC cycle
    Transfer Phase Length: 1 divided CAPOSC cycle
    Resulting Charge Transfer Frequency (measurable at the electrode): 1 MHz
    Conversion Count: 100
    Conversion Time (without stabilization): 100 microseconds (100 charge transfers at 1MHz rate)
    Stabilization Time: 20 microseconds (320 cycles at 16 MHz rate)
    Total Time: 120 microseconds (conversion time + stabilization time)

    Please let me know if this makes sense.  The confusion is around the stabilization time and how it is applied.  It's in CAPOSC cycles, not charge transfer cycles.  With short conversions (100 counts at high charge transfer frequency) the stabilization time can be significant.  With long conversions (say, 500 counts at a low charge transfer frequency) the stabilization time becomes small relative to the total time.

    You are correct in your identification of an inconsistency between the GUI tool and the documentation.  The documentation is correct.  I will file a bug report on our end for the CapTIvate Design Center's implementation.

    Regards,
    Walter

  • In reply to Walter Schnoor:

    I should also note that the scan time is somewhat non-deterministic at runtime. Because the number of charge transfers is a function of the external capacitance being measured, the scan time is also a function of the capacitance. In self-mode, a touch decreases the measurement time (higher capacitance, fewer counts). In mutual-mode, a touch increases the measurement time (lower capacitance, more counts). Keep this in mind when using the estimated times to set up an application.

    Walter

  • In reply to Walter Schnoor:

    Hi Walter,

    Yes, it makes sense, I think now it's clear to me. Thanks a lot!

    P.S. I still think that there is a little mistake in EQN2: CAPOSC[f] and STABLETIME should be interchanged in the fraction.
  • In reply to Andrejs Gasilovs:

    Andrei Gasilovs
    Hi Walter,

    P.S. I still think that there is a little mistake in EQN2: CAPOSC[f] and STABLETIME should be interchanged in the fraction.

    Good catch again.  It should be displayed as 1/F * STABLETIME.  We'll get it corrected.

    Thanks,
    Walter

  • In reply to Walter Schnoor:

    Hi Walter,

    Thank you once again!

    I have noted some other mistakes/typos in the design guide and design center. If you have some specific place or tracker (or at least an e-mail) I would gladly share them with TI team.

    I also have some other questions regarding the layout but will open a new thread for them.

    P.S.

    In the BOOSTXL-CAPKEYPAD tool page the design files archive actually includes the design files for CAPTIVATE-PGMR board and not for the booster pack.

  • In reply to Andrejs Gasilovs:

    Andrei,

    We definitely want your feedback on the ecosystem.  If you prefer to work with me directly I can reach out to you via your myTI account and we can get any issues filed accordingly to be updated in future releases, which we tend to do quarterly.  Please also feel free to post any concerns to the forum as well if you feel that is appropriate.

    Regards,
    Walter

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