MSP430FR2676: Scan Time Estimation value

Part Number: MSP430FR2676


My customer is evaluating their sensors with FR2676.
In CapTIvate Design Center GUI, "Scan Time Estimation" value is less than 10msec.
But when customer measured actual scan time on board, it takes ~15msec.
The time is measured for CAPT_MANAGER_UPDATE_SENSOR().

Customer understood CAPT_MANAGER_UPDATE_SENSOR() calls callback function, so the time used by CAPT_MANAGER_UPDATE_SENSOR() 
is not exactly equal to the scan time.
But the callback function customer uses takes less than 1msec. So still there is ~4msec difference.

Are there any reason why measured scan time is longer than "Scan Time Estimation"?

Thanks and regards,
Koichiro Tashiro

  • Hi Koichiro-san,

    The Scan Time Estimator only takes into account the conversion frequency and target conversion count value to estimate the time.  It does not consider the processing time and communications that occur in the CAPT_MANAGER_UPDATE_SENSOR().

    For example, if target conversion count = 500 and conversion frequency in the GUI is set = f/2, then conversion clock is 8MHz, but it takes 2 conversion clock cycles to perform one conversion, so the effective conversion rate is 4MHz.  The conversion rate period is 1/4MHz , or 250nsec, so the time it takes to measure one sensor is 250nsec * 500 (conversion counts) = 125 usec. This time will increase or decrease slightly, depending on if the sensor is self or mutual, when there is a finger touching.

    Keep in mind that this is the measurement time for just1 cycle.  The FR2676 has 4 measurement blocks, which are measured in parallel.  If there are 4 buttons and they are in the same "button group", and each button is connected to a different measurement block, then because they are measured in parallel it still only take 125usec to measure all 4 sensors.  If there are more than 4 sensors then there will be additional measurement cycles.  Each sensor group can have different target conversion counts, so that will also impact the total measurement time.

    Have the customer click the help button in the scan time estimator window for more details.

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