Changing resolution of CapSense proximity sensor in MBR3 does not decrease scan time - KBA229990

Version 2

    Version: **

     

    Translation - Japanese: MBR3のCapSense 近接センサーの分解能を変更してもスキャン時間が短縮されない - KBA229990 - Community Translated (JA)

     

    Question:

    Why is the scan time for CapSense proximity sensor not affected when the resolution is changed in MBR3?

     

    Answer:

    MBR3 allows setting the resolution for proximity sensors (register 0x2e and 0x2f). Resolution values from 12 bits to 16 bits can be set. While decreasing resolution results in decreased sensing distance, the scan time remains constant.

    This is because the SmartSense algorithm, which tunes the sensors, always sets proximity resolution to the maximum value (16 bits) for best sensing distance. The resolution set in the registers will only modify the raw counts that are obtained after scanning. The raw counts are modified, that is, right shifted, to make it compatible with the resolution value set. This means, if the resolution set in the registers is 13 bits, the difference is 3, (16 – 13 = 3) and the new raw count = raw counts >>3.

    Hence, raw count is modified as:

    Raw Counts = (Raw Counts >> n), where n = 16 – (resolution set)

     

    To decrease the scan time of the sensors, the proximity sensors can be configured as button widgets in MBR3. The sensitivity parameter, registers 0x08, 0x09, 0x0a, and 0x0b can be modified to suit the sensing distance and scan time. Setting sensitivity parameter to 0x0, 50 counts/0.1 pF will result in the best sensing distance and highest scan time while setting it to 0x3, 50 counts/0.4 pF will result in reduced sensing distance and low scan time. The sensitivity is adjusted by changing the resolution in the case of buttons.

    One limitation of configuring buttons as proximity sensors to reduce scan time is that the Look-for-proximity mode cannot be implemented. After MBR3 exits active mode, it will stay in Look-for-touch mode. However, the device will switch to Active mode as soon as there is a presence of a hand in the proximity sensing region for faster response.