Why do I get a warning message when I select Sense clock source for CapSense® as PRS12 or PRS8, as Fig-1 shows?
Irrespective of the selection (PRS or SSC), the sense clock requires a longer time than Direct sense clock to finish an LFSR sequence.
For the PRS sense clock, the time to complete PRS(N) equals to (2N-1)/(Sense Clock Frequency). If the time calculated is longer than the sensor scan time, you will get the warning message as shown in Fig-1. At least one full-spread spectrum polynomial should be completed during the scan time to guarantee CapSense performance.
ModClock = 12 MHz
SenseClock (Average) = 300 kHz
Scan Resolution = 14 bits
Sensor Scan Time (TSCAN) = (214-1)/ 12 MHz = 1365.25 µs
Time to complete PRS8 (TPRS8) = (28-1)/ 300 kHz = 850 µs
Time to complete PRS12 (TPRS12) = (212-1)/ 300 kHz = 13650 µs
Because TSCAN < TPRS12 and TSCAN > TPRS8, you can choose PRS8 as the sense clock source for the CapSense Component without any warning, but will receive warning message if you choose PRS12. It is recommended to use Auto mode to ensure that the best clock source is selected for the given configuration.