1 2 Previous Next 16 Replies Latest reply on Aug 21, 2019 10:05 PM by AH_96

    PSoC5LP CapSense "Scan Speed" definition

    MaMi_1205306

      Hi,

       

      The PSoCLP CapSense component datasheet includes the following:


      We understand the relationship between scan speed and resolution scan speed and resolution.

       

      For example,

      CapSense input clock : 24MHzScan Speed : Fast

       

      Divides the component input clock by 2 = 12MHz

       

      What does this frequency indicate?
      Is it a digital circuit clock?

      Is this clock is "Clock from UDB"?

       

      Regards,

        • 1. Re: PSoC5LP CapSense "Scan Speed" definition
          AH_96

          Hello MaMi_1205306

           

          This clock frequency indicates the modulator clock frequency that is provided to the counter. The scan time of a widget is calculated as (2^N - 1)*mod_clk_frequency, where N is the resolution of the widget.

          This is a digital clock and is derived from MASTER_CLK.

          You can check the Clocks tab in design wide resources for more details on the clock source.

           

          Thanks and regards

          Harigovind

          • 2. Re: PSoC5LP CapSense "Scan Speed" definition
            MaMi_1205306

            Thank you for your reply.

             

            We have calculated the formula you have been taught.

             

            (2^N - 1)*mod_clk_frequency

             

            For example,

            Resolution : 13bit

            Master clock : 48MHz

            Scan clock input for Clock settings : 24MHz

            Mod clk frequency : fast = 24MHz / 4 = 6MHz

             

            Scan time = ( 2^13 - 1 ) * 24MHz / 4 = 4.9 * 10^10

             

            If the frequency is calculated in terms of time,

            6MHz = 167ns,

             

            Scan time= 1.37ms

             

            However, the PSoC Creator display is 1.405ms,
            It is 1400us (1.400ms) in the data sheet.

             

            What is the effect of this difference?

             

            Regards,

            • 3. Re: PSoC5LP CapSense "Scan Speed" definition
              AH_96

              Hello MaMi_1205306

               

              The scan time shown in this tab includes scanning time and estimated setup and pre-processing time. If you check, the difference between the calculated value and the value shown in the tab is 0.041ms. This is the estimated setup and pre-processing time of the sensor.

               

              Please note that this value is the approximate sensor scan time and the actual value might be slightly different. This is given in the "Total Scan Time" section in the component datasheet.

               

              Do let us know in case of any further queries.

               

              Thanks and regards
              Harigovind

              • 4. Re: PSoC5LP CapSense "Scan Speed" definition
                MaMi_1205306

                Harigovind-san,

                 

                It turns out that the scan time is determined by the speed of the Mod frequency.
                I thought that the RAW count would not change even if the scan time changed.

                 

                Scan Speed : Raw count
                NOMAL : 6612
                FAST : 6537

                The RAW count value changes as described above.
                Do you understand this factor?

                Is setup and pre-processing time affected?

                 

                Regards,

                • 5. Re: PSoC5LP CapSense "Scan Speed" definition
                  AH_96

                  Hi MaMi_1205306

                   

                  The Mod clock is the input to the counter which actually produces the count value that is interpreted as raw counts. Therefore, changing the mod cock frequency will result in a difference in raw counts.

                   

                  The setup and pre-processing time are generally not affected. This can be observed as the difference in theoretical calculated value of scan time and the scan time shown in component configuration window remains same even when scan speed is varied. However, the actual time might be slightly different based on factors such as Cp.

                   

                  Thanks and regards

                  Harigovind

                  • 6. Re: PSoC5LP CapSense "Scan Speed" definition
                    MaMi_1205306

                     

                    Faster Mod clock shortens scan time,
                    Is it correct to understand that the raw count value will decrease?

                    Because we have large fluctuations in power
                    I would like to take measures by slowing down the scan time.
                    However, I don't understand the relationship between scan time and raw count.

                     

                    1. Power on sysytem
                    2. Initialize the baseline
                    3. Long press the Caos sensor
                    4. Release finger from the Caps sensor

                    The RAW count does not return to the "2. Initialize the baseline" because the power supply voltage has dropped when the finger is released.
                    Since the DIFF value (= RAW - Baseline) is large, there is a problem that the baseline does not follow.

                    Tuner Helper has no problem, but Bridge Contorl Panel can confirm the problem.
                    We understand that the longer the scan time, the less susceptible to power fluctuations.

                    • 7. Re: PSoC5LP CapSense "Scan Speed" definition
                      AH_96

                      Hi MaMi_1205306

                       

                      Raw counts are not directly related to mod clock frequency. Raw counts are proportional to the resolution, sense clock frequency, parasitic capacitance of the sensor, reference voltage and inversely proportional to Imod.

                       

                      The scan time is calculated as the time required for the counter to reach the maximum counts (2^N -1) with a particular mod clock frequency. Therefore, while there will be slight variations in raw counts when mod clock is changed, we cannot establish a direct relationship.

                       

                      In the CSD component, there is an option for sensor auto reset (in advanced tab). This causes the baseline to always update. Please note that this limits the long press duration but solves the problem of high raw counts due to power fluctuations.

                       

                      Thanks and regards
                      Harigovind

                      • 8. Re: PSoC5LP CapSense "Scan Speed" definition
                        MaMi_1205306

                        Harigovind-san,

                         

                        Auto reset cannot be used because there is a mode to use by pressing and holding the button.

                        The above description is in the data sheet.

                         

                        It has been confirmed that if the scan speed is delayed, the raw count error due to the voltage fluctuation decreases.

                        Do you have a document that shows the relationship between scan speed and voltage fluctuation(IDAC)?

                         

                        Regards,

                        • 9. Re: PSoC5LP CapSense "Scan Speed" definition
                          MaMi_1205306

                          When reading various documents, we understand that "Scan Speed" is the discharge time.

                          We thought that the raw count would change greatly because the slew rate of the voltage drop during discharge was the same.

                           

                           

                          If it is the same period, the count of FAST will increase,
                          However, since the counting time of FAST is shortened, the count values are almost the same FAST and NOMAL..

                          Because it is "resolution - count of counter", The FAST and NOMAL raw counts are similar.

                           

                          When the power supply voltage is high, the IDAC current will increase and the RAW count decrease.
                          When the power supply voltage is low, the IDAC current decreases and the RAW count increases.

                          FAST has low voltage amplitude. In other words, it is susceptible to changes in IDAC.
                          NOMAL has a large voltage amplitude. Therefore, it is hard to be affected by changes in IDAC.

                          Is it correct with the above recognition?

                          • 10. Re: PSoC5LP CapSense "Scan Speed" definition
                            AH_96

                            Hello MaMi_1205306

                             

                            As long as the sensor charges and discharges completely (which is a requirement for correct operation of the device), the Cmod will be discharged to the same level. And moreover, the Cmod charging and discharging is determined by the sense clock frequency(not mod clock) and this is decided by the analog switch divider value that is set.

                             

                            When the mod clock is set to a low value, the scan time increases and results in lower peak to peak noise. This is the reasoning behind the statement given in the component datasheet.

                             

                            For the issue that you are facing, you can try to increase the noise threshold so that the baseline tracks even for slight change in the raw counts.

                             

                            Please let me know the difference counts for a button press and the difference counts during the power change.

                             

                            Thanks and regards

                            Harigovind

                            • 11. Re: PSoC5LP CapSense "Scan Speed" definition
                              MaMi_1205306

                              Harigovind-san,

                               

                              /* Scan Speed : Fast */

                              When the sensor is not pressed.

                              6536 at 4.7V

                              6357 at 5.0V

                              Raw count difference : 6536 - 6357 = 179

                               

                              /* Scan Speed : Normal */

                              When the sensor is not pressed

                              6708 at 4.7V

                              6612 at 5.0V

                              Raw count difference : 6709 - 6612 = 97

                               

                              For example,

                              It is initialized at 5V in fast of scan speed, baseline and raw count are 6357.

                              Press the sensor, raw count is up and baseline is stop.

                              Take finger off the sensor when the voltage is 4.7V, raw count is 6536,

                              However baseline is 6357.

                              In other words, the noise threshold must be set to 180 for Fast of scan speed.

                               

                              > When the mod clock is set to a low value,

                              > the scan time increases and results in lower peak to peak noise.

                              > This is the reasoning behind the statement given in the component datasheet.

                               

                              I think the reason you mentioned above is the reason why you are not affected by noise,

                              but we would like to know the cause and effect of fluctuations in power supply voltage.

                               

                              (a )As the scan speed decreases, the scan time increases, so the charge and discharge times increase.

                              => We understand the above.

                               

                              (b) IDAC current changes when power supply voltage changes.

                              => CapSense component data sheet also shows.

                              (c) Since the charge / discharge time becomes longer, it was considered less susceptible to changes in IDAC current.


                              This is shown in the previous diagram. Is this correct recognition?

                               

                              Regards,

                              • 12. Re: PSoC5LP CapSense "Scan Speed" definition
                                AH_96

                                Hello MaMi_1205306

                                 

                                (a)  The charge and discharge time of the sensor (and Cmod) is not determined by scan speed. It is determined by Sense Clock Frequency. This parameter is set by choosing the Analog switch divider.

                                 

                                (b) Yes This is the correct understanding. It is the fluctuation in IDAC that appears as noise and results in incorrect raw counts.

                                 

                                (c) There will be more charge discharge cycles when the mod clock is set to low frequency. Hence the noise will average out and will result in lower peak to peak noise. The charge and discharge time (per cycle) will still remain same as long as the sense clock frequency is constant.

                                 

                                Thanks and regards

                                Harigovind

                                • 13. Re: PSoC5LP CapSense "Scan Speed" definition
                                  MaMi_1205306

                                  I have new questions in your explanation.

                                  The charge / discharge of the sensor ( Cmod ) is a sense clock, which is a fixed value of  ( Component Clock / Analog switch Divider ).
                                  The scan speed is mod Clock, which is the component clock divided by each mode.
                                  Of course, the component clock is constant.

                                  It can be understood from the data sheet that the scan time changes when the scan speed is changed.

                                   

                                  Assuming that one charge / discharge cycle is the same in the sense clock, If the scan time simply doubles, does the counter value also double?

                                  It has been answered that the raw count value does not affect the mod clock.

                                   

                                  Could you tell us more about this relationship?

                                  • 14. Re: PSoC5LP CapSense "Scan Speed" definition
                                    MaMi_1205306

                                    In your description below,

                                     

                                    (c) There will be more charge discharge cycles when the mod clock is set to low frequency. Hence the noise will average out and will result in lower peak to peak noise. The charge and discharge time (per cycle) will still remain same as long as the sense clock frequency is constant.

                                     

                                    "There will be more charge discharge cycles when the mod clock is set to low frequency"

                                    "The charge and discharge time (per cycle) will still remain same as long as the sense clock frequency is constant"

                                     

                                    We don't understand the two opposite meanings.

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