4 Replies Latest reply on Jul 30, 2019 1:56 AM by user_3696751

    Buttons's rawcout behavior with shield in wet application

    user_3696751

      Hello,

       

      I have mylar keyboard (2 layers) with some buttons of different sizes on top side and the shield on bottom side (all around the keys - no under the buttons).

      The keyboard is covered by a glass.

      The application is the remaking of a previous one already in operation and involves the replacement of the previous micro (CY8C21543) with a PSoC of the 4000S family (CY8C4045AZI-S413).

      I have a behavior that I find strange and not good:

      when I touch the shield the Raw-Conts of all the keys are raised (see Graph image). A phenomenon that is amplified in the presence of water on the glass.

      In the image below the configurations of the CapSense module (v6.0). These are the main information regarding the shield:

      • Inactive sensor connection: shield

      • IDAC discovery configuration: IDAC sourcing

      • Shield SW resistance: low EMI

      Through the Self-Test functions I exceeded the Cmod value and the key capacities that are:

      Shield capacity ~ 50 pF

      Capacity of keys 8 ÷ 11pF

       

      Is the behavior normal? If not, can you give me some suggestions on how to solve?

       

      Thanks

      Best regards

      Samuele

      Graph.jpgCSD Settings.jpg

        • 1. Re: Buttons's rawcout behavior with shield in wet application
          BragadeeshV_41

          Hi user_3696751,

           

          1. Are you observing this issue when you touch the bottom layer of the PCB? It is usually recommended to make sure that the bottom layer is not touched.

          2. Can you please probe the shield pin and check if it is charging and discharging correctly? Also check if it is in phase with the sensor waveform. If there is a phase shift, use the shield electrode delay to make shield waveform in phase with sensor waveform. If there is phase delay between the two waveforms, then touching the shield might give some high signal.

          3. Please check if there is any conductive object near the sensors. If they are there, please remove them.

          4. When the shield is touched, the buttons raw count will increase, but the noise threshold and finger threshold should be set in such a way that your sensor is not triggered when the shield is touched.

          5. Is it possible for you to share the schematic and layout file of your design for a review?

          6. Also, can you please check if you have placed the correct Cmod capacitor? 2.2nF

          7. Have you placed Csh capacitor, 10nF? If not, can you add that to your schematic and check if you still face this issue.

           

          Regards,

          Bragadeesh

          • 2. Re: Buttons's rawcout behavior with shield in wet application
            user_3696751

            Hi Bragadeesh,

            1. Are you observing this issue when you touch the bottom layer of the PCB? It is usually recommended to make sure that the bottom layer is not touched.

            the two layers, which are very close in mylar keyboard (70x200 mm) , are protected by a 6 mm glass. The keyboard are at 1 cm the pcb with the psoc and is connected by two flats 5 cm longer.

             

            2. Can you please probe the shield pin and check if it is charging and discharging correctly? Also check if it is in phase with the sensor waveform. If there is a phase shift, use the shield electrode delay to make shield waveform in phase with sensor waveform. If there is phase delay between the two waveforms, then touching the shield might give some high signal.

             

            yes, if Csh is enabled. We also tried lowering the sensor clock frequency. Comparing the signals of the shied and of the various sensors the phase shift is just over 300/400 pSec with the shield that is slightly ahead.

            The waveform is shown below.

            3. Please check if there is any conductive object near the sensors. If they are there, please remove them.

            The screws and other conductive parts are at the edges of the circuit far enough from the keys, as in the previous version.

            4. When the shield is touched, the buttons raw count will increase, but the noise threshold and finger threshold should be set in such a way that your sensor is not triggered when the shield is touched.

            The level is very high.

            5. Is it possible for you to share the schematic and layout file of your design for a review?

            would it be possible to have a confidential channel to send layout of mylar keyboard and board?Pins.jpg

            6. Also, can you please check if you have placed the correct Cmod capacitor? 2.2nF

            Yes we checked. The self test library funcion indicates 2.4nF for the Cmod and 10.2nF for the Ctank

            7. Have you placed Csh capacitor, 10nF? If not, can you add that to your schematic and check if you still face this issue.

            Yes, with csh the system works better but the problem remains

             

            Thanks

            Best Regards,

            Samuele

            • 3. Re: Buttons's rawcout behavior with shield in wet application
              BragadeeshV_41

              Hi,

               

              This looks like a cross talk issue between shield electrode and the CapSense sensors. What is the spacing between the CapSense sensor and the shield electrode? Also gap between shield trace to capsense trace.

              Also, I'm finding it difficult to understand stack up (different layers and overlay) of your design. Can you please send us a picture of your stack up?

              What is the signal you get for button "A" when you touch the button "A" vs when you touch the shield?

              Also your system seems oversensitive. Can you try decreasing the resolution and check the performance?

              It is recommended to use the max mod clock frequency as supported by your device.

               

              Regards,

              Bragadeesh

              • 4. Re: Buttons's rawcout behavior with shield in wet application
                user_3696751

                Hi Bragadeesh,

                I apologize for the late reply but I was absent,

                 

                The space between the keys and shield area is greater than 0.5mm. In any case the keyboard is the same as the existing one, only few icons change, and in the existing project it's working quite well.

                For the stack up: there are two layers, two maylar keyboards, superimposed.

                 

                The tests on other circuits gave us better results. We are verifying, but it would seem to be a problem related to the first camopioni of the keyboards.

                 

                I will update you to the outcome of the tests

                 

                Thank you and best regards

                Samuele