SNR value below to one when interface with PSoc 4 and Capsense Key

[Anonymous]

Hello Member,

I am new of this group. Currently we used PSoC 4 interfaced with Capsense key. In PSoc side we configured this pin like as capsense analog.

This scenario it will work some time and SNR value goes to above 20 and widget picture indicate touch / non touch correctly.

But some time issue occur and Widget working fine and SNR shows below 1. For concluding this i had split the connection between  PSoc and Capsense and find some noise in the PSoc Pin. Please  find the image. Is this happend when this pin configured as capsense ? or dont probe this pin as flating. Please confirm.

Note: When configured the PSoc pin as analog only  the noise doesn't appear. Capacitive sensing method used for self capacitance.

KRK

[Anonymous]

Can you provide a high level system overview and which chip you are using?

How are you measuring SNR?

When you probe a sensor, it is expected that the voltage of the sensor might not reach the full value. This is because your probe adds ~10pF cap to the sensor and the sensor clock frequency so high that it is not able to charge the sensor to full value.

I don't understand what you mean by "When configured the PSoC pin as analog only  the noise doesn't appear."

[Anonymous]

Thanks for your reply.

We used CY8C4045LQI-S412T - PSoC 4 chipset.  We used P4.2 as CSD Mode and P4.3 used as capsense key pad.

Capsense Key pad directly connected to P4.3 pin through 560 ohm resistor ( as per mentioned in the datasheet )

In this scenario, when touch the pad, it will work as shown in the PSOc Software capsense tuner ( Widget view + Validate SNR Tab ). Please find the image.

Same kind of test when touch the pad, the widget view will work and SNR value shows below to 1. In This time i have split the connection between PSoc and Touch pad and measure the PSOc pin ( Configured as Capsense Input ) shows some signal  as attached earlier.

Normally is  this happened in PSoc side when the pin is floating and configured as capsense input ?

[Anonymous · Accepted solution]

There is something wrong with your project configuration or the board.

If you look at the left side window, it says compensation IDAC is 0, which means your IDAC calibration failed. Because of this, your raw count (a measure of your sensor capacitance) is also incorrect. Ideally rawcount should be 85% of 2^Resolution. In your case, you are seeing a rawcount of 0 before touch and 115 counts when touched.

Can you attach your board schematic and layout file? It would help to understand if there is anything wrong with the board design.

Also, attach your project so that i can check if there is any incorrect setting.

I am assuming you are not using the CY8CKIT-041 development kit  because you are using P4.3 for a sensor (On the dev kit, there is a 470pF cap connected to this pin).

When you say "We used P4.2 as CSD Mode and P4.3 used as capsense key pad." do you mean you have connected CMOD capacitor to P4.2 and sensor to P4.3?

Why does your tuner show 14 sensors instead of just 1?

[Anonymous]

Thank you so much for your frequent reply.

This is customer project and confidential.

I am assuming you are not using the CY8CKIT-041 development kit  because you are using P4.3 for a sensor (On the dev kit, there is a 470pF cap connected to this pin).

>> Yes. We don,t used development kit. CY8C4045LQI-S412T - PSoC 4 chipset in our customer design.

When you say "We used P4.2 as CSD Mode and P4.3 used as capsense key pad." do you mean you have connected CMOD capacitor to P4.2 and sensor to P4.3?

>>Yes. 2.2nF connected P4.2 to ground and P4.3 connected to sensor.

Why does your tuner show 14 sensors instead of just 1?

>> Yes. We used 14 touch sensors in our project. But currently we have to verified single sensor for validation.

I need one more clarification. In Hardware design we used sensor pin  for P4.3. When this pin configured as capsense - analog input and left floating for validation ( ie no other connection to sensor ). What will happend? Is noise will occur due to capsense analog configuration? Is it. ?  Because we observed some kind of signal when floating the input analog pin.( Please find the 1st conversation attachment for your Inofrmation )

[Anonymous]

I need one more clarification. In this design customer used P4.2 used as CMOD and used 2.2nF capacitor connected with ground. We used 14 sensor for our application.

My question is the above mentioned capacitor value is varied to our application and is this value fixed one ? Whether this is varied one what is maximum range for the same. Please confirm.

[Anonymous]

The CMOD value is fixed to 2.2nF.

I want to see your project's CapSense settings.

Open CapSense project and click on Save Configuration button at the top. Attach the CapSense_Configuration.xml here. This will let me know what setting are used for CapSense.