PSOC 4 Trackpad Performance While Touching Battery

[alba_4439266]

Hello, 

I am designing a capacitive sensing trackpad using the CYBLE-214015-01 module. I notice that when I have my finger also touching the battery, the trackpad works great to control the cursor of my computer use the BLE HID Mouse Trackpad code provided by Cypress. Conversely, when I am not physically touching the battery, the trackpad works poorly. Do you have any insights on why this is the case? I have added a hatched ground plane to the front and back layer of the PCB as well as solid ground planes to the two middle layers of the  PCB. Originally, I thought it was a grounding issue, however, I have maximized the amount of ground planes I can add to my design. I also added the 560 ohm resistors to each of the CapSense pins.

Thank you.

[Hari · Accepted solution]

Hi @alba_4439266, 

The issue is due to the capacitive coupling between the device ground and the earth ground. Maximizing the ground plane is a solution. However, you can still tune the sensor so that you get good sensing.

1. Use Manual Tuning method instead of SmartSense. This will allow you to set the firmware thresholds and hardware resolution such that you get a good signal. 

2. Use Tuner. CapSense tuner is a GUI tool that you can use to visualize the CapSense performance. You can refer to any CapSense example for this, such as CE210709 – CapSense Linear Slider and Buttons. Note that you will need an I2C interface to communicate with the PSoC. This will help you understand the difference in signal level in both cases. 

Using the correct tuning parameters would fix the issue.

1. Observe the signal level that you are getting when you touch the sensors directly.

2. Go to the SNR tab in Tuner and make sure that you get atleast 5:1 SNR. 

3. Set the finger threshold parameter to 80% if the signal that you see.

4. Set the noise threshold to 40% of the signal level. 

Best regards, 
Hari

[alba_4439266]

Hello,

Thank you for the information. I used the Launch Tuner to view my signals today and found that my SNR is 6.91, so that should be sufficient for my application. Also, in the widget view, I can see that the cursor is moving well on the touchpad.

When I try to use the same parameters (as set for Launch Tuner, which evidently works well) also for the BLE HID cursor code, my cursor does not move at all. Do you have any insights on this? Why is it that these parameters control the Launch Tuner touchpad well, but not as a BLE HID cursor ?

Thank you again.

[Hari]

Hi @alba_4439266, 

The reason for this could be the extra coupling to ground that is formed when the I2C interface is connected to the touchpad. When the grounds are shorted, the coupling between earth ground and device ground increases. You can confirm this by simply leaving the I2C interface connected without actually running the Tuner.

You can tune the sensors to a higher SNR such as 10:1 to compensate for this effect as well. Since the cursor moves well with the Tuner connected, we can conclude that the hardware is correct and with tuning, the sensors would work as expected. 

Best regards, 
Hari

[alba_4439266]

Thank you very much for the information. I was able to successfully get the trackpad to work by tuning at a higher SNR. I appreciate all your help!