What are CintA and CintB capacitors and why are they needed for CSX?

[JoJo_4453281]

Hello there,

I'm working on designing a touchpad using PSoC 4100s plus, and am just before the prototype design & development phase. I know that CintA and CintB are external capacitors at 470pF that are connected to pins P4[2] & P4[3] respectively, but I'm having trouble understanding conceptually why this is needed for CSX?

My understanding of how a CSX touchpad works is that rows/columns are scanned one at a time. This means electrical current comes out of a single GPIO (Tx electrode) the adjacent row/column receives the charge (Rx) and each row/column is done one by one and I get my raw counts for each intersection, or something like that! I'm not sure where in all of that, that a dedicated external capacitor (that is simply connected to ground in the schematics of the prototyping kits) fits into all of this.

Many thanks, this is probably a no brainer question

[Hari]

Hi JoJo_4453281​

The function of the external integrating capacitors, Cint, is mentioned in the CapSense design guide, under CapSense CSX Sensing method section. Adding a snippet of the same for you reference:

In short, the external capacitors are charged to an initial reference voltage (dependent on the VDDA supply). The charge is transferred from the Rx electrodes to the CInt capacitors (CintA for one half cycle and CintB for the other), and the increase/decrease in voltage levels of the Cint capacitors is detected as raw counts by the CapSense IP.

Best regards,
Hari

[JoJo_4453281]

Thanks AH! I'll be honest and say that I still don't understand why I need them or what exactly they're used for. I understand this portion of the document when it states a multiplexer is used to select Rx pins and feed them to the Current to Digital Converter though.

I still don't know where Cint fits into this explanation, but I'm guessing my lack of understanding is okay in this case? I'm not missing something big? All I need to know going forward is that the Cint capacitors out of those specific pins at 470 pF are a necessary part of the Current to Digital Converter to work?

[Hari · Accepted solution]

Hi JoJo_4453281​

That is correct. The Cint capacitors of 470pF are necessary and must be assigned to the specified ports.

The actual function, in simple terms, is that the Cint capacitors act as a charge bank. When sensing a particular sensor, the charge is taken from (or added to in the other half cycle) from the Cint capacitor. Since Q = C*V (equation of a charge stored in a capacitor) the change in charge causes a change in voltage and this is the change detected by CapSense. So, whenever a finger comes in contact with the sensor, this changes the Cm, therefore, the charge taken by the sensor. This further causes a change in the voltage of Cint, and hence, the raw counts.

Best regards,
Hari