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Problematic tuning MK20 with 4 Capacitive Key TSI module

Question asked by Luca Campanale on Jan 10, 2017
Latest reply on Feb 16, 2017 by Luca Campanale

Hi all,

I need the opinion of someone expert with TSI Module and capacitive key.

 

I have a board with with different devices including a MK20DX256VLK7 cpu and 4 capacitive keys connected to cpu using TSI Module. Key shape is a rectangular about 1.65(b)x1.2(h) cm and they are separated from each other with 0.2cm distance. Above each key is placed a copper tower about 0.4cm in order to reduce non-conducting strate and finally there is a plexiglass front panel about others 0.4cm. The follow figure shows the layout for one key.

Stack layout

The finger's distance is about 0.8cm.

 

I can successfully implement TSI Module with Keyboard control on Processor Expert both Kinetis Design Studio 3.1 and CodeWarrior 10.6, I can also use FreeMaster GUI.

In order to tune keys the parameter I can change are:

_Nsamples and Resolution to modify senisibility;

_Signal multiplier, divider, electrode senitivity to validate the touch.

 

The problem is the follow, in this enviroment changing Nsample or Resolution does not follow a logical behavior.

For example:

_with Resolution=14 bit and Nsample=6, all 4 Keys work but not good;

_with Resolution=14 bit and Nsample=7, 3 Keys work good and 1 Key does NOT work good, it seems deaf, freezed to finger although the  Status is OK;

_Changing again the Nsample, another key is freezed and often the previous one starts again to sense the finger.

 The same behavior appears with change Resolution, for a certain resolution all keys are sensing, for another one not all keys are working good.

 

Application note says that increasing Nsample improve sensibility. In my case it is false, increasing Nsample or Resolution, causes aleatory changing of sensiblity and baseline.

 

In this situation for me is impossibile to tune fine every sensors, besides I have to tune dozens of these boards. I would like to know, if it is an hardware layout problem (due to finger distance, pcb layout etc etc), or there is also a software component.

 

It is hard to explain well the problem, in case I can post FreeMaster screen, to shows this behavior.

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