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MPR121 capacitive touch tuning for baseline filtering
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MPR121 capacitive touch tuning for baseline filtering

by omi on Mon Nov 09, 2020 1:07 pm

Hi,

I am trying to tune the MPR121 12 output board for a specific use case. I have tuned the touch sensitivity well and have no issues there. I changed MPR121_CONFIG1, and MPR121_CONFIG2 to 0x3F and 0X3A as these values work well for me.

What I am having issues with is when someone touches the sensor (through a leather sheet) in a gradual slow manner. What I noticed is that the baseline voltage measurement starts gradually going down along with the actual touch measurement. So after the touch is at full pressure, both the touch and baseline signals are parallel. I used the data sheet here on page 7.
https://www.sparkfun.com/datasheets/Com ... MPR121.pdf

Can someone from Adafruit, give me some understanding of the Baseline Filtering Control Register (0x2B~0x40) which will allow me to reduce (?) the speed at which the baseline voltage is averaged? Which register can help me on my specific issue.
omi
 
Posts: 2
Joined: Tue Aug 20, 2013 2:00 am

Re: MPR121 capacitive touch tuning for baseline filtering

by siddacious on Wed Nov 11, 2020 12:17 am

From what I can tell, the FDL bitfield within the baseline filtering control registers is responsible for setting the update rate of the baseline filter. That said, there is going to be a notable amount of interplay between this value and the other baseline filter configuration variables.

The datasheet references this application note for configuring the baseline system:
https://www.nxp.com/docs/en/application-note/AN3891.pdf

Reading that will likely help narrow down the configuration options.

siddacious
 
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Joined: Fri Apr 21, 2017 3:09 pm

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