MPR121 Touch Detection

[Mohit_Hirani]

Hello,

I am using MPR121 for my touch switchboard design. I have question with hatched ground shielded of circular touch pad. also i am using wifi module with mpr121 I2C.

my layout is like attached file which have 8 electrod and 4 led connected to MPR121 and both layer top and bottom both are fill with hatch ground 45 dgree.

track width of electrod is minimum as possible 0.1524 mm

touch pad size is 12mm and ground hatch and touch pad clearance is approx 1.23mm something.

and the spacing between electrod is littel more then 6mm or approx 7 mm something.

i just want to know this layout is ok for touch switch borad haveing 1mm or 2mm acrylic dielectric?

also i have other confusion can i use electrod 11,10,9,8 as a gpio and other electrod as a touch electrod for same application?

here below is my schematic and layout design.

[Mohit_Hirani]

Thanks Jozef for your valuable reply.

I just process my pcb for prototyping. if i have trouble with prototype i will back to you.

Thanks & Regarads,

Mohit Hirani

[JozefKozon · Accepted solution]

Hi Mohit Hirani,

I just want to inform you, that this layout is ok for touch switch board having 1mm or 2mm acrylic dielectric?

[A] Please refer to section 3.4 in an AN3863 attached. "A ground plane prevents coupling of external electromagnetic interference to the touch sensing electrodes, and acts as a shield for undesired electric fields. However, abusing the ground planes or simply filling open areas with ground planes might affect the sensor's sensitivity."

"For the electrodes room, an x-hatch pattern or no ground plane is recommended to avoid adding load capacitance to the capacitive electrodes. On the other hand, a 100% filled plane helps digital circuitry EMI and signal integrity. It is recommended to have digital and analog circuitry with these filled planes. The electrode wires can be considered as digital traces, therefore it is recommended to reference them to a completely filled ground plane during their way to the electrodes. Figure 12 shows an example of the two ground patterns existing in an application."

Also I have other confusion can I use electrodes 11,10,9,8 as a gpio and other electrodes as a touch electrode for same application?

[A] Do you mean the pins 11,10,9,8 or ELECTRODEs ELE8, ELE9, ELE10, ELE11?
"MPR121 has 12 input sensing channels ELE0~ELE11, which occupies pin 8 to pin 19. Among these, pin 12 to pin 19 are multifunction pins. When these multifunction pins are not configured as electrodes, they may be used to drive LED or for general GPIO purpose."

Pins 12,13,14,15,16,17,18,19 are multipurpose pins and can by set as GPIO. Please refer to section MULTIFUNCTION PINS in an AN3894 attached.
The number of touch sensing electrodes (and therefore the number of GPIO ports available) is configured by the Electrode Configuration register (0x5E) and GPIO Enable Register (0x77), but electrode configuration has higher priority than GPIO feature. Please refer to AN3894, AN3893 and MPR121 Touch Sensing Design Guidelines attached.

With Best Regards,

Jozef