Member #314135

Member Since: April 2, 2012

Country: United States

  • Has anyone experienced fluctuation of the pin 11? I can't get a stable value from any of the 8 boards I have. I really believe Sparkfun's design of this board is to blame, since pin 11 routing is very close to the 3.3v one. I've tried with different I2C bus frequencies, from 20kHz to 400kHz, hoping this would change any noise inducted on the pin, but it didn't.

  • Just use a MOSFET for each of the MPR121 - 8051 lines and pull up resistors, like explained in NXP application note AN10441 (

  • Ah, and don't forget, in this specific board, to cut the jumper on the backside of the PCB that connects the ADDR pin to VSS.

  • It is actually 4 different addresses!

  • How wet? if it is just moisture, I believe it will work fine, but in the rain it certainly will not.

  • You can access the Electrode Filtered Data Register (0x04~0x1D), section 5.3 of the datasheet, to get a proportional value of the capacitance.

  • Hi. Yes, it makes sense, and I have done it already. MPR121 has one ADDR pin which can be connected to VCC, GND, SDA and SCL, each assigning a different I2C address. So, for example, if you connect eight MPR121 on the same I2C bus, you should connect all the SDA together, and all the SCL together too. Now you have eight ADDR pins, so, you can connect each of them to a different pin of your microcontroller. In the startup routine on the microcontroller, reset all the GPIO pins connected to the ADDRx. This will assign all the MPR121 the address 0x5A. This means you should never send a package to this address, otherwise all the ICs will try to respond at the same time, crashing your I2C bus. Instead, every time you want to communicate to an MPR121, you must set the corresponding ADDR pin, assigning address 0x5B only to that MPR121, then you send the I2C packages to address 0x5B, read whatever you need, and when the communication is over, it is necessary to restore the 0x5A address to that MPR121, by reseting the respective ADDR pin.

  • Hi. Refer to section 5 of the datasheet, where they explain one by one the registers of the MPR121, including all the built in filters, namely Baseline Value (0x1E~0x2A), Baseline Filter (0x2B~0x40), Touch/Release thresholds (0x41~0x5A), and Debounce (0x5B). I don't know what the application end is, but with based on my experience, I would suggest: 1) put the MPR121 as close as possible to the pads; 2) use the thinnest possible wire to connect the pads to MPR121; 3) Avoid any other wires close to the capacitive wires for a couple of inches, specially digital connections with high frequencies; 4) avoid any other conductor materials close to the capacitive wires.

  • I get a very clear voice sound, but there is a constant high frequency noise. Then I connected the probe directly to the output of the ADMP401 MEMS device, bypassing the OPA334 OpAmp, and all the noise vanished. Any suggestions without bypassing it?

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