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Thread: mpr121 sensor test code won't work

  1. #1
    Junior Member
    Join Date
    Dec 2017

    mpr121 sensor test code won't work

    I've downloaded an arduino compatible library for the MPR121 sensor. This sensor comunicates using i2c protocol. Here are pictures of my wiring:

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    I have modified the code a bit compared to the original code where I downloaded this from. I changed the i2c pins to the pins 37 and 38, and I added a "Serial.println("hello world");" in the setup function.
    The problem I am having is that nothing is being printed in the console, not even the hello world i printed in the setup function. here is the arduino code:

    #include "mpr121.h"
    #include <Wire.h>
    int irqpin = 2;  // Digital 2
    int SDApin = 38;
    int SCLpin = 37;
    boolean touchStates[12]; //to keep track of the previous touch states
    void setup(){
      pinMode(irqpin, INPUT);
      digitalWrite(irqpin, HIGH); //enable pullup resistor
      Serial.println("hello world");
    void loop(){
    void readTouchInputs(){
        //read the touch state from the MPR121
        byte LSB =;
        byte MSB =;
        uint16_t touched = ((MSB << 8) | LSB); //16bits that make up the touch states
        for (int i=0; i < 12; i++){  // Check what electrodes were pressed
          if(touched & (1<<i)){
            if(touchStates[i] == 0){
              //pin i was just touched
              Serial.print("pin ");
              Serial.println(" was just touched");
            }else if(touchStates[i] == 1){
              //pin i is still being touched
            touchStates[i] = 1;      
            if(touchStates[i] == 1){
              Serial.print("pin ");
              Serial.println(" is no longer being touched");
              //pin i is no longer being touched
            touchStates[i] = 0;
    void mpr121_setup(void){
      set_register(0x5A, ELE_CFG, 0x00); 
      // Section A - Controls filtering when data is > baseline.
      set_register(0x5A, MHD_R, 0x01);
      set_register(0x5A, NHD_R, 0x01);
      set_register(0x5A, NCL_R, 0x00);
      set_register(0x5A, FDL_R, 0x00);
      // Section B - Controls filtering when data is < baseline.
      set_register(0x5A, MHD_F, 0x01);
      set_register(0x5A, NHD_F, 0x01);
      set_register(0x5A, NCL_F, 0xFF);
      set_register(0x5A, FDL_F, 0x02);
      // Section C - Sets touch and release thresholds for each electrode
      set_register(0x5A, ELE0_T, TOU_THRESH);
      set_register(0x5A, ELE0_R, REL_THRESH);
      set_register(0x5A, ELE1_T, TOU_THRESH);
      set_register(0x5A, ELE1_R, REL_THRESH);
      set_register(0x5A, ELE2_T, TOU_THRESH);
      set_register(0x5A, ELE2_R, REL_THRESH);
      set_register(0x5A, ELE3_T, TOU_THRESH);
      set_register(0x5A, ELE3_R, REL_THRESH);
      set_register(0x5A, ELE4_T, TOU_THRESH);
      set_register(0x5A, ELE4_R, REL_THRESH);
      set_register(0x5A, ELE5_T, TOU_THRESH);
      set_register(0x5A, ELE5_R, REL_THRESH);
      set_register(0x5A, ELE6_T, TOU_THRESH);
      set_register(0x5A, ELE6_R, REL_THRESH);
      set_register(0x5A, ELE7_T, TOU_THRESH);
      set_register(0x5A, ELE7_R, REL_THRESH);
      set_register(0x5A, ELE8_T, TOU_THRESH);
      set_register(0x5A, ELE8_R, REL_THRESH);
      set_register(0x5A, ELE9_T, TOU_THRESH);
      set_register(0x5A, ELE9_R, REL_THRESH);
      set_register(0x5A, ELE10_T, TOU_THRESH);
      set_register(0x5A, ELE10_R, REL_THRESH);
      set_register(0x5A, ELE11_T, TOU_THRESH);
      set_register(0x5A, ELE11_R, REL_THRESH);
      // Section D
      // Set the Filter Configuration
      // Set ESI2
      set_register(0x5A, FIL_CFG, 0x04);
      // Section E
      // Electrode Configuration
      // Set ELE_CFG to 0x00 to return to standby mode
      set_register(0x5A, ELE_CFG, 0x0C);  // Enables all 12 Electrodes
      // Section F
      // Enable Auto Config and auto Reconfig
      /*set_register(0x5A, ATO_CFG0, 0x0B);
      set_register(0x5A, ATO_CFGU, 0xC9);  // USL = (Vdd-0.7)/vdd*256 = 0xC9 @3.3V   set_register(0x5A, ATO_CFGL, 0x82);  // LSL = 0.65*USL = 0x82 @3.3V
      set_register(0x5A, ATO_CFGT, 0xB5);*/  // Target = 0.9*USL = 0xB5 @3.3V
      set_register(0x5A, ELE_CFG, 0x0C);
    boolean checkInterrupt(void){
      return digitalRead(irqpin);
    void set_register(int address, unsigned char r, unsigned char v){
    and here is the contents of MPR121.h:

    // MPR121 Register Defines
    #define MHD_R	0x2B
    #define NHD_R	0x2C
    #define	NCL_R 	0x2D
    #define	FDL_R	0x2E
    #define	MHD_F	0x2F
    #define	NHD_F	0x30
    #define	NCL_F	0x31
    #define	FDL_F	0x32
    #define	ELE0_T	0x41
    #define	ELE0_R	0x42
    #define	ELE1_T	0x43
    #define	ELE1_R	0x44
    #define	ELE2_T	0x45
    #define	ELE2_R	0x46
    #define	ELE3_T	0x47
    #define	ELE3_R	0x48
    #define	ELE4_T	0x49
    #define	ELE4_R	0x4A
    #define	ELE5_T	0x4B
    #define	ELE5_R	0x4C
    #define	ELE6_T	0x4D
    #define	ELE6_R	0x4E
    #define	ELE7_T	0x4F
    #define	ELE7_R	0x50
    #define	ELE8_T	0x51
    #define	ELE8_R	0x52
    #define	ELE9_T	0x53
    #define	ELE9_R	0x54
    #define	ELE10_T	0x55
    #define	ELE10_R	0x56
    #define	ELE11_T	0x57
    #define	ELE11_R	0x58
    #define	FIL_CFG	0x5D
    #define	ELE_CFG	0x5E
    #define GPIO_CTRL0	0x73
    #define	GPIO_CTRL1	0x74
    #define GPIO_DATA	0x75
    #define	GPIO_DIR	0x76
    #define	GPIO_EN		0x77
    #define	GPIO_SET	0x78
    #define	GPIO_CLEAR	0x79
    #define	GPIO_TOGGLE	0x7A
    #define	ATO_CFG0	0x7B
    #define	ATO_CFGU	0x7D
    #define	ATO_CFGL	0x7E
    #define	ATO_CFGT	0x7F
    // Global Constants
    #define TOU_THRESH	0x06
    #define	REL_THRESH	0x0A
    Any ideas of what I can try next would be apreciated.

  2. #2
    Senior Member
    Join Date
    Nov 2015
    Cold hollow VT
    Might help to put a delay() after the serial begin ir use 'while(serial.begin(9600))' to let it connect with your windows (?) port.

    Then sprinkle a few more serial.println() statements thru the code to find where it it getting hung up.

  3. #3
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Pins 37 & 38 are SCL1 & SDA1. That means you need to use "Wire1" rather than "Wire" to access them.

    I'm not familiar with this particular library (and I don't see any link to its code), but if it is designed like most Arduino libs, it likely has "Wire" written throughout its code. You would need to edit that code to change every "Wire" to "Wire1". Some newer sensor libraries offer a way to configure which Wire instance they use, but those are relatively rate. The vast majority of sensor libs are hard-coded to use only Wire, since most Arduino boards offer only a single Wire port.

    Instead of Wire1, you could use pins 33 & 34, or pins 16 & 17, or 18 & 19. Those are all SDA0 & SCL0, meaning they are controlled by "Wire". I'm pretty sure you'll find everything works fine if you use those pins.

  4. #4
    Senior Member+ defragster's Avatar
    Join Date
    Feb 2015
    It may work to just put :
    //#include "Wire.h"
    #include <i2c_t3.h>
    #define Wire Wire1
    In the library header file - that worked for when for an adafruit display.

  5. #5
    Junior Member
    Join Date
    Dec 2017
    Thanks Paul, I changed every "Wire" to "Wire1" and it worked. But I don't understand the reason why there are a setSDA and setSCL functions when you just need to use Wire1, Wire2, or Wire3... kinda confusing...

  6. #6
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Yeah, these chips are packed with tons of features. There are 4 separate I2C ports. Any feedback how we could make the card easier to understand and still show the info?

  7. #7
    Junior Member
    Join Date
    Dec 2017
    I woulden't change the card, it is very lisable and shows clearly that there are 4 i2c ports. I was just confused by this page:

    Where it says that you can use the functions setSCL and setSDA to change the i2c port you are using, but in reality you just need to use Wire1, Wire2, and Wire3 to use different i2c ports. You should update your documentation.

  8. #8
    Senior Member+ manitou's Avatar
    Join Date
    Jan 2013
    if you look at the pin maps you'll see grayed SCL/SDA numbers. Those are alternate pins for the I2C port and are selected with setSDA and setSCL. So there are both alternate I2C ports and pins
    Last edited by manitou; 01-31-2018 at 07:22 PM.

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