Sparkfun MPR121 Technical document

MPR121 Hookup Guide

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CONTRIBUTORS: TONI_K

MPR121 Overview

If you are interested in adding the ‘magic’ of touch to control your

electronics project, a capacitive touch sensor might be the way to go. This

hookup guide will show you how to use the MPR121QR2 sensor.

The MPR121QR2 is a capacitive touch sensor controller that makes it very

easy to integrate capacitive touch sensing into your project. It

communicates via I C, and works by measuring the capacitance of twelve

electrode points. When an object comes close to the electrode connector,

the measured capacitance changes. This signals the MPR121 that

something has touched a ‘button’. The IC is also capable of driving LEDs or

basic GPIO functionality on electrode pins 4 through 11, giving you a lot of

freedom for setting up your project. The sensor works from 1.6V to 3.3V.

The sensor isn’t very current-hungry, drawing only around 29 µA when

sampling every 16 milliseconds.

Materials

To work through this tutorial, you are going to need one of the three

versions of the MPR121 sensor:

• MPR121 Capacitive Touch Sensor Breakout Board

• Touch Shield

• MPR121 Capacitive Touch Keypad

You will also want a soldering iron, some hookup wires and a

microcontroller capable of I C communication. For our examples, we will be

using an Arduino Uno. You will also need some kind of material to act as a

capacitive sensing surface (also known as an electrode, which is not to be

confused with the character Electrode). Generally, aluminum foil works well.

However, you could also use coins, conductive paint, or copper tape.

Page 1 of 2

Suggested Readin

The MPR121 is very easy to get started using, especially with the example

code. However, if you haven’t worked with Arduino previously or aren’t

familiar with I C communication, you should check out the tutorials below.

• What is an Arduino

• I C Communication

• How to Use a Breadboard

• How to Solder

Capacitive Touch Sensor Breakout

Board

The breakout board is the most versatile option of the three MPR121

products. You can wire it up to any kind of electrode you want, and, as it is

a simple breakout board, does not have a particular microcontroller footprint

it favors.

The breakout board has 4 pins that need to be connected to your

microcontroller at a minimum to get communication going: the power lines

and the I C lines. However, for our example, we are going to be also

connecting the IRQ pin to more easily detect a change on one of the

electrodes.

Connections

MPR121 Breakout →Arduino Uno

•3.3V→3.3V

•SCL→A5

•SDA→A4

Page 2 of 2

•GND→GND

•IRQ→D2

You will also want to connect the Electrode/LED pins to your electrode

material you selected previously. You will want to make sure you have a

good, solid connection between your material and your board, so make

sure you thoroughly solder your connections.

Check out the Fritzing diagram below for how your connections should look.

The yellow squares represent whatever material you decide to use for your

electrodes.

Communicating with the Breakout

Board

To communicate with your breakout board, you will need the Arduino

sketch available as a zip file here. Alternatively, you can also find the most

up-to-date firmware for working with the breakout board available on

GitHub. Let’s take a look and see exactly what the code is doing.

#include"mpr121.h"

#include<Wire.h>

int irqpin=2;//Digital2

boolean touchStates[12];//tokeeptrackoftheprevioust

ouchstates

In this first section of the code, the MPR121 library and the Wire library are

initialized. The Wire library makes I C communication easy to use on the

Arduino. The sketch also defines digital pin 2 as the IRQ pin connection,

and creates 12 instances of the boolean variable touchStates.

For the second section of the code, we define the irqpin as an INPUT,

telling the Arduino to monitor the digital signal coming in over that pin.

Serial communication is also started at 9600 bps, s well as the Wire and

mpr121 libraries.

Page 3 of 2

void setup(){

pinMode(irqpin,INPUT);

digitalWrite(irqpin,HIGH);//enablepullupresistor

Serial.begin(9600);

Wire.begin();

mpr121_setup();

}

The main loop of the code is incredibly simple, as it only calls a single

function.

void loop(){

readTouchInputs();

}

The function is actually described in the next section of the code. The

Arduino requests the electrode states from the sensor in the first section,

and the least significant bits and most significant bits are defined for the

sensor.

Page 4 of 2

void readTouchInputs(){

if(!checkInterrupt()){

//readthetouchstatefromtheMPR121

Wire.requestFrom(0x5A,2);

byteLSB= Wire.read();

byteMSB= Wire.read();

uint16_ttouched= ((MSB<< 8)| LSB);//16bitsthatmake

upthetouchstates

for (int i=0;i< 12;i++){//Checkwhatelectrodeswer

epressed

if(touched& (1<<i)){

if(touchStates[i]== 0){

//piniwasjusttouched

Serial.print("pin");

Serial.print(i);

Serial.println("wasjusttouched");

}else if(touchStates[i]== 1){

//piniisstillbeingtouched

}

touchStates[i]=1;

}else{

if(touchStates[i]== 1){

Serial.print("pin");

Serial.print(i);

Serial.println("isnolongerbeingtouched");

//piniisnolongerbeingtouched

}

touchStates[i]=0;

}

}

}

}

The Arduino scans each electrode and prints out a message over serial if

an electrode is triggered as being touched. The Arduino will then print out a

message as soon as the electrode is no longer being touched.

Page 5 of 2

The last major section of the code defines the threshold values for each

electrode. Each electrode must have a touch threshold and a release

threshold for the Arduino to compare the current state of the electrode.

Page 6 of 2

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 el

ectrode

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);

Page 7 of 2

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 Electr

odes

// 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); // LS

L = 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);

}

It looks like a lot of code, but it simply repeating the procedure of setting the

threshold values for each electrode pin.

The last two functions in the example sketch simply check the status of the

irqpin to determine if the IC is signaling that an electrode has been

touched. The very last function set_register simply runs the Arduino

through the standard steps in the Wire library to write the registers to the

IC.

Page 8 of 2

boolean checkInterrupt(void){

return digitalRead(irqpin);

}

void set_register(int address, unsigned char r, unsigned char

v){

Wire.beginTransmission(address);

Wire.write(r);

Wire.write(v);

Wire.endTransmission();

}

Now that you have a basic understanding of the code, let’s start reading the

data. Open up the sketch from the file you downloaded earlier, and upload

it to your Arduino. Once it is uploaded, open the Serial monitor in the

Arduino IDE. You should start seeing the Arduino print out sentences if you

press any electrodes or release any electrodes.

Touch Shield

The Touch Shield is an Arduino R3 compatible shield that enables

capacitive touch capabilities for your project using the MPR121 IC. The

shield itself has 9 touch pads on it (conveniently numbered 1-9 in a 3x3

grid), and has headers for 3 additional electrode connections.

The shield connects directly to the 3.3V pin on the Arduino. There is no on-

board voltage regulation on the VCC line, so keep that in mind if you intend

to use this with a 5V Arduino Pro board. However, there is an onboard logic

level converter to step down 5V levels to the 3.3V for the sensor’s I C lines.

This shield is designed to interface with an Arduino Uno R3. You can use

other microcontrollers as well with the same footprint, such as the Arduino

Mega. You will need access to the I C pins, SDA and SCL. Other than that,

you will need access to the 3.3V and GND pins and to digital pin 2. This

connects to the INT pin on the MPR121 sensor.

Page 9 of 2

We generally use stackable headers for attaching shields to Arduino

boards, but you can use standard male headers if you prefer. The R3

Stackable Header Kit is the easiest option to use if you are planning on

stackable headers. For detailed instructions on how to assemble your

shield, have a look at our Shield Tutorial.

The shield also has 3 pins labeled ELE9, ELE10, and ELE11. These

correspond to electrodes 9,10, and 11 on the MPR121 chip. You can solder

additional buttons or connections on to these pins at this time if you want

more than the 9 buttons already available on the shield. However, you don’t

have to do this to get the shield to function. It’s up to you!

Check out the Fritzing diagram below to see how your shield should look if

you have added buttons on to your shield on pins ELE9, ELE10, and

ELE11.

Note: The yellow squares represent whatever material you’ve chosen to

use as your electrode.

Once you’ve got your shield all hooked up, let’s start pulling data from the

shield!

Communicating with the Shield

Once you’ve got your shield assembled, it’s time to start talking to it! You

can download the example code here, or find the most up-to-date version in

the GitHub repository.

Open up the Touch_Sensor_Shield_Example.ino file. The other files set the

with these at all.

Page 10 of 2

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