MPR121

Sensors

Freescale Semiconductor

39

AN3893: MPR121 Proximity Detection

INTRODUCTION

MPR121 is a feature rich, second generation touch sensor controller after Freescale’s initial release of the MPR03x series

device. Like MPR03x, MPR121 has a unique feature that all the electrode inputs can be internally connected together so that all

the surface touch sensing area on the inputs are “summed” together to act as a single large electrode pad. This can effectively

increase the total area of the sensing conductor for non-contact near proximity detection for hand approaching.

OVERVIEW

Capacitive proximity detection uses the same principle as capacitive touch sensing. Each MPR121 input sensing channel can

be used as contactless proximity detection as well as finger touch detection if each sensing pad is designed properly and relevant

register are set properly.

Typically a smaller pad size is used for finger touch button detection; while a larger pad size is necessary for contactless near

proximity detection. On the other side, it’s true that in most portable application design there is no dedicated big surface area left

for proximity detection as the touch sensing buttons occupy all the available surface area. To make proximity detection at the

same time of touch detection without additional dedicated large sensing pad, MPR121 has an internal input multiplexor which

can connect all input sensing channels together so that all the touch sensing surface areas on the input pads are “summed”

together effectively acting as a single large sensing pad.

Using this scheme in typical applications, the 12 channels can be used for 12 key buttons touch sensing, and the surface area

of all the pads can also used for proximity detection (e.g., hand approaching).

PROXIMITY DETECTION REGISTER SETTING

Like each independent touch sensing detection, the 13th Proximity Detection electrode also has its own register configura-

tions, other than that, all the concepts applied to the proximity detection are the same as touch sensing detection.

1.0

Enable Proximity Sensing

Proximity detection (a/k/a area detection mode) is enabled by configuring the Electrode Configuration Register (0x5E), see

sensing detect sequence. Once configured, we refer to this area detection as the 13th Proximity Detection electrode.

2.0

Proximity Data Register and Baseline Register

Eleprox Electrode Registers (0x1C, 0x2D) contain the 10-bit raw data of the capacitance-voltage measurement value for the

Table 34. Electrode Configuration Register 0x5E (Reset Default: 0x00)

Bit

Bit7

Bit6

Bit5

Bit4

Bit3

Bit2

Bit1

Bit0

Read

CL[1]

CL[0]

AD[1]

AD[0]

EC[3]

EC[2]

EC[1]

EC[0]

Write

Reset

00

00

0

000

AD1

AD0

EC3

EC2

EC1

EC0

Description

01

xx

xx

Area Detection by connecting ELE0~1.

10

xx

xx

Area Detection by connecting ELE0~3.

11

xx

xx

Area Detection by connecting ELE0~11.

Table 35. Eleprox Electrode Register 0x1C, 0x2D (Reset Default: 0x00, 0x00)

0x1C

Bit7

Bit6

Bit5

Bit4

Bit3

Bit2

Bit1

Bit0

D7

D6

D5

D4

D3

D2

D1

D0

0x1D

Bit7

Bit6

Bit5

Bit4

Bit3

Bit2

Bit1

Bit0

——

——

——

D9

D8