MPR121

Sensors

Freescale Semiconductor

33

AN3892: MPR121 Jitter and False Touch Detection

INTRODUCTION

Touch acquisition takes a few different parts of the system in order to detect touch. The baseline filter and touch detection are

tightly coupled. The purpose of the touch detection block is to use the baseline value and the 2nd level filter data to determine

when a user has touched an electrode. The electrodes are independently configured using the Touch Threshold and Release

Threshold registers. The global Debounce register also controls when a touch is detected by adding some minimal delay. The

data is then output through a couple of registers: Filtered Data High, Filtered Data Low, Baseline Data and two touch output reg-

isters.

Figure 37. Data Flow in the MPR121

First, the MPR121 touch sensor detects touch by the methods in this application note, and the data is output through the first

two registers in the map. The two touch status registers both trigger an interrupt on any change of the data. Thus, as a touch

happens (bit is set) an interrupt will be triggered, and when a touch is released (bit is cleared) it will also trigger. To clear the

interrupt all you must do is initiate a I2C communication, with the intent that you read register 0x00 and 0x01 to determine which

electrodes are touched.

AFE AQUISITION

RAW

DATA

2nd

FILTER

1st

FILTER

BASELINE

FILTER

STATUS REGISTER

IRQ

TOUCH

1 - 32

μs

1 - 128

μs

4 - 2048

μs