MPR083

Sensors

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Freescale Semiconductor

3

Touch Detection

3.1

Introduction

When using a capacitive touch sensor system the raw data must be filtered and interpreted. This process can be done many

different ways but the method used in the MPR083 is explained in this chapter.

3.2

Understanding the Basics

The rotary interface has to distinguish touch status through varying user conditions (different finger sizes in bare hands or gloves)

and environmental conditions (electrical and RF noise, sensor contamination with dirt or moisture).

The rotary circuitry reports touch status as one of the following two conditions:

1.

Rotary untouched

2.

Rotary touched in one of eight positions.

The rotary is only touched in one position, ideally near the middle of one of the eight pads. If a touch occurs between pads,

untouched will be reported.

3.3

Conditional Output Scenarios

Since it is unlikely that in a real world case a single independent touch will occur two specific multi-touch response cases are

outlined. Methods for changing the sensitivity of the device will be discussed in another Chapter, but the important part is that the

sensitivity is determined by the strength of an input signal. If more than one input signal is above the selected sensitivity then the

touch sensor controller interprets this in a specific way. This functionality is broken down into two different cases.

3.3.1

Simultaneous Touches

Any time two touches are detected at the same time the touch sensor controller recognizes this case and accounts for it. Any

time more than one key is pressed the touches are ignored. Thus the touch sensor controller will show the rotary as untouched.

In most cases one of the two electrodes will receive a stronger signal than the other. If the difference in capacitance is statistically

significant between the pad with the stronger signal will be reported.

This functionality is sometimes called 1-Key Lockout.

3.3.2

Sequential Touches

Another case is when one rotary pad is touched and held and a second rotary pad is then touched and held. For this situation

the second touch will be ignored and the first touch will continue to be reported.

If the second touch is released before the first touch then the second touch will be completely ignored. But, if the first touch is

released before the second then the system will report that the first key is released and that the second key is now touched. This

functionality is sometimes called 2-Key Rollover.

3.4

Rotary Configuration Register

The Rotary Configuration Register configures a variety of the MPR083 features. Each of these features is described in following

sections. The I2C slave address of the Rotary Configuration Register is 0x03.

Figure 14. Rotary Configuration Register

7

6

543210

R

RSE

00

ACE

RRBE

RTBE

0

RE

W

Reset:

1

0

000001

= Unimplemented