MPR084

Sensors

10

Freescale Semiconductor

Preliminary

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 MPR084 is explained in this chapter.

3.2

Understanding the Basics

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

1.

Touch Pad untouched

2.

Touch Pad touched on one of eight pads.

The touch pad 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 multiple touches are detected at the same time the touch sensor controller recognizes this case and accounts for it. The

number of allowed reported touches is settable using the Maximum Number of Touched Positions Register (Section 3.6). In the

case where this register is set to 1, all touches past the first will be ignored and unreported.

A special case is when exact 2 keys are involved in the interaction. 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-Keyed Lockout. by changing the Maximum Number of Touched Positions Register

(Section 3.6). this value can be set. Thus the n-key lockout is determined by this register.

3.3.2

Sequential Touches

Another case is when one touch pad is touched and held and a second touch 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

Touch Pad Configuration Register

The Touch Pad Configuration Register configures a variety of the MPR084 features. Each of these features is described in

Figure 14. Touch Pad Configuration Register

7

6

543210

R

TPE

0

BKA

ACE

TPRBE

TPTBE

0

TPE

W

Reset:

1

0

000001

= Unimplemented