MPR03X

Sensors

Freescale Semiconductor

23

9

Touch Detection

9.1

Introduction

The MPR03X uses a threshold based system to determine when touches occur. This section will describe that mechanism.

9.2

Thresholds

When a touch pad is pressed, an increase in capacitance will be generated. The resulting effect will be a reduction in the ADC

counts. When the difference between the second level filter value and the third level filter value is significant, the system will

detect a touch. When a touch is detected, there are a couple of effects: the third level filter output becomes fixed (refer to

Section 8.4), an interrupt is generated (refer to Section 6), and the touch status register (Section 5.2) is updated.

The touch detection system is controlled using two threshold registers for each independent electrode. The Touch Threshold

register represents the delta at which the system will trigger a touch. The Release Threshold represents the difference at which

a release would be detected. In either case the system will respond by changing the previously mentioned items.

9.2.1

Touch Threshold Register

The Touch Threshold Register is used to set the touch threshold for each of the electrodes. The address of the ELE0 Touch

Threshold Register is 0x29. The address of the ELE1 Touch Threshold Register is 0x2B. The address of the ELE2 Touch

Threshold Register is 0x2D.

Figure 30. Touch Threshold Register

9.2.2

Release Threshold Register

The Release Threshold Register is used to set the release threshold for each of the electrodes. The address of the ELE0 Release

Threshold Register is 0x2A. The address of the ELE1 Release Threshold Register is 0x2C. The address of the ELE2 Release

Threshold Register is 0x2E.

Figure 31. Release Threshold Register

7

6

543210

R

TTH

W

Reset:

0

0

000000

= Unimplemented

Table 17. Touch Threshold Register Field Descriptions

Field

Description

7:0

TTH

Touch Threshold – The Touch Threshold Byte sets the trip point for detecting a

touch.

00000000 Encoding 0

~

11111111 Encoding 255

7

6

543210

R

RTH

W

Reset:

0

0

000000

= Unimplemented

Table 18. Release Threshold Register Field Descriptions

Field

Description

7:0

RTH

Release Threshold – The Release Threshold Byte sets the trip point for detecting

a touch.

00000000 Encoding 0

~

11111111 Encoding 255