MPR084

Sensors

Freescale Semiconductor

25

Preliminary

9

Sensitivity

9.1

Introduction

The MPR084 can operate in a variety of environments with a variety of different electrode patterns. Because of this it is necessary

to adjust the relative sensitivity of the sensor controller. Usually this requires fine tuning in any final application.

There are many factors that must be taken into account, but much of the time this value is relative to the capacitance changes

generated by a touch. Since capacitance is directly proportional to the dielectric constant of the material and the area of the pad,

while inversely proportional to the distance between pads these are the primary factors.

Equation 8

As the relative capacitance rises the sensitivity setting of the MPR084 should be adjusted accordingly. Thus a very high sensitivity

value represents a large A and a small d.

9.2

Adjusting the Sensitivity

The sensitivity of the MPR084 is adjusted by varying the Sensitivity Threshold Registers.

9.2.1

Sensitivity Threshold Registers

Sensitivity Threshold Registers is 0x04 - 0x0B.

Figure 28. Sensitivity Threshold Register Format

7

6

543210

R

SL

W

Reset:

0

0

000000

= Unimplemented

Table 17. Sensitivity Threshold Register Format Descriptions

Field

Description

7:0

ST

Sensitivity Threshold – The Sensitivity Threshold selects or reports the sensitivity

setting of the Sensor Controller. The resulting value must be in the range 1 to 64

units. If the value is outside of this range the ST will be set to 00111111.

00000000 Encoding 0 – Sets the sensitivity to level 1

~

00111111 Encoding 63 – Sets the sensitivity to level 64

C

d

------------

=