MPR03X

Sensors

20

Freescale Semiconductor

8Filtering

8.1

Introduction

The MPR03X has three levels of filtering. The first and second level filters will allow the application to condition the signal for

undesired input variation. The third level filter can be configured to reject touch stimulus and be used as a baseline for touch

detection. Each level of filtering will be further described in this section.

8.2

First Level

The first level filter is designed to filter high frequency noise by averaging samples taken over short periods of time. The number

of samples can be configured to equal a set of values ranging from 6 to 34 samples. This value is set by the FFI in the AFE

Configuration Register (Section 7.4.1). The timing of this filter is determined by the configuration of the electrode charge time in

the Filter Configuration Register (Section 8.3.1).

Note that the electrode charge time must be configured for the capacitance in the application. The resulting value will affect the

period of the first level filter.

8.3

Second Level

The second level filter is designed to filter low frequency noise and reject false touches due to inconsistent data. The number of

samples can be configured to equal a set of values ranging from 4 to 18. This value is set by the SFI in the Filter Configuration

Register (Section 8.3.1). The timing of this filter is determined by the configuration of ESI in the Filter Configuration Register

(Section 8.3.1).

Note that the ESI (Electrode Sample Interval) must be configured to accommodate the low power requirements of a system.

Thus, the resulting value will affect the period of the second level filter.

The raw data from the second level of filtering is output in the Filtered Data High and Filtered Data Low registers, as shown in

Section 5.3.

8.3.1

Filter Configuration Register

The Filter Configuration register is used to set. The address of the Electrode Configuration Register is 0x43.

Figure 26. Filter Configuration Register

7

6

543210

R

CDT

SFI

ESI

W

Reset:

0

0

000000

= Unimplemented