MPR03X

Sensors

Freescale Semiconductor

21

8.4

Third Level Filter

The Third Level Filter is designed for varying implementations. It can be used as either an additional low pass filter for the

electrode data or a baseline for touch detection. For it to function as a baseline filter, it must be used in conjunction with the touch

detection system described in the next chapter. To use the filter as an additional layer for low pass filtering, the touch detection

system must be disabled by setting all of the touch thresholds to zero (refer to Section 9.2). Although, in most cases the third

level of filter will be used as a baseline filter. The primary difference between these implementations is this: if a touch is detected

the baseline filter will hold its current value until the touch is released. The touch/release configuration will be described in

Chapter 9.

When a touch is not currently detected, the baseline filter will operate based on a few conditions. These are configured through

a set of registers including the Max Half Delta Register, the Noise Half Delta Register, and the Noise Count Limit.

8.4.1

Max Half Delta Register

The Max Half Delta register is used to set the Max Half Delta for the Third Level Filter. The address of the Max Half Delta Register

is 0x26.

Figure 27. Max Half Delta Register

Table 13. Filter Configuration Register Field Descriptions

Field

Description

7:5

CDT

Charge Discharge Time – The Charge Discharge Time field selects the amount

of time an electrode charges and discharges.

000 Encoding 0 – Invalid

001 Encoding 1 – Time is set to 0.5

μs

010 Encoding 2 – Time is set to 1

μs

~

111 Encoding 7 – Time is set to 32

μs.

4:3

SFI

Second Filter Iterations – The Second Filter Iterations field selects the number of

samples taken for the second level filter.

00 Encoding 0 – Number of samples is set to 4

01 Encoding 1 – Number of samples is set to 6

10 Encoding 2 – Number of samples is set to 10

11 Encoding 3 – Number of samples is set to 18

2:0

ESI

Electrode Sample Interval – The Electrode Sample Interval field selects the

period between samples used for the second level of filtering.

000 Encoding 0 – Period set to 1 ms

001 Encoding 1 – Period set to 2 ms

~

111 Encoding 7 – Period set to 128 ms

7

6

543210

R0

0

MHD

W

Reset:

0

0

000000

= Unimplemented

Table 14. Max Half Delta Register Field Descriptions

Field

Description

5:0

MHD

Max Half Delta – The Max Half Delta determines the largest magnitude of

variation to pass through the third level filter.

000000 DO NOT USE THIS CODE

000001 Encoding 1 – Sets the Max Half Delta to 1

~

111111 Encoding 63 – Sets the Max Half Delta to 63