MPR121 datasheet(23/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

The properties of the filter are determined by these two settings, but the CDT is determined by the capacitance being mea-

sured, as discussed in AN3889. The FFI sets the number of samples being measured. The result of an FFI setting of 6 or 0x00

would be to take 6 samples, toss the maximum and minimum, then average the remaining 4 samples. The results of an oscillo-

scope output on an electrode with the setting of FFI = 0x00 and CDT is shown in Figure 20.

Figure 20.

The first level of filtering delivers data to a second filter stage. The second filter stage averages samples over more time, in

this example anywhere from 1 ms to 128 ms. Then a value can be selected for how many samples should be averaged.

FILTER CONFIGURATION REGISTER

Figure 21. Filter Configuration Register

Table 20. AFE Configuration Register Field Descriptions

Field

Description

7:6

FFI

First Filter Iterations – The first filter iterations field selects the number of samples

taken as input to the first level of filtering.

00 Encoding 0 – Sets samples taken to 6

01 Encoding 1 – Sets samples taken to 10

10 Encoding 2 – Sets samples taken to 18

11 Encoding 3 – Sets samples taken to 34

5:0

CDC

Charge Discharge Current – The Charge Discharge Current field selects the supply

current to be used when charging and discharging an electrode.

000000 Encoding 0 – Disables Electrode Charging

000001 Encoding 1 – Sets the current to 1

μA

111111 Encoding 63 – Sets the current to 63

μA

543210

CDT

SFI

ESI

Reset:

000000

= Unimplemented

TIME

CDT

μs

μs4 μs6 μs8 μs

μs12 μs

FFI

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