MPR121 datasheet(24/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

Note: In most cases the CDT in this register is not used. It will normally be auto-configured as described in AN3889

While the 1 ms to 128 ms does affect the filtering, the main purposed of adjusting the sample rate would be to change the

average current consumption of the device. Figure 22 illustrates this adjustment.

Figure 22.

From this, it can be seen that the 12

μs up time from the 1 ms samples results in a very low percent of duty cycle. This results

in a very low average current consumption.

Table 21. 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

TIME (ms)

01 ms

3 ms

First Filter

2 ms

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