MPR121QR2 datasheet(36/57 Pages) FREESCALE

36 / 57 page

MPR121

Sensors

Freescale Semiconductor

FILTERED DATA LOW

Figure 41. Filtered Data Low Register

BASELINE VALUE

Figure 42. Filtered Data High Register

In this system, a touch is defined as any time the difference between the Filtered Data and the Baseline Value is greater than

the threshold. Since this calculation is done totally internal to the part, it is unnecessary for the user to actually do this math in

the software. If it were being done, the steps would be to first combine the Filtered Data Low and Filtered Data High values into

a single 10-bit number. Thus,

Data = Filtered Data High • 256 + Filtered Data Low

The baseline is then shifted to the left to make it equal scale to the Data.

Baseline = Baseline Value • 4

Internally to the device, the full 10-bit value is stored, but as this level of precision is not necessary as the low two bits are

disregarded for output. The Touch Threshold is a user defined value. There is both a touch and an un-touch threshold to provide

hysteresis.

543210

RFDLB

Reset:

000000

= Unimplemented

Table 29. Filtered Data Low Register Field Descriptions

Field

Description

7:0

FDLB

Filtered Data Low Byte – The Filtered Data Low Byte displays the lower 8 bits of

the 10 bit filtered A/D reading.

00000000 Encoding 0

11111111 Encoding 255

54321

RBV

Reset:

0000

= Unimplemented

Table 30. Filtered Data High Register Field Descriptions

Field

Description

7:0

Baseline Value – The Baseline Value byte displays the higher 8 bits of the 10 bit

baseline value.

00000000 Encoding 0 – The 10 bit baseline value is between 0 and 3.

11111111 Encoding 255 – The 10 bit baseline value is between 1020 and 1023.

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