MPR121 datasheet(22/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

AN3890: MPR121 Capacitance Sensing — Filtering and

Timing

INTRODUCTION

The capacitance sensing front end of the MPR121 produces data at extremely high rates, which significantly improves the

capabilities of a filtering system. The capacitance engine described in AN3889 act on a 1

μs - 32 μs per sample data rate. THis

application note will discuss the first and second level filters in the MPR121 and how they impact timing and power consumption.

Figure 18. Data Flow in the MPR121

The first level filter is configured through the use of the First Filter Iterations (FFI) and the Charge Discharge Time (CDT).

AFE CONFIGURATION REGISTER

The AFE Configuration Register is used to set both the CDC and the number of samples taken in the lowest level filter. The

address of the AFE Configuration Register is 0x5C.

Figure 19. AFE Configuration Register

543210

FFI

CDC

Reset:

000000

= Unimplemented

AFE AQUISITION

RAW

DATA

2nd

FILTER

1st

FILTER

BASELINE

FILTER

STATUS REGISTER

IRQ

TOUCH

1 - 32

μs

1 - 128

μs

4 - 2048

μs

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