MPR121 datasheet(14/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

Figure 7.

CONFIGURATION

From the implementation above, there are two elements that can be configured to yield a wide range of capacitance readings

ranging from 0.455 pF to 2874.39 pF. The two configurable components are the electrode charge current and the electrode

charge time. The electrode charge current can be configured to equal a range of values between 1

μA and 63 μA. This value is

set in the Charge Discharge Current (CDC) in the Analog Front End AFE Configuration register. The electrode charge time can

be configured to equal a range of values between 500 ns and 32

μS. This value is set in the Charge Discharge Time (CDT) in

the Filter Configuration Register.

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 8. AFE Configuration Register

543210

FFI

CDC

Reset:

000000

= Unimplemented

Table 9. 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

Sensitivity vs. Capacitance for VDD =1.8 Vand I=36 μA and T=.5 μS

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

Maximum

Minimum

C/ADC

Capacitance

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