Data Sheet

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MPR121 Sensor

13 Freescale Semiconductor

8. AFE Configuration Register (0x5C, 0x5D)

AFE Configuration 1 Register (0x5C), default =0x10

Bit

Read

FFI

CDC

Write

This register holds the first level filter configuration, and the charge current global setting for all 13 channels. For each channel

the global CDC will be used if the individual charge discharge current CDCx is zero. If there is none zero data in 0x5F~0x6B,

then this individual CDCx value will be used for that respective channel instead of the global CDC value.

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 (Default)

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. Programmable in 1uA step.

000000 Encoding 0 – Disables Electrode Charging??

000001 Encoding 1 – Sets the current to 1μA

010000 Encoding 16 – Sets the current to 16μA (Default)

111111 Encoding 63 – Sets the current to 63 μA

AFE Configuration 2 Register (0x5D), default =0x24

Bit

Read

CDT

SFI

ESI

Write

This register holds global charge and discharge time setting, and the second level filter configuration. The CDT bits set the

charge/discharge time global setting for all the 13 channels. For each channel the global CDT will be used if the individual

charge discharge time CDTx is zero. If there is none zero data in 0x6C~0x72, then this individual CDTx value will be used for

that respective channel instead of the global CDT value.

Field

Description

7:5

CDT

Charge Discharge Time – selects the amount of time an electrode charges. Programmable

to 0.5 (2

n-1

) uS

000 Encoding 0 – Invalid

001 Encoding 1 – Time is set to 0.5 μs (Default)

010 Encoding 2 – Time is set to 1 μs

111 Encoding 7 – Time is set to 32 μs

4:3

SFI

Second Filter Iterations – selects the number of samples taken for the second level filter.

00 Encoding 0 – Number of samples is set to 4 (Default)

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 – selects the period between samples used for the second level

of filtering. Programmable to 2

ms.

000 Encoding 0 – Period set to 1 ms

001 Encoding 1 – Period set to 2 ms

100 Encoding 4 – Period set to 16 ms (Default)

111 Encoding 7 – Period set to 128 ms