Data Sheet

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MPR121 Sensor

12 Freescale Semiconductor

5. Baseline Filtering Control Register (0x2B~0x40)

Background baseline value tracking is operated by the internal 3

level filter controlled by registers 0x2B to 0x40. All the 12

electrodes’ baseline are controlled by the same set of registers 0x2B to 0x35, while the 13

channel ELEPROX is controlled by

another independent set of registers 0x36 to 0x40.

Both above two sets have the same structure using 3 different scenarios called rising, falling, and touched. The rising situation

is defined when electrode data > baseline value, the falling is defined when electrode data < baseline value, and the touched is

when the electrode is in touched status.

For each scenario, the filtering characteristic is further defined by 4 parameters: the maximum half delta, noise half delta, noise

count limit and filter delay count limit. There is no maximum half delta for touched scenario.

Maximum Half Delta (MHD): Determines the largest magnitude of variation to pass through the baseline filter. The range of

the effective value is 1~63.

Noise Half Delta (NHD): Determines the incremental change when non-noise drift is detected. The range of the effective

value is 1~63.

Noise Count Limit (NCL): Determines the number of samples consecutively greater than the Max Half Delta necessary

before it can be determined that it is non-noise. The range of the effective value is 0~255.

Filter Delay Count Limit (FDL): Determines the rate of operation of the filter. A larger number makes it operate slower. The

range of the effective value is 0~255.

The setting of the filter is depended on the actual application. For the details on how to set these registers, refer to application

note AN3891.

6. Touch and Release Threshold (0x41~0x5A)

Touch Threshold ExTTH

Bit

Read

ExTTH

Write

Release Threshold ExRTH

Read

ExRTH

Write

ExTTH, ExRTH: Each of the 13 channels has its own set of touch threshold and release threshold registers. The threshold is

defined as a deviation value from the baseline value, so it remains constant even baseline value changes. Typically the touch

threshold is a little bigger than the release threshold to touch debounce and hysteresis.

The range of the value is 0~255. For typical touch application, the value can be in range 0x05~0x30 for example. The setting

of the threshold is depended on the actual application. For the operation details and how to set the threshold refer to

application note AN3892 and MPR121 design guidelines.

7. Debounce Register (0x5B)

Touch and Release Debounce

Bit

Read

Write

The register holds debounce setting affecting the final touch and release status change in 0x00 and 0x01. The status change

will only take place after the number of consecutive touch or release detection meets the debounce number. If the number

detected does not meet the debounce number, the status will not change.

DT: Debounce number for touch. The value range is 0~7.

DR: Debounce number for release. The value range is 0~7.

When an electrode data deems consecutive touch with the debounce number, the status becomes “touch”. When an electrode

data deems consecutive release with the debounce number, the status becomes “release”. The status appears on Status