MPR121
Sensors
Freescale Semiconductor, Inc.
13
Noise Count Limit (NCL): Determines the number of samples consecutively greater than the Max Half Delta value. This is
necessary to determine that it is not noise. The range of the effective value is 0~255.
Filter Delay Count Limit (FDL): Determines the operation rate of the filter. A larger count limit means the filter delay is operating
more slowly. The range of the effective value is 0~255.
The setting of the filter is depended on the actual application. For more information on these registers, refer to application note
AN3891.
5.6
Touch / Release Threshold (0x41~0x5A)
ExTTH: Electrode touch threshold, in range of 0~0xFF.
ExRTH: Electrode release threshold, in range of 0~0xFF.
Each of the 13 channels can be set with its own set of touch and release thresholds. Touch and release are detected by
comparing the electrode filtered data to the baseline value. The amount of deviation from the baseline value represents a
immediate capacitance change detected by possible a touch/release action.
Touch condition: Baseline - Electrode filtered data > Touch threshold
Release condition: Baseline - Electrode filtered data < Release threshold
Threshold settings are dependant on the touch/release signal strength, system sensitivity and noise immunity requirements. In
a typical touch detection application, threshold is typically in the range 0x04~0x10. The touch threshold is several counts larger
than the release threshold. This is to provide hysteresis and to prevent noise and jitter. For more information, refer to the
application note AN3892 and the MPR121 design guidelines.
5.7
Debounce Register (0x5B)
DT: Debounce number for touch. The value range is 0~7.
DR: Debounce number for release. The value range is 0~7.
All 13 channels use the same set of touch and release debounce numbers. The status bits in Status Register 0x00 and 0x01 will
only take place after the number of consecutive touch or release detection meets the debounce number setting. The debounce
setting can be very useful in avoiding possible noise glitches. Using the debounce setting, the status bit change will have a delay
of {ESI x SFI x DR (or DT)}.
ELEx, ELEProx Touch Threshold (0x41,0x43,...,0x59)
Bit
D7
D6
D5
D4
D3
D2
D1
D0
Read
ExTTH
Write
ELEx, ELEProx Release Threshold (0x42,0x44,...,0x5A)
Bit
D7
D6
D5
D4
D3
D2
D1
D0
Read
ExRTH
Write
Debounce Register (0x5B)
Bit
D7
D6
D5
D4
D3
D2
D1
D0
Read
—
DR
—
DT
Write