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MPR121 Quick Start Guide
INTRODUCTION
The MPR121 is Freescale Semiconductor’s top of the line touch sensor and can fit into a wide range of applications. These applications can all be accommodated by having a device a with a very large range of flexibility. While all of these added features can allow for a wide range of flexibility, they can also add an unnecessary layer of complication. For advanced users who want to do more than basic touch detection, additional information can be found in other application notes.
To start, the device is configured through an I2C serial interface. The following table lists the registers that are initialized. The order they are written in is not significant except that register 0x05E, the Electrode Configuration Register must be written last.
Register Address
0x2B0x2C0x2D0x2E0x2F0x30 0x31 0x320x410x420x430x440x450x460x470x480x490x4A0x4B0x4C0x4D0x4E0x4F0x50
MHD RisingNHD Amount RisingNCL RisingFDL RisingMHD FallingNHD Amount FallingNCL FallingFDL Falling
ELE0 Touch ThresholdELE0 Release ThresholdELE1 Touch ThresholdELE1 Release ThresholdELE2 Touch ThresholdELE2 Release ThresholdELE3 Touch ThresholdELE3 Release ThresholdELE4 Touch ThresholdELE4 Release ThresholdELE5 Touch ThresholdELE5 Release ThresholdELE6 Touch ThresholdELE6 Release ThresholdELE7 Touch ThresholdELE7 Release Threshold
Register Name
Value0x010x010x000x000x010x010xFF0x020x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A
Application Note
AN3891AN3891AN3891AN3891AN3891AN3891AN3891AN3891AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892
SectionAAAABBBBCCCCCCCCCCCCCCCC
© Freescale Semiconductor, Inc., 2009, 2010. All rights reserved.
Register Address
0x510x520x530x540x550x560x570x580x5D0x5E0x7B0x7D0x7E0x7F
Register Name
ELE8 Touch ThresholdELE8 Release ThresholdELE9 Touch ThresholdELE9 Release ThresholdELE10 Touch ThresholdELE10 Release ThresholdELE11 Touch ThresholdELE11 Release ThresholdFilter ConfigurationElectrode Configuration
AUTO-CONFIG Control Register 0AUTO-CONFIG USL RegisterAUTO-CONFIG LSL RegisterAUTO-CONFIG Target Level Register
Value0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A0x040x0C0x0B0x9C0x650x8C
Application Note
AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3890AN3890AN3889AN3889AN3889AN3889
SectionCCCCCCCCDEFFFF
The following sections describe what each of the defaults do and recommendations for variations.Section A
Register Address
0x2B0x2C0x2D0x2E
MHD RisingNHD Amount RisingNCL RisingFDL Rising
Register NameValue0x010x010x000x00
Application Note
AN3891AN3891AN3891AN3891
Description:This group of setting controls the filtering of the system when the data is greater than the baseline.
The setting used allow the filter to act quickly and adjust for environmental changes. Additionally, if calibration happens to take place while a touch occurs, the value will self adjust very quickly. This auto-recovery or snap back prevents repeated false negative for a touch detection.Variation:As the filter is sensitive to setting changes, it is recommended that users read AN3891 before
changing the values. In most cases these default values will work
Section B
Register Address
0x2F0x30
MHD FallingNHD Amount Falling
Register NameValue0x010x010xFF0x02
Application Note
AN3891AN3891AN3891AN3891
0x31 NCL Falling0x32
FDL Falling
Description:This group of setting controls the filtering of the system, when the data is less than the baseline. The
settings slow down the filter as the negative charge is in the same direction as a touch. By slowing down the filter, touch signals are “rejected” by the baseline filter. While at the same time lon term environmental change that occur slower than at a touch are accepted. This low pass filter both allows for touches to be detected properly while preventing false positive by passing environmental change through the filter.Variation:As the filter is sensitive to setting changes, it is recommended that users read AN3891 before
changing the values. In most cases these default values will work
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Section C
Register Address
0x410x420x430x440x450x460x470x480x490x4A0x4B0x4C0x4D0x4E0x4F0x500x510x520x530x540x550x560x570x58
Register Name
ELE0 Touch ThresholdELE0 Release ThresholdELE1 Touch ThresholdELE1 Release ThresholdELE2 Touch ThresholdELE2 Release ThresholdELE3 Touch ThresholdELE3 Release ThresholdELE4 Touch ThresholdELE4 Release ThresholdELE5 Touch ThresholdELE5 Release ThresholdELE6 Touch ThresholdELE6 Release ThresholdELE7 Touch ThresholdELE7 Release ThresholdELE8 Touch ThresholdELE8 Release ThresholdELE9 Touch ThresholdELE9 Release ThresholdELE10 Touch ThresholdELE10 Release ThresholdELE11 Touch ThresholdELE11 Release Threshold
Value0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A0x0F0x0A
Application Note
AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892AN3892
Description:The touch threshold registers set the minimum delta from the baseline when a touch is detected
0x0F or 15 in decimal is an estimate of the minimum value for touch. Most electrodes will work with this value even if they vary greatly in size and shape. The value of 0x0A or 10 is the release threshold register allowed for hysteresis in the touch detection.Variation:For very small electrodes, smaller values can be used and for very large electrodes the reverse is
true. One easy method is to view the deltas actually seen in a system and set the touch at 80% and release at 70% of delta for good performance.
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3
Section D
Register Address
0x5D
Filter Configuration
Register Name
Value0x04
Application Note
AN3890
Description:There are three settings embedded in this register so it is only necessary to pay attention to one.
The ESI controls the sample rate of the device. In the default, the setting used is 0x00 for 1 ms sample rate. Since the SFI is set to 00, resulting in 4 samples averaged, the response time will be 4 ms. Variation:To save power, the 1 ms can be increased to 128 ms by increasing the setting to 0x07. The values
are base 2 exponential thus 0x01 = 2 ms; 0x02 = 4 ms; and so on to 0x07 = 128 ms. Most of the time, 0x04 results in the best compromise between power consumption and response time.
Section E
Register Address
0x5E
Register Name
Electrode Configuration
Value0x0C
Application Note
AN3890
Description:This register controls the number of electrodes being enabled and the mode the device is in. There
are only two modes, Standby (when the value is 0x00) and Run (when the value of the lower bit is non-zero). The default value shown enables all 12 electrodes by writing decimal 12 or hex 0x0C to the register. Typically other registers cannot be changed while the part is running so this register should always be written last.Variation:During debug of a system, this register will change between the number of electrodes and 0x00
every time a register needs to change. In a production system, this register will only need to be written when the mode is changed from Standby to Run or vise versa.
Section F
Register Address
0x7B0x7D0x7E0x7F
Register Name
AUTO-CONFIG Control Register 0AUTO-CONFIG USL RegisterAUTO-CONFIG LSL RegisterAUTO-CONFIG Target Level Register
Value0x0B0x9C0x650x8C
Application Note
AN3889AN3889AN3889AN3889
Description:These are the settings used for the Auto Configuration. They enable AUTO-CONFIG and
AUTO_RECONFIG. In addition they set the target range for the baseline. The upper limit is set to 190, the target is set to 180 and the lower limit is set to 140.Variation:In most cases these values will never need to be change, but if a case arises, a full description is
found in application note AN3889.
CONCLUSION
In many applications for the MPR121, the default settings presented in this document will be sufficient for both design time activities as well as in the production implementation.
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AN3944Rev. 103/2010
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