MPR121
Sensors
4 Freescale Semiconductor, Inc.
3 Device Operation Overview
Power Supply
The VDD pin is the main power supply input to the MPR121 and is always decoupled with a 0.1 μF ceramic capacitor to the VSS.
Excessive noise on the VDD should be avoided.
The VDD pin has an operational voltage range specification between 1.71V to 3.6V. The internal voltage regulator, which
generates current to internal circuitry, operates with an input range from 2.0V to 3.6V. To work with a power supply below 2.0V
and to avoid the unnecessary voltage drop, the internal voltage regulator can be bypassed, refer to Figure 1 and Figure 2.
When a power supply is in the range of 1.71V to 2.75V, the VDD and VREG pins can be connected together (Figure 1) so that
internal voltage regulator is bypassed. In this configuration, the supply voltage cannot be higher than 2.75V as this is the
maximum voltage limit for VREG pin.
When a power supply is higher than 2.75V, it must be connected to the VDD, i.e. configuration as in Figure 2. In this configuration,
a separate 0.1 μF decoupling ceramic capacitor on VREG to VSS is applied as a bypass cap for internal circuitry. This
configuration can work with a VDD supply voltage down to 2.0V. For a typical two dry cell 1.5V batteries application, this
configuration covers the entire expected working voltage range from 2.0V to 3.0V.
Capacitance Sensing
The MPR121 uses a constant DC current capacitance sensing scheme. It can measure capacitances ranging from 10 pF to over
2000 pF with a resolution up to 0.01 pF. The device does this by varying the amount of charge current and charge time applied
to the sensing inputs.
The 12 electrodes are controlled independently; this allows for a great deal of flexibility in electrode pattern design. An automatic
configuration system is integrated as part of the device, this greatly simplifies the individual register setup. Please refer to the
Freescale application note, AN3889, for more details.
The voltage measured on the input sensing node is inversely proportional to the capacitance. At the end of each charge circle,
this voltage is sampled by an internal 10-bit ADC. The sampled data is then processed through several stages of digital filtering.
The digital filtering process allows for good noise immunity in different environments. For more information on the filtering system,
refer to application note AN3890.
Touch Sensing
Once the electrode capacitance data is acquired, the electrode touch/release status is determined comparing it to the
capacitance baseline value. The capacitance baseline is tracked by MPR121 automatically based on the background
capacitance variation.
The baseline value is compared with the current immediate electrode data to determine if a touch or release has occurred. A
designer has the ability to set the touch/release thresholds, as well as a touch/release debounce time. This is to eliminate jitter
and false touches due to noise. Additional information on baseline capacitance system is covered in application notes AN3891
and AN3892.
Proximity Sensing
One new feature of the MPR121 is the near proximity sensing system. This means that all of the system’s electrodes can be
summed together to create a single large electrode. The major advantage of the large electrode is that is can cover a much larger
sensing area. The near proximity sensing system can be used while at the same time having separate electrodes by using touch
button sensing.
Proximity detection is read as an independent channel and has configuration registers similar to the other 12 channels. When
proximity detection is enabled, this “13
th
” measurement channel will be included at the beginning of a normal detection cycle.
This system is described in application note AN3893.
LED Driver
Among the 12 electrode inputs, 8 inputs are designed as multifunctional pins. When these pins are not configured as electrodes,
they may be used to drive LEDs or used for general purpose input or output. For more details on this feature, please refer to
application note AN3894.
Serial Communication
The MPR121 is an Inter-Integrated Circuit (I
2
C) compliant device with an interrupt IRQ pin. This pin is triggered any time a touch
or release is detected. The device has a configurable I
2
C address by connecting the ADDR pin to the VSS, VDD, SDA or SCL
lines This results in I
2
C addresses of 0x5A, 0x5B, 0x5C and 0x5D. The specific details of this system are described in AN3895.
For reference, the register map of the MPR121 is included in Table 2.
