Data Sheet

MPR121 Sensor

7 Freescale Semiconductor

3. Electrode Auto-Configuration

Given by a user defined target charge level, MPR121 can automatically run to get an optimized setting of charge current and

charge time for each electrode without knowing the specific capacitance value on the electrode input. This smart auto-

configuration feature greatly reduces the trial and fine tune time during the real system development period. The independent

control and setting for each electrode allow for a great deal of flexibility in electrode pattern design. For information on how to

use the auto-configuration function refer to application note AN3889.

4. Environment Auto-Calibration

MPR121 incorporates auto-calibration function which continually learns the background baseline capacitance of each

individual electrode, so the system only has to program the amount of small change from these baselines that represents a

touch or release. The 3

level filtered result is used as the baseline value representing the capacitance variation of each

electrode over the long term and slow environment change such as atmospheric moisture and dirt. For information on how to

set up the background baseline tracking system refer to application note AN3891 and AN3892.

5. Touch and Release Detection

Touch and release is determined by comparing the immediate capacitance deviation that is the electrode 2

level filtered

output data deviation to the baseline value. If the deviation passed the setting threshold, then a touch or release status is

detected and reported in the status register. The touch threshold and release threshold are independent and individually

programmable for each electrode, providing hysteresis and electrode independence. Debounce setting can be used for further

noise filtering to provide glitch free touch and release detection.

Figure 5. MPR121 Touch and Release Detection

6. Proximity Sensing

MPR121 can be used for near proximity sensing. By internally multiplexing all the sensing inputs so that they are connected

together and all separated touch pads are “combined together” to create a single large electrode covering a larger sensing

area. When enabled, this “13

” proximity detection channel will be included at the beginning of a normal electrode detection

cycle. This 13

proximity detection has its own independent set of configuration registers. This system is described in

application note AN3893.

Run Mode and Stop Mode

After completing power on reset (POR), MPR121 is default in Stop Mode without any capacitance measurement. MPR121

can be set into Run Mode when bit D5~D0 in Electrode Configuration Register (ECR, address 0x5E) are not all zeros

which indicates at least one of the 13 measurement channels is enabled. MPR121 can be set into Stop Mode by writing

2bXX000000 to ECR register.

In Run Mode, MPR121 uses a state machine which wakes up periodically to measure the electrodes, filter and analyze the

data, and determine whether a touch pad has been touched or released. During the waiting period between measurements

MPR121 draws negligible current, so the average current consumption is determined by the frequency and quantity of

measurements channels enabled. MPR121 can be configured for a sampling interval between 1ms and 128ms. The user can

make the tradeoff between noise rejection, touch response time, and power consumption. Refer to application note AN3890

on how to set up these configurations.