MPR121_10 FREESCALE [Freescale Semiconductor, Inc], MPR121_10 Datasheet - Page 11

• Manufacturer Part Number: MPR121_10
• Description: Proximity Capactive touch Senosr controller
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

Sensors
Freescale Semiconductor
AN3889: MPR121 Capacitance Sensing Settings
INTRODUCTION
the pad capacitance. Freescale’s MPR121 utilizes the principle that a capacitor holds a fixed amount of charge at a specific elec-
tric potential. Both the implementation and the configuration will be described in this application note.
CAPACITANCE MEASUREMENT
current I for a time T (the charge time). Before measurement, the electrode input is grounded, so the electrode voltage starts from
0 V and charges up with a slope,
trodes are grounded during this measurement. At the end of time T, the electrode voltage is measured with a 10 bit ADC. The
voltage is inversely proportional to capacitance according to
same rate it was charged.
Touch acquisition takes a few different parts of the system in order to detect touch. The first stage of this process is to capture
The basic measurement technique used by the MPR121 is to charge up the capacitor C on one electrode input with a DC
AFE AQUISITION
1 - 32 μs
DATA
RAW
Equation
1, where C is the pad capacitance on the electrode
Figure 3. Data Flow in the MPR121
1 - 128 μs
FILTER
1st
V
dV
------ -
dt
=
=
I
---------- -
×
C
--- -
C
I
Equation 2
T
FILTER
2nd
.
The electrode is then discharged back to ground at the
4 - 2048 μs
STATUS REGISTER
TOUCH
IRQ
BASELINE
(Figure
FILTER
4). All of the other elec-
Equation 1
Equation 2
MPR121
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