ML51/ML54/ML56
Sep. 01, 2020
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Manual
6.19.5 Functional Description
When a finger touches on a key pad, capacitance of the key pad sensed by Touch Key controller will
be bigger than it is not touched. The capacitance is measured by Touch Key controller analog front
end circuit, user can read the sensed capacitance value to distinguish if a finger touch event occurs or
not.
There is a sensing threshold control for each channel, it supports keys scanning automatically, until
any threshold setting condition is met. Thus processor can keep its work or sleep state to save power
consumption without interrupt.
The sensitivity level of Touch Key controller is programmable. The key scanning time is programmable
for higher sensitivity or scanning speed.
Any key wake up function and proximity detection are also supported.
In this chapter the letter x within register name denotes channel 0 to 14.
Capacitive Touch Key Fundamentals
The capacitance of the touch key without a finger touch is called as
“parasitic capacitance”, C
P
.
Parasitic capacitance results from the electric field between the touch key (including the sensor pad,
traces, and vias) and other conductors in the system such as the ground planes, traces, any metal in
the product’s chassis or enclosure, etc. The touch key pin and internal capacitances of NuMicro
®
ML56 series also contribute to the parasitic capacitance.
C
F
is known as the finger capacitance. When a finger touches the touch key, the parasitic capacitance
C
P
and finger capacitance C
F
are parallel to each other because both represent the capacitance
between the touch key pin and ground. Therefore, the total capacitance C
T
of the sensor is the sum of
C
P
and C
F
.
C
T
= C
P
+ C
F
The touch key controller converts the capacitance C
T
into equivalent digital counts called raw data.
Because a finger touch increases the total capacitance of the touch key, an increase in raw data
indicates a finger touch.
Capacitive Touch Key Sensing Method
Touch key sensing method as shown in Figure 6.19-2, it implements two switching capacitor banks for
injecting charges to C
P
(or C
T
) and C
R
which is the parasitic capacitance of reference channel.