STC micro STC8A8K64D4 Series Reference Manual page 893

STC8A8K64D4 Series Manual
Appendix S PCB design guidance for touch keys
The touch key has strict requirements on PCB design, otherwise its effect will be greatly reduced or even fail.It is
recommended that users follow the following principles when designing PCB:
1. Follow the basic principles of common digital-analog hybrid circuit design.
The capacitive touch button module integrates an analog circuit for precise capacitance measurement, so it
should be treated as an independent analog circuit when designing the PCB. Follow the basic principles of
common digital-analog hybrid circuit design.
2. Use star grounding
The ground wire of the touch chip should not be shared with other circuits. It should be connected to the ground
point of the board's power input separately, which is usually called "star grounding".
3. The impact of noise generated on the power supply on the touch chip
The power ripple and noise should be as small as possible. It is best to use an independent trace to take power
from the power supply point of the board and add filtering measures. Do not share the power circuit with other
circuits.
4. The connection between the IC and the sensing board should be as long as possible, so that it has an
approximate distributed capacitance, as shown in the figure below.
5. The size and gap of the key induction plate (capacitive sensor)
In the case of meeting the aesthetic design requirements of the panel, the optimal touch sensing effect must be
obtained through a reasonable arrangement of the size of the sensing plate and the interval size. The induction
plate is placed on the bottom layer, and the IC is also placed on the bottom layer. There should be no vias
between the induction plate and the IC. The distance between the edges of adjacent sensing plates is preferably
at least 1.5mm (dimension D in the figure below). If the PCB area allows, try to use a larger distance. The
distance between the copper paving and the induction plate is 0.5mm (dimension E in the figure below).
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