Cypress CapSense CY8C20x36 Betriebsanweisung PDF herunterladen (Seite 53)

Getting Started with CapSense

Document No. 001-64846 Rev. *I 53

3.7.8 Trace Routing

Route sensor traces on the bottom layer of the PCB, so that the only user interaction with the CapSense sensors is

with the active sensing area. Do not route traces directly under any sensor pad unless the trace is connected to that

sensor.

Do not run capacitive sensing traces in close proximity to communication lines, such as I

C or SPI masters. If it is

necessary to cross communication lines with sensor pins, make sure the intersection is at right angles, as illustrated

in Figure 3-31.

Figure 3-31. Routing of Sensing and Communication Lines

3.7.9 Crosstalk Solutions

A common backlighting technique for panels is to mount an LED under the sensor pad so that it shines through a

hole in the middle of the sensor. When the LED is switched on or off, the voltage transitions on the trace that drives

the LED can couple into the capacitive sensor input, creating noisy sensor data. This coupling is referred to as

crosstalk. To prevent crosstalk, isolate CapSense and non-CapSense traces from one another. In the case of

CY8C21x34/B, the crosstalk can also occur due to the coupling of the LED voltage transitions with the R

resistor. To

avoid this, isolate the R

trace from the non-CapSense traces. A minimum separation of 4 mm is recommended. A

hatched ground plane also can be placed between those traces to isolate them. LED drive traces and CapSense

traces (including R

trace) should not be routed together.

Figure 3-32. Not Recommended - LED and CapSense in Close Proximity

Figure 3-33. Recommended - LED and CapSense with Wide Separation

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Cypress CapSense CY8C20x36 Betriebsanweisung Seite 53