Datasheet

CY8C21x34/B CapSense
®
Design Guide Document No. 001-66271 Rev. *B 47
A guard sensor is a copper trace, as shown in Figure 6-5, that surrounds all the sensors on the PCB, which is used to
detect the presence of a continuous water stream. When a water stream is present on the sensing surface, a large-
capacitance C
ST
is added to the system, as shown in Figure 6-6. This capacitance may be several times larger than
C
WD
. Because of this, the effect of the shield electrode is completely masked and the raw counts measured by the
sensor will be the same as or even higher than a finger touch. In this situation, a guard sensor will help; when it
detects a water stream, it will block the other sensors from triggering.
Figure 6-6. Capacitance Measurement with Water Stream
 C
WD
- Capacitance between the water stream and shield electrode.
 C
ST
- Capacitance between the water stream and the system ground.
 C
WG
- Capacitance between the water stream and the guard sensor
The guard sensor should be implemented in firmware. One CapSense pin and a counter (hardware/software) are
required to implement the guard sensor.
When a water stream is applied to the board, the guard sensor detects this event and disables the touch sensor
processing logic. Additional guard sensor "dead" time prevents unlocking the sensor prematurely. When the water
stream is gone, the guard counter suppresses touch processing for a short time (Guard Sensor counter). This
eliminates false touch detection from any water that remains on the board.
Figure 6-7
is a flowchart that shows how the guard sensor should be implemented in firmware.