Design Considerations
CY8C20xx7/S CapSense
®
Design Guide
Doc. No. 001-78329 Rev. *E
55
the amplitude of the GPIO load transient-noise to increase. Therefore, it is recommended to minimize the sensor
C
P
by following the layout guidelines mentioned in the
Reduce LED sink current
The GPIO load transient noise is directly proportional to the LED sink current. It is recommended to keep the
LED sink current within the limits as specified in the
. If the GPIO has to sink a current, which is
more than the maximum value specified in datasheet, use an external transistor or a driver IC.
Select appropriate pins for LED
All CapSense controllers provide high-current sink- and source-capable port pins. When using high-current sink
or source from port pins, you should use the ports recommended in
5.7.2 Firmware Guidelines to Compensate GPIO Load Transient Noise
To prevent sensor false triggers due to GPIO load transients, the sensor baseline can be updated using rule-based
algorithms. One of the methods to compensate the baseline is explained here.
shows a condition in which false triggers is seen due to GPIO load transient.
1. At instant 1, there is no finger on the sensor and the LED is in the OFF condition.
2. At instant 2, a finger is on the sensor and the shift in raw count is greater than the finger threshold.
3. Because the shift in raw count is greater than the finger threshold, the LED is turned ON at instant 3.
4. When the LED is turned ON, because of GPIO load-transient, the raw count further shifts.
5. At instant 4, even if the finger is removed, the raw count will not return to initial value because of the shift in the
raw count due to the GPIO load-transient. If this shift is greater than the finger threshold, the LED will remain ON
permanently indicating a sensor false-trigger.
To prevent the sensor and the LED from remaining in ON condition permanently, the sensor baseline should be
compensated, which is explained in the below steps.