CapSense Performance Tuning with User Modules
CY8C20xx7/S CapSense
®
Design Guide
Doc. No. 001-78329 Rev. *E
44
This means that, when a CapSense sensor is activated, firmware produces the sensor ON status within 7.35 ms (the
sensor should be active for debounce number of consecutive scans). This is often referred to as the response time of
a CapSense system.
If the scan time varies with respect to the parasitic capacitance to maintain consistent, what is the impact on
response time if the parasitic capacitance of the sensor changes because of the process variation? Response time
may be increased (slow response) in this case. This can have a negative impact on sensor performance. Guidelines
to build a robust firmware design are provided in the next section.
4.3.8 Method to Ensure Minimum SNR Using the SmartSense_EMCPLUS UM
SmartSense_EMCPLUS is an advanced electromagnetic compliance design of a CSD-based SmartSense user
module that does not require a tedious tuning process. However, there are two simple steps to ensure robustness of
the design while using the SmartSense_EMCPLUS UM.
1. Set up a real-time monitoring tool to monitor the CapSense user module parameters to measure sensor signals.
The
sensor
raw
count
(SmartSense_EMCPLUS_waSnsResult),
sensor
normalized
signal
(SmartSense_EMCPLUS_baSnsSignal),
and
sensor
finger
threshold
(SmartSense_EMCPLUS_baBtnFThreshold) must be observed during the tuning process. Do not use the LCD
or any other numerical displays to monitor data because they are slow and do not allow visualizing the data
dynamics. Recommended data monitoring tools are multi-chart or the I
2
C USB Bridge Control Panel.
2. Set the sensitivity level to 0.4 pF (Low), and calculate the SNR.
shows a typical raw count graph with
a finger touch. According to CapSense best practices, SNR for a robust design should be greater than 5:1. If
measured SNR is more than 10:1, reduce sensitivity level value to the next possible step until SNR is more than
5:1 and less than 10:1.
Figure 4-6. Raw Count Graph for a Typical Sensor with a Finger Touch
3. If you are using the automatic finger threshold feature in the design, the process is complete after the previous
step. If you are using the flexible finger threshold feature, you should also set individual finger thresholds for
each
sensor
to
complete
the
process.
To
set
finger
threshold,
monitor
sensor
signal
(SmartSense_EMCPLUS_baSnsSignal) and set finger threshold value to roughly 80 percent of the sensor signal
value when the sensor is touched. This completes the process.
shows a typical sensor signal and
finger threshold value.
