Datasheet

CY8C20xx6A/H CapSense Design Guide Doc. No. 001-65973 Rev. *A 35
Table 4-3. Scan Time for a Single Sensor in цs Based on Resolution and Scanning Speed
Resolution
(bits)
Scanning Speed
Ultra Fast Fast Normal Slow
9 57 78 125 205
10 78 125 205 380
11 125 205 380 720
12 205 380 720 1400
13 380 720 1400 2800
14 720 1400 2800 5600
15 1400 2800 5600 11000
16 2800 5600 11000 22000
4.3.1 High-Level API Parameters
High-level API parameters determine the behavior of high-level firmware algorithms that discriminate between sensor
activations and noise, and compensate for signal drift caused by environmental conditions. In order to determine
proper values for these parameters, you must establish a digital communication interface with the system to monitor
raw counts, baseline, and difference counts during a finger activation event for each sensor. This data is stored in
arrays named CSD_waSnsBaseline[], CSD_waSnsResult[], and CSD_waSnsDiff[], respectively. The high-level API
parameter settings are based primarily on ambient noise and finger signal strength, as indicated by this data. Noise
and signal strength depend on EMI environment, PCB layout, overlay thickness, and other physical characteristics of
the system. Therefore, the data used as the basis for setting these parameters must be taken in its original position
with the system in its final assembled state and in the same EMI environment as will exist in use.
Figure 4-8 shows the typical raw counts obtained from a sensor during a finger activation cycle; that is, the sensor is
activated then deactivated. Labels are superimposed over the data that indicate how noise and signal are to be
calculated based on the raw data. Where appropriate, the high-level parameter descriptions that follow include
information about how to set each parameter based on these noise and signal values. According to AN2403, the ratio
of signal to noise (SNR) must be at least 5:1 for robust CapSense system operation. If SNR is less than 5:1, the
hardware parameters must be adjusted, the PCB layout changed according to the guidelines of AN2292 and AN2318
to raise SNR to at least 5:1, or both.
Figure 4-8. Typical Raw Counts from a Sensor During Finger Activation Cycle
Noise
Signal