Cypress CY8C20xx7/S CapSense Series, Design Manual

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CapSense Performance Tuning with User Modules
CY8C20xx7/S CapSense
®
Design Guide Doc. No. 001-78329 Rev. *E 39
higher SNR at the expense of longer scan time and more power consumption. Table 4-3 shows the actual scan time
in microseconds for a single sensor based on resolution and scanning speed.
Table 4-3. Scan Time for a Single Sensor in µs 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
4.2.10 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. 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 CSDPLUS_waSnsBaseline[], CSDPLUS_waSnsResult[], and CSDPLUS_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 4-5 shows the typical raw counts obtained from a sensor during a finger activation cycle; that is, the sensor is
activated and 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 CapSense design
best practice, the 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
Getting Started with
CapSense to raise SNR to at least 5:1, or both.
Figure 4-5. Typical Raw Counts from a Sensor During Finger Activation Cycle
Noise
Signal