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 36
Figure 4-3. Tuning the CSD/CSDPLUS User Module
Measure C
p
of sensors
using EP64134
Start
If max C
p
< 45 pF set
Idac Rang e to 4X.
Otherwise set to 8X.
Enable Autocalibration
Set Precharge Source to
PRS
Set Prescaler according
to Table 4-1
Set Resolution according
to Figure 29 and Table 4-2
Set Scanning Speed to
Fast
Set PRS Resolution to
12 bits if scan time >
380 µs according to
Table 4-3. Otherwise set
to 8 bits.
Set Shield
Electrode Out
according to needs
of design
Establish digital
communication interface and
obtain raw counts from each
sensor during finger
activation cycle, for example.
Figure 30
Is SNR from all sensors
> 5:1?
Increase Resolution and/or
experiment with Scanning
Speed until optimal SNR is
obtained
Is SNR from all sensors
> 5:1?
Revise PCB design
according to AN2292 and
AN2318
No
No
Does scan time meet
design requirements?
Yes
Yes
Reduce Resolution or
use faster Scanning
Speed
No
Yes
Set Finger Threshold
to 80% of signal
Set Noise Threshold
to 40% of signal
Set BaselineUpdate
Threshold to double
the Noise Threshold
Set Sensors
Autoreset according to
needs of design
Set Hysteresis to 10%
of signal
Set Debounce
according to needs of
design
Set Negative Noise
Threshold to same
value as Noise
Threshold
Set Low Baseline
Reset to 10
End
Hardware
Parameters
High-Level API
Parameters
Hardware parameters configure the hardware that the CSD/CSDPLUS method uses to convert the physical
capacitance of each sensor into a digital code. This section describes these parameters and provides guidance about
how each should be tuned based on system characteristics and other parameters.
By default, hardware parameters are global settings that apply to all CapSense sensors in a design. In designs where
total parasitic capacitance of each sensor (C
P
), sensor sensitivity, or both, vary over a wide range, there may not be
global hardware parameter settings that are suitable for all sensors. In such cases, the SetIdacValue(i),
SetPrescaler(i), and SetScanMode(i) API functions can be used to configure the respective hardware parameters for
each sensor, where (i) is the sensor index before calling the ScanSensor(i) API function.
Table 4-1 and Table 4-3 provide tuning recommendations for several key hardware parameters based on sensor C
P
.
C
P
values depend on the characteristics of the PSoC, PCB layout, and proximity of other components in the
assembled product. Because of this, C
P
must be measured in its original position with the system in its final
assembled state; that is, in the same enclosure and with the same overlay as the system will have in service. The
best way to measure C
P
is to use the code example "Measuring Absolute Sensor Capacitance with a CY8C20xx6A
CapSense Controller" available in the CapSense Controller Code Examples Design Guide . This project measures the
absolute capacitance of each sensor in a system using the PSoC itself, thus taking into account all factors affecting
C
P
. See the documentation associated with the code example for instructions on its setup and use.
4.2.1 Recommended C
MOD
Value for CSD/CSDPLUS
The recommended C
MOD
value for a CSD-based design is 2.2 nF . X7R or NPO type capacitors are recommended for
C
MOD
stability over temperature and the capacitor should have a voltage rating not less than 5 V.