Datasheet

CY8C21x34/B CapSense
®
Design Guide Document No. 001-66271 Rev. *B 39
 The recommended series resistance for communication lines, such as I
2
C, and SPI is 330 Ω.
 Trace Length: Minimize trace length whenever possible.
 Current Loop Area: Minimize the return path for current. Provide hatched ground instead of solid fill within 1 cm
of the sensors and traces to reduce the impact of parasitic capacitance.
 RF Source Location: Partition systems with noise sources such as LCD inverters and switched-mode power
supplies (SMPS) to keep them separated from CapSense inputs. Shielding the power supply is another common
technique for preventing interference.
5.3.2 Radiated Emissions
To reduce radiated emissions from the CapSense sensor, select a low frequency for the switched capacitor clock.
This clock is controlled in firmware using the Prescaler option. Increasing the Prescaler value decreases the
frequency of the switching clock
5.3.3 Conducted Immunity and Emissions
Noise entering a system through interconnections with other systems is referred to as conducted noise. These
interconnections include power and communication lines. Because CapSense controllers are low-power devices,
conducted emissions must be avoided. The following guidelines will help reduce conducted emission and immunity:
 Use decoupling capacitors as recommended by the datasheet.
 Add a bidirectional filteron the input to the system power supply. This is effective for both conducted emissions
and immunity. A pi-filter can prevent power-supply noise from effecting sensitive parts, while also preventing the
switching noise of the part itself from coupling back onto the power planes.
 If the CapSense controller PCB is connected to the power supply by a cable, minimize the cable length and
consider using a shielded cable.
 You can place a ferrite bead around the power supply or communication lines to filter out high frequency noise.
5.4 Software Filtering
Software filters are one of the techniques for dealing with high levels of system noise. Table 5-3 lists the types of
filters that have been found useful for CapSense.
Table 5-3. Table of CapSense Filters
Type Description Application
Average Finite impulse response filter (no feedback) with
equally weighted coefficients
Periodic noise from power supplies
IIR Infinite impulse response filter (feedback) with a
step response similar to an RC filter
High-frequency white noise (1/f noise)
Median Nonlinear filter that computes median input value
from a buffer of size N
Noise spikes from motors and switching power supplies
Jitter Nonlinear filter that limits current input based on
previous input
Noise from thick overlay (SNR < 5:1), especially useful for
slider centroid data
Event-Based Nonlinear filter that causes a predefined
response to a pattern observed in the sensor
data
Commonly used during nontouch events to block CapSense
data transmission
Rule-Based Nonlinear filter that causes a predefined
response to a pattern observed in the sensor
data
Commonly used during normal operation of the touch surface
to respond to special scenarios such as accidental
multibutton selection
A code example that uses all the above listed filters can be downloaded here.