Cypress CapSense AN64846, Getting Started

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Design Considerations
AN64846 - Getting Started with CapSense
®
Doc. No. 001-64846 Rev. *X 50
3.3.1.3.4 Slew Rate Control
Figure 3-16 shows the impact of rise/fall time of a square wave on radiated emissions. Note that slowing the transitions
introduces the cutoff point and damps the radiated-energy level. Internal clock signals of the CapSense controller are
slew-controlled to reduce the radiated emission.
Figure 3-16. Impact of Slew Rate on Emissions
T
t
ON
A
A
m
p
lit
u
d
e
Frequency
At
ON
/T
0 dB/decade
-20 dB/decade
1/(
π
t
ON
)
t
ON
t
r
t
f
A
A/2
T
A
m
p
lit
u
d
e
Frequency
At
ON
/T
0 dB/decade
-20 dB/decade
1/(
π
t
ON
) 1/(
π
t
r
)
-40 dB/decade
0
3.3.1.3.5 Sensor Scan time
The scan time of sensors impacts radiated emissions. Figure 3-17 shows the impact of the sensor-scan time on radiated
emissions. An increase in the sensor-scan time results in increased emissions. Table 3-5 shows the parameter settings
and associated sensor scan times.
Table 3-5. Sensor Scan time
Parameter Value
Scan resolution 8 bits 10 bits
Individual sensor scan time 0.021 ms 0.085 ms
Total scan time for five buttons 0.105 ms 0.425 ms