Datasheet

CY8C20xx6A/H CapSense Design Guide Doc. No. 001-65973 Rev. *A 28
Figure 4-3. Charge/Discharge Waveforms
V
s
V
x
Vdd
Vref
Vdd
Vref
t
t
R
x
C
x
V
s
V
x
5*R
x
*C
x
5*R
x
*C
x
Ts
min
>= 10*R
x
*C
x
Set the charge/discharge rate to a level that is compatible with this RC time constant. The rule of thumb is to allow a
period of 5RC for each transition, with two transitions per period (one charge, one discharge).The equations for
minimum time period and maximum frequency are:


= 10 × 



Equation 6


=

×



Equation 7
For example, assume the series resistor includes a 560-Ω external resistor and up to 800 Ω of internal resistance,
and the sensor capacitance is typical:
R
X
C
= 1.4 kΩ
X
The value of the time constant and maximum front-end switching frequency in this example would be:
= 24 pF
Ts
min
fs
= 0.34 µs
max
4.1.3 Importance of Baseline Update Threshold Verification
= 3 MHz
Temperature and humidity both cause the average number of counts to drift over time. The baseline is a reference
count level for CapSense measurements that plays an important role in compensating for environmental effects.
High-level decisions, such as Finger Present and Finger Absent states, are based on the reference level established
by the baseline. Because each sensor has unique parasitic capacitance associated with it, each capacitive sensor
has its own baseline.
Baseline tracks the change in counts at a rate set by the Baseline Update Threshold parameter. Make sure to match
the update rate to the intended application. If the update rate is too fast, the baseline will compensate for any
changes introduced by a finger, and the moving finger will not be detected. If the update rate is too slow, relatively
slow environmental changes may be mistaken for fingers. During development, you should verify the Baseline
Update Threshold settings.