Datasheet

CY8C21x34/B CapSense
®
Design Guide Document No. 001-66271 Rev. *B 31
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
Make sure to 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:
Equation 5
Equation 6
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
= 1.4 kΩ
C
X
= 24 pF
The value of the time constant and maximum front-end switching frequency in this example would be:
Ts
min
= 0.34 µs
fs
max
= 3 MHz
4.1.3 Importance of Baseline Update Threshold Verification
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 is an important part of 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.