Cypress CE66058 Podręcznik - Strona 2
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The following equation provides the relationship between the C
2
∗ ∗ [
=
Raw counts
In this equation:
N = Resolution
Rb = Bleed resistor (Ω)
Sysclk = Internal main oscillator (IMO) frequency (Hz)
Period = Prescaler period (Period = 1 if prescaler is not used)
REF = Voltage reference set in CSD User Module parameters
C
= Sensor parasitic capacitance (F)
p
From
Equation
1, if N, Rb, Sysclk, and REF have the same value for all the sensors, then the raw count is directly proportional
to the sensor's C
. If the sensor's C
p
to 70 percent of the maximum value to achieve the best sensitivity, tuning one sensor for 60 to 70 percent can yield hugely
different raw counts for the other sensor because of the difference in their C
If this difference is not great, you can use different V
will result in lower sensitivity). However, if the difference in raw counts is very great, then using different V
you to tune the sensors for the same raw counts (60 to 70 percent of the maximum value). For instance, this may happen in a
project where a matrix and buttons are implemented at the same time, and the sensing elements of the matrix show a higher
capacitance compared to a single button's capacitance.
As a result, if a group of sensing elements cannot be properly tuned, even with different values of V
bleed resistors for different sensors.
To find the value of a bleed resistor, use the following equations:
Rbmax =
2
Rbmin =
2
∗[
4∗
In these equations, the bleed resistor, Rb
a raw count value that is 60 percent of the maximum value.
Figure 3
shows a CapSense design in which two bleed resistors, Rb1 and Rb2, of different values are used to tune all the
sensors to 60 to 70 percent of the maximum value.
www.cypress.com
1
] ∗ [
− 1] ∗
1
4∗
+
4
16
value is different, then the raw count value will also differ. As the raw counts must be 60
p
ref
0.70∗2
1
∗[
]∗[
−1]∗
1
4∗
+
4
16
0.60∗2
1
]∗[
−1]∗
1
4 +
16
, gives a raw count value that is 70 percent of the maximum value, and Rb
max
Figure 3. CapSense System with Two Different Bleed Resistors
Rb2
Rb1
Cmod
Document No. 001-66058 Rev.*E
and the raw counts in the CY8C24x94 device:
p
Equation 1
values.
p
values while scanning different sensors (choosing a high value for V
Equation 2
Equation 3
Vdd
CY8C24x94
Dynamically Switch CSD Bleed Resistor
values cannot help
ref
, you must use different
ref
CS0
CS1
CSn
ref
gives
min
2