Cypress CapSense CY8C20x36 Guia do Utilizador Página 48
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48 Document No. 001-64846 Rev. *I Getting Started with CapSense
®
active state (scan all sensors, update all baselines, check if any sensor is in the TOUCH state, and so on), and then
returns to sleep state. The resulting instantaneous current graph is shown in Figure 3-23.
Figure 3-23. Instantaneous Current
I(t)
t
I
Act
I
Slp
t
Act
t
Slp
T
Where:
I(t) = Instantaneous current
I
Act
= Active current
I
Slp
= Sleep current
t
Act
= Active time
t
Slp
= Sleep time
T = Time period of a cycle
The average current consumed by the device over a long period can be calculated by using the following equation.
Equation 24
The average power consumed by the device can be calculated as follows:
Equation 25
3.5.3 Response Time versus Power Consumption
As illustrated in Equation 25, the average power consumption can be reduced by decreasing I
AVE
or V
DD
. I
AVE
may be
decreased by increasing sleep time. Increasing sleep time to a very high value leads to poor response time of the
CapSense button. Because of this tradeoff between response time and power consumption, the application developer
must carefully select the sleep time based on system requirements.
In any application, if both power consumption and response time are important parameters to be considered, then, an
optimized method can be used that incorporates both continuous-scan and sleep-scan modes. In this method, the
device spends most of its time in sleep-scan mode where it scans the sensors and goes to sleep periodically as
explained in the previous section and thereby consuming less power. When you touch a sensor to operate the
system, the device jumps to continuous-scan mode where the sensors are scanned continuously without invoking
sleep and thereby giving very good response time. The device remains in continuous-scan mode for a specified
time-out period. If you do not operate any sensor within this time-out period, the device returns to the sleep-scan
mode.
3.6 Pin Assignments
An effective method to reduce interaction between CapSense sensor traces and communication and non-CapSense
traces is to isolate each by port assignment. Figure 3-24 shows a basic version of this isolation for a 32-pin QFN
package. Because each function is isolated, the CapSense controller is oriented such that there is no crossing of
communication, LED, and sensing traces.
- Getting Started with CapSense 1
- Contents 3
- 1. Introduction 6
- 1.4 Document Conventions 9
- 2. CapSense Technology 10
- 2.1.2 Mutual-Capacitance 11
- 2.3.1 Sensing Methods 12
- CY8C21x34/B 13
- = External Connection 14
- 2.4 CapSense Tuning 15
- 2.4.3 Measuring SNR 16
- 2.4.4.1 What is SmartSense? 17
- 2.5 Sensor Types 19
- 2.5.1.2 Matrix Buttons 20
- 2.5.2.1 Linear Sliders 20
- 2.5.2.2 Diplexed Sliders 21
- Area contacted by finger 22
- Proximity Sensor 22
- 2.6 Sensor Construction 23
- 2.7.1 Visual Feedback 24
- OFFON OFF 26
- 2.7.2 Haptic Feedback 28
- 2.7.3 Audible Feedback 29
- 2.8 Water Tolerance 31
- CapSense Controller 32
- 3. Design Considerations 33
- 3.2 ESD Protection 34
- Non Conductive 35
- Material 35
- 3.2.2 Redirect 36
- 3.2.3 Clamp 36
- 3.3.1.1 Ground Plane 37
- 3.3.1.2 Series Resistor 37
- 3.3.1.3 Trace Length 37
- 3.3.1.4 Current Loop Area 37
- CapSense 38
- Controller 38
- 3.3.2 Radiated Emissions 39
- 3.4 Software Filtering 41
- 3.4.2 IIR Filter 42
- Equation 18 43
- 3.4.3 Median Filter 44
- 3.4.4 Jitter Filter 45
- Unfiltered 46
- Filtered 46
- 3.5 Power Consumption 47
- 3.6 Pin Assignments 48
- 3.7 PCB Layout Guidelines 51
- 3.7.5 Slider Design 52
- 3.7.8 Trace Routing 53
- 3.7.9 Crosstalk Solutions 53
- 3.7.10 Vias 54
- 3.7.11 Ground Plane 54
- Sensor-1 55
- Shield Electrode 56
- Guard Sensor 56
- Sensor Pads 56
- 4.1.1.1 CY8CMBR20xx 57
- 4.1.1.2 CY8C201xx 57
- 4.1.2.1 CY8C20x34, CapSense 57
- 5. CapSense Selector Guide 59
- Design Requirements 60
- 7. Resources 65
- 7.4.5 Evaluation Pods 66
- 7.5 Demonstration Kit 67
- 7.6 PSoC Designer 67
- 7.7 PSoC Programmer 68
- C-to-USB Bridge Kit 69
- 7.10 Design Support 71
- 8. Appendix 72
- Introduced Capacitance, pF 73
- 8.1.4 Design Examples 74
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