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Getting Started with CapSense
®
Document No. 001-64846 Rev. *I 15
Figure 2-8. CSA_EMC Block Diagram
Cx
isensor
Single
Slope ADC
Precharge
Clock
Cmod
High-Z
input
CY8C20x34
Gnd
= External Connection
AMUX Bus
Vref
Rbus
Sw1
Sw2
iDAC
idiff
For a detailed discussion of Cypress‘s CSD and CSA_EMC sensing methods, see the respective device design
guides. Table 5-1 shows the CapSense controller offerings and the sensing method supported for each. Table 5-2
compares these two CapSense sensing technologies in detail.
2.4 CapSense Tuning
Optimal CapSense system performance depends on board layout, button dimensions, overlay material, and
application requirements. These factors are discussed in Design Considerations. In addition to these factors,
switching frequency and threshold levels must be carefully selected for robust and reliable performance. Tuning is the
process of determining the optimum values for these parameters. Tuning is required to maintain high sensitivity to
touch and to compensate for process variations in the sensor board, overlay material, and environmental conditions.
Before we move further, a number of terms need to be defined to understand the tuning process.
2.4.1 Definitions
Raw Count: As seen in Figure 2-9, sensor capacitance is converted into a count value by the CapSense
algorithm. The unprocessed count value is referred to as raw count. Processing of the raw count results in
ON/OFF states for the sensor.
Baseline: The baseline is an estimate of the average sensor count level when the sensor is in the OFF state.
The baseline provides a reference level for the ON/OFF comparison.
Difference Count: Subtracting the baseline level from the raw count produces the difference count that is used
in the ON/OFF decision process. The actual baseline is dynamically adjusted by the user module to compensate
for environmental changes through a process called baseline update.
The thresholds are offset by a constant amount from the baseline level. The thresholds have the following functions:
Noise Threshold: If the difference count is below the noise threshold, then the baseline is updated.
ON Threshold (Finger Threshold + Hysteresis): If the difference count is increasing and exceeds the level of
(Finger Threshold + Hysteresis), then the sensor state changes from OFF to ON.
OFF Threshold (Finger Threshold - Hysteresis): If the difference count is decreasing and drops below the
level of (Finger Threshold - Hysteresis), then the sensor state changes from ON to OFF.
- 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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