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Capacitance Sensing - Layout Guidelines for PSoC CapSense
 

Capacitance Sensing - Layout Guidelines for PSoC CapSense

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Adopting capacitive sensing as an interface technology in high-volume, high-visibility applications such as portable media players and mobile handsets has created demand for the same technology in ...

Adopting capacitive sensing as an interface technology in high-volume, high-visibility applications such as portable media players and mobile handsets has created demand for the same technology in more conventional consumer electronics. This demand has led to significant innovation and several competitive technologies are available. The PSoC architecture allows designers to incorporate multiple capacitive sensing design elements into an application. Buttons, sliders, touchpads, and proximity detectors are supported simultaneously with the same device in the same circuit. Use PSoC to scan capacitive sensors and use the activation status to drive LEDs, control a motor, drive a speaker, and so on. A concept called dynamic reconfiguration allows the CapSense application to use more than 100% of the system resources by reconfiguring as needed on-the-fly. With so many potential applications, design rules must function more as guidelines. This document describes some layout and system design guidelines for PSoC CapSense.

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    Capacitance Sensing - Layout Guidelines for PSoC CapSense Capacitance Sensing - Layout Guidelines for PSoC CapSense Document Transcript

    • Capacitance Sensing—Layout Guidelines for PSoC CapSense AN2292 Authors: Mark Lee Associated Project: No Associated Part Family: CY8C21x34, CY8C24794 GET FREE SAMPLES HERE Software Version: None Associated Application Notes: AN2233a, AN2277, AN2318, AN2403 Application Note Abstract This application note describes layout guidelines for PSoC® CapSense™. Figure 1. Application with CapSensePlus—Motor, LED, Introduction and Speaker with a Single PSoC Adopting capacitive sensing as an interface technology in high-volume, high-visibility applications such as portable media players and mobile handsets has created demand for the same technology in more conventional consumer electronics. This demand has led to significant innovation and several competitive technologies are available. The PSoC architecture allows designers to incorporate multiple capacitive sensing design elements into an application. Buttons, sliders, touchpads, and proximity detectors are supported simultaneously with the same device in the same circuit. Use PSoC to scan capacitive sensors and use the activation status to drive LEDs, control a motor, drive a speaker, and so on, as illustrated in Figure 1. A concept called dynamic reconfiguration allows the CapSense application to use more than 100% of the system resources by reconfiguring as needed on-the-fly. PSoC Placement With so many potential applications, design rules must It is good practice to minimize the distance between PSoC function more as guidelines. This document describes and the sensors. This is especially true for thin flex some layout and system design guidelines for PSoC circuits. The PSoC is usually mounted on the bottom layer CapSense. along with other components and the CapSense sensor pads are placed on the top layer. The PSoC is usually PCB Guidelines centered relative to the sensors so that the parasitic capacitance is balanced among the sensors. In the typical CapSense application, the capacitive sensors are formed by the traces of a printed circuit board Board Layers (PCB) or flex circuit. This section contains PCB guidelines for CapSense. The most common application involves a two layer board with sensor pads and hatched ground plane on top and everything else on the bottom. The two layer stack up is shown in Figure 2. Four layer boards are used when board area must be minimized. January 11, 2008 Document No. 001-41439 Rev. *A 1 [+] Feedback
    • AN2292 Figure 2. Two Layer Stack Up for CapSense Boards Figure 3. Multi Layer Treatment of Sensing and Communication Lines COM COM Board Thickness FR4-based designs are found to perform well with B ad G ood standard board thickness ranging from 0.020quot; (0.5 mm) to 0.063quot; (1.6 mm). Flex circuits work well with CapSense. All guidelines PSoC Pin Assignment presented for PCBs also apply to flex. A flex circuit is An effective method to reduce the interaction between typically much thinner than a PCB. The flex circuit is communication and sensor traces is to isolate each by ideally no thinner than 0.01quot; (0.25 mm). One good feature port assignment. Figure 4 shows a basic version of this of flex is the high breakdown voltage provided by the isolation for a 32-pin QFN package. Because each Kapton material (290 KV/mm), which provides built in ESD function is isolated, the PSoC is oriented such that there is protection for the CapSense sensors. no crossing for communication and sensing traces. Trace Length and Width Figure 4. Port Isolation for Communication and Sensing The parasitic capacitance of the traces and sensor pad is T o S ensors minimized to make the dynamic range of the system as large as possible. Trace capacitance is minimized by short and narrow traces. Traces must be less than 12quot; (300 mm) for a standard PCB and less than 2quot; (50 mm) on flex To Sensors To Sensors circuits. Trace width also plays a role in the parasitic capacitance. Decreasing the trace width decreases the parasitic capacitance. Trace widths of 0.0065quot;–0.008quot; (0.17–0.20 mm) suffice for most applications. Trace Routing T o C om m unication Route sensor traces on the bottom layer of the PCB. With Vias this approach to routing, the only user interaction with the CapSense sensors is with the active sensing area and not Use the minimum number of vias consistent with routing of with the traces to the sensor. Do not route traces directly the CapSense inputs to minimize parasitic capacitance. under any sensor pad unless the trace is connected to that The placement of the via is done anywhere on the sensor sensor. pad, as shown in Figure 5. Do not run capacitive sensing traces in close proximity Figure 5. Via to Sensor Pad is Anywhere on the Pad with and parallel to high frequency communication lines, (Trace on Bottom Layer, Sensor Pad on Top Layer) such as an I2C or SPI master. If it is necessary to cross communication lines with sensor pins, make sure the intersection is at right angles, as illustrated in Figure 3. January 11, 2008 Document No. 001-41439 Rev. *A 2 [+] Feedback
    • AN2292 ground fill area separates the traces of each button group. Ground Plane This prevents coupling between the independent Ground fill is added to both the top and bottom of the CapSense groups. sensing board. When ground fill is added near a CapSense sensor pad, there is a tradeoff between Buttons maintaining a high level of CapSense signal and increasing noise immunity of the system. Typical hatching A button determines the presence or absence of a for the ground fill is 15% on the top layer (7 mil line, 45 mil conductive object. A typical application of a CapSense spacing) and 10% on the bottom layer (7 mil line, 70 mil button is to sense the presence of a finger. spacing), as shown in Figure 6. Sensor Shape Figure 6. Partial Ground Fill to Minimize Parasitic The recommended shape for sensing a finger press is a Capacitance solid round pattern as shown in Figure 7. Figure 7. Large Button with Ground Plane The Buttons section discusses how the clearance between the sensor pad and ground affects capacitance and sensitivity. Other Board Considerations Programming Pins Sensing lines in capacitive sensing applications must be Figure 8 shows the shape recommendations for buttons. A connected to sensors only. Sensing lines that are attached square or rectangular button works if the layout does not to other board elements, such as ISSP programming support a round shape. Buttons should not be triangular or headers, are more sensitive to external noise and have a include other pointed features with angles less than 90 higher parasitic capacitance due to increased surface area degrees. Interdigitated sensor traces do not work well for of the conductive path. Avoid placing sensors on the CapSense buttons implemented with the CSD and CSA programming pins, P1[0] and P1[1]. sensing methods. EMC Figure 8. Shape Recommendations for Buttons. Resistors placed in series with the CapSense input dampen the resonance of each trace. This is an effective way to increase the RF immunity of the system. These series resistors must be placed close to the PSoC to be effective against RF interference. The recommended series resistance added to the CapSense inputs is 560 ohms. Communication lines, I2C, and SPI, also benefit from series resistance. 300 ohms is the recommended series resistance for communication lines. CapSense sensors and their associated traces are Sensor Size isolated from other circuit elements. This is especially true of antennae and other signal sending and receiving All things being equal, larger buttons are typically better. elements. Refer to application note AN2318, EMC Design Two buttons connected to the PSoC with identical traces Considerations for PSoC CapSense(TM) Applications for have different sensitivities if they are different in size. information on this topic. Clearance Between Sensor and Ground LED Backlighting The ground plane is placed on the same layer of the board CapSense works well with LED backlighting. Cut a hole in as the buttons as shown in Figure 7. The clearance the sensor pad; keep LED traces on the bottom layer of between the button and ground plane plays an important the board. role in the performance of the button. Electric field lines Multiple PSoCs on One PCB fringing between a button and the ground plane are illustrated in Figure 9. The parasitic capacitance of the For systems with many buttons, such as a keyboard, the sensor, Cp, is related to this electric field. system design may require two or more PSoCs dedicated to CapSense. In this case, partition buttons so that a January 11, 2008 Document No. 001-41439 Rev. *A 3 [+] Feedback
    • AN2292 Figure 9. Button-Ground Plane Fringing Fields Figure 11. Sensor Capacitance, Csensor, as a Function of Button-Ground Clearance and Button Diameter B utton G round G round The capacitance Cp decreases as the clearance surrounding the button is increased. An example of this dependence of Cp on the gap is shown in Figure 10 through Figure 13. In these plots, the board material is FR4 with a thickness of 62 mils (1.57 mm), and the acrylic overlay has a thickness of 2 mm. Each plot contains data for three button sizes (5 mm, 10 mm, and 15 mm diameter). The Cp in Figure 10 does not include the effect of the traces or vias. It is only the parasitic capacitance of the sensor pad itself. Figure 10. Parasitic Capacitance, Cp, as a Function of Button-Ground Clearance and Button Diameter Note The finger is not on the sensor. Sensor capacitance decreases with the size of the gap. The capacitance Cf in Figure 12 is the capacitance added by the touch of the finger. Total capacitance of the sensor pad and the finger is Cp + Cf. Figure 12. Finger Capacitance, Cf, as a Function of Button-Ground Clearance and Button Diameter. Note The finger is not on the sensor. Capacitance increases with sensor size, but decreases with the gap. The capacitance Csensor is the total sensor capacitance when the finger is not on the sensor. It includes the effect of the sensor pad, the traces, and vias. Figure 11 shows the sensor capacitance for a board routed with 50 mm trace length, 8 mil (0.3 mm) trace width, and 20 mil (0.8 mm) spacing from trace to the co-planar ground. Note The finger is on the sensor. Capacitance increases with both sensor size and gap. January 11, 2008 Document No. 001-41439 Rev. *A 4 [+] Feedback
    • AN2292 Figure 13 plots finger capacitance as a percentage of the Slider Segment Size and Spacing sensor capacitance. This is the sensitivity of the sensor. When the finger is placed on the slider, a signal must be The sensitivity of the system changes with the routing of produced on at least three adjacent sensor segments for the CapSense traces. For example, increasing the trace proper operation of the centroid algorithm. This guides the length between the PSoC and the sensor pad decreases sizing of the slider segments. The jagged edges of the the button’s sensitivity. slider pattern enable more segments to be active at each finger location along the slider. Figure 13. Sensitivity, Cf,/Csensor as a Function of Button- Ground Clearance and Button Diameter As with buttons, the total surface area of each slider element influences the signal. Bigger slider segments lead to a higher signal level. The same guidelines given for button to ground clearance apply to sliders. Slider segments are scanned one at a time, with all other CapSense inputs grounded. Neighboring segments present ground to the segment being scanned, so the segment to segment gap must be the same as the gap from segment to ground. Figure 14. CapSense Slider Note The sensitivity increases with both button size and gap. The CapSense signal results from converting the sensor capacitance changes into digital count values. How large should the signal be? This depends on the noise in the system. The signal and noise levels both vary with the operating point established by firmware parameters. What is important for proper performance of a CapSense button is a large Signal-to-Noise ratio (SNR). The minimum Slider Diplexing recommended SNR for CapSense buttons is 5:1. Refer to If IO pins are at a premium, connecting two slider application note AN2403, Signal-to-Noise Ratio elements to a single PSoC pin increases the number of Requirement for CapSense Applications for more details slider elements that are sensed by the PSoC (and thus the on CapSense SNR. linear distance) two-fold. This technique is called diplexing. This option is selected in the User Module Sliders Wizard. However, the pin assignment for elements in the slider is prescribed by a diplexing table. An example of a Sliders are used for controls requiring gradual diplexed slider with 12 slider elements (6 PSoC input pins) adjustments. Examples include a lighting control (dimmer), is shown in Figure 15. volume control, and speed control. The CapSense sensors in a slider are mechanically adjacent to one Figure 15. Diplexed Slider Basic Example (Six Pins) another. Actuation of one sensor results in partial actuation of physically adjacent sensors. The actual position in the slider is found by computing the centroid location of the set of activated sensors as shown in Figure 14. The practical lower limit number for sensor sliders is five. The upper limit is the number of sensor pins available on the selected PSoC device. 0 1 2 3 4 5 0 3 1 4 2 5 Large parasitic capacitance is a negative feature of diplexing. The parasitic capacitance approximately doubles with diplexing and this technique is not recommended for use with thick overlays. January 11, 2008 Document No. 001-41439 Rev. *A 5 [+] Feedback
    • AN2292 Table 1 shows some basic diplexing tables. Touchpads Table 1. Common Diplexing Tables The CapSense User Module does not directly support touchpads. Touchpads are implemented as two 6 Pins, 8 Pins, 10 Pins, independent sliders. All guidelines for sliders also apply to 12 Elements 16 Elements 20 Elements touchpads. 0 0 0 Figure 18. Touchpads Use Two CapSense Sliders (for X 1 1 1 and Y) 2 2 2 3 3 3 4 4 4 5 5 5 0 6 6 3 7 7 1 0 8 4 3 9 2 6 0 5 1 3 4 6 Touchpad Resolution 7 9 An example of a commercially successful CapSense 2 1 touchpad design implements a 20 position column slider 5 4 (X) and a 10 position row slider (Y). A total of 30 pins are assigned as CapSense inputs. The dimensions of the 7 active area are 3.9quot; x 1.9quot; (99 mm x 47 mm). The overlay 2 is 0.010quot; (0.25 mm) ABS plastic. The row and column 5 sensors are spaced with a pitch of 0.2quot; (5 mm). The baseline noise level is a single count in the finger-absent 8 state. A finger on the touchpad produces a difference Figure 16. Diplex Slider Data from Finger Press on Left signal of 15 counts, which results in an SNR of 15:1. Side of Slider Setting the centroid algorithm to resolve 20 positions between each row pair and each column pair, this touchpad system has a resolution of 100 counts per inch. Touchpad Layout Patterns The CapSense User Module does not directly support touchpads. Touchpads are implemented as two independent sliders. All the guidelines that apply to sliders also apply to touchpads. Two example touchpad layouts are shown in Figure 19 and Figure 20. Figure 17. Diplex Slider Data from Finger Press on Right Side of Slider Figure 16 and Figure 17 represent the data collected by the PSoC relative to finger position. The User Module firmware determines finger position by analyzing the pattern of the signals in the slider array. January 11, 2008 Document No. 001-41439 Rev. *A 6 [+] Feedback
    • AN2292 Figure 19. Hexagon Touchpad Layout The geometry of this simple system is captured in the ratio A/d. A is the area of the conductive plates, d is the distance between the plates, r is the dielectric constant Y1 (permittivity) of the material between the sensors, and 0 is the permittivity of free space. Y2 Y3 The geometry of the CapSense system is more complex than the parallel plate capacitor. The conductors in the Y4 sensor include the finger and PCB copper. In general, the Y5 geometry of this capacitive system is captured by the function f(A,d). Equation 2 states the relation between geometry, the dielectric constant, and the system X1 X2 X3 capacitance. C   r  0 f ( A, d ) Equation 2 Figure 20. Octagon Touchpad Layout Similar to the parallel plate capacitor, the finger capacitance of the sensor is directly proportional to r. High dielectric constants lead to high sensitivity. Air has the lowest dielectric constant, and any air gaps between the sensor pad and overlay must be eliminated. Y1 Dielectric constants of some common overlay materials Y2 are listed in Table 2. Materials with dielectrics between 2.0 and 8.0 are well suited to capacitive sensing applications. Sensing through a metal surface is challenging and is not Y3 recommended. X1 X2 X3 Table 2. Dielectric Constants of Common Materials r In both layouts and for touchpads in general, it is good Material practice to surround the touchpad with a ground plane that Air 1.0 follows the contours of the sensing elements. Formica 4.6–4.9 Proximity Sensors Glass (Standard) 7.6–8.0 Glass (Ceramic) 6.0 A proximity sensor is implemented as a CapSense button with large CP and small difference counts. A dedicated PET Film (Mylar®) 3.2 proximity sensor is best implemented as a single length of Polycarbonate (Lexan®) 2.9–3.0 wire. Connecting button and slider sensors already on the Acrylic (Plexiglass®) 2.8 CapSense PCB into a single large sensor is another technique of implementing a proximity sensor. CSD is the ABS 2.4–4.1 best method to use for proximity sensing. CSD performs Wood Table and Desktop 1.2–2.5 better than CSA with large CP values and the shield Gypsum (Drywall) 2.5–6.0 feature of CSD is used to extend the detection distance of the sense wire. Protective Overlay An overlay is the covering that is placed over the sensor pads of the sensor PCB. The overlay material and thickness are both important design considerations. Overlay Material The influence of the dielectric properties of the overlay material on system performance is understood by considering a simple parallel plate capacitor. The capacitance of a parallel plate capacitor is given in Equation 1.  r 0 A C Equation 1 d January 11, 2008 Document No. 001-41439 Rev. *A 7 [+] Feedback
    • AN2292 Table 3. Maximum Overlay Thickness with a Plastic Overlay Thickness Overlay Material Sensitivity is inversely proportional to overlay thickness, as  illustrated in Figure 21. Refer to the Buttons section of this Design Element document for a specific example of this characteristic. Button <5 mm Figure 21. Sensitivity vs.Overlay Thickness Slider <2 mm Touchpad <0.5 mm The overlay must be thick enough to prevent dielectric breakdown due to electrostatic voltage on the human body. Table 4 shows the minimum overlay thickness required to withstand 12 KV for common overlay materials. The overlay in the CapSense system protects the PSoC from permanent damage when the thickness guidelines of the table are followed. A layer of Kapton tape works well in applications requiring extra ESD protection. Table 4. Breakdown Voltage of Overlay Materials and Minimum Thickness to Prevent Breakdown Minimum Overlay Breakdown Material Thickness at Voltage [V/mm] 12 KV [mm] Air 1200–2800 10 Glass–Common 7900 1.5 Both signal and noise are affected by the overlay properties. As thickness of the overlay increases, signal Glass–Borosilicate 13,000 0.9 and noise both decrease. A representative plot of (Pyrex) CapSense signal versus overlay thickness is shown in Formica 18,000 0.7 Figure 22. ABS 16,000 0.8 Figure 22. Signal Level Drops as Overlay Thickness Acrylic 13,000 0.9 Increases (Plexiglass®) Polycarbonate 16,000 0.8 (Lexan®) PET Film (Mylar®) 280,000 0.04 Polyimide Film 290,000 0.04 (Kapton®) FR–4 28,000 0.4 Wood–Dry 3900 3 Overlay Adhesives Overlay materials must have good mechanical contact with the sensing PCB. This is achieved using a nonconductive adhesive film. This film increases the Table 3 lists the recommended maximum overlay sensitivity of the system by eliminating any air gaps between overlay and the sensor pads. 3M™ makes a high thicknesses for PSoC CapSense applications (plastic overlay). As stated previously, the dielectric constant plays performance acrylic adhesive called 200MP that is widely a role in the guideline for maximum thickness of the used in CapSense applications in the form of adhesive overlay. Common glass has a dielectric constant around εr transfer tape (product numbers 467MP and 468MP). = 8, while plastic is around εr = 2.5. The ratio of εr/2.5 is an CapSense Operation with a Gloved Hand estimate of the overlay thickness relative to plastic for the same level of sensitivity. Using this rule of thumb, a If the sensors must work with a gloved hand, add the common glass overlay is about three times as thick as a thickness of the glove material to the total overlay stack up plastic overlay for the same sensitivity. when sizing the buttons. Dry leather and rubber are similar to plastic with a dielectric constant of 2.5–3.5. Ski gloves have a dielectric constant of two or less, depending on the air content of the glove’s thermal insulation. January 11, 2008 Document No. 001-41439 Rev. *A 8 [+] Feedback
    • AN2292 Summary About the Author Name: Capacitive sensing may take multiple design revisions to Mark Lee find the best layout for the application. However, Title: Principal Applications Engineer knowledge of what has worked in the past and an understanding of the physics at work reduces design Background: PhD, University of Washington revisions and accelerates design time. Electrical Engineering, 1992 Contact: olr@cypress.com In March of 2007, Cypress recataloged all of its application notes using a new documentation number and revision code. This new documentation number and revision code (001-xxxxx, beginning with rev. **), located in the footer of the document, will be used in all subsequent revisions. PSoC is a registered trademark of Cypress Semiconductor Corp. quot;Programmable System-on-Chip,quot; PSoC Designer, and PSoC Express are trademarks of Cypress Semiconductor Corp. All other trademarks or registered trademarks referenced herein may be the property of their respective owners. Cypress Semiconductor 198 Champion Court San Jose, CA 95134-1709 Phone: 408-943-2600 Fax: 408-943-4730 http://www.cypress.com/ © Cypress Semiconductor Corporation, 2005-2008. The information contained herein is subject to change without notice. Cypress Semiconductor Corporation assumes no responsibility for the use of any circuitry other than circuitry embodied in a Cypress product. Nor does it convey or imply any license under patent or other rights. Cypress products are not warranted nor intended to be used for medical, life support, life saving, critical control or safety applications, unless pursuant to an express written agreement with Cypress. Furthermore, Cypress does not authorize its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress products in life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges. This Source Code (software and/or firmware) is owned by Cypress Semiconductor Corporation (Cypress) and is protected by and subject to worldwide patent protection (United States and foreign), United States copyright laws and international treaty provisions. Cypress hereby grants to licensee a personal, non-exclusive, non-transferable license to copy, use, modify, create derivative works of, and compile the Cypress Source Code and derivative works for the sole purpose of creating custom software and or firmware in support of licensee product to be used only in conjunction with a Cypress integrated circuit as specified in the applicable agreement. Any reproduction, modification, translation, compilation, or representation of this Source Code except as specified above is prohibited without the express written permission of Cypress. Disclaimer: CYPRESS MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARD TO THIS MATERIAL, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. Cypress reserves the right to make changes without further notice to the materials described herein. Cypress does not assume any liability arising out of the application or use of any product or circuit described herein. Cypress does not authorize its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress’ product in a life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges. Use may be limited by and subject to the applicable Cypress software license agreement. January 11, 2008 Document No. 001-41439 Rev. *A 9 [+] Feedback