AN Extended Range Proximity with SMSC RightTouch Capacitive Sensors

Transcription

1 AN Extended Range Proximity with SMSC RightTouch Capacitive Sensors 1 Overview 2 Audience 3 References SMSC s RightTouch 1 capacitive sensor family provides exceptional touch interfaces, and now expands to extended range proximity systems (1cm to 28+cm). Traditional extended range proximity designs have used infrared (IR) technology which has high costs and high power consumption. RightTouch capacitive sensing offers a new low cost, low power alternative to traditional IR systems. This application note provides software and hardware guidelines to achieve an extended range proximity system with SMSC RightTouch capacitive sensing devices. Initially physical requirements for the design of a proximity antenna (also know as a capacitive pad) will be discussed. Software configurations of SMSC devices will then be addressed. Several examples of proximity systems with results will complete this application note. This application note assumes that the reader is familiar with capacitive touch sensing designs using the SMSC RightTouch devices. The following documents should be referenced when using this application note: SMSC RightTouch capacitive touch sensor device datasheets CAP1114, CAP1214, CAP1188, CAP1166, CAP1133, CAP1128, CAP1126, CAP1106, and CAP1105 SMSC AN 19.16, Physical Design and Layout Guidelines for Capacitive Sensor Systems SMSC AN 22.8, RightTouch CAP1188 Family EVB Tutorial SMSC AN 22.9, RightTouch CAP1105/CAP1106 EVB Tutorial 1. SMSC, the SMSC logo and RightTouch are registered trademarks and the RightTouch logo is a trademark of Standard Microsystems Corporation ( SMSC ). SMSC AN Revision 1.0 ( )

2 4 Proximity System Design Capacitance sensing for touch detection and proximity detection require different design considerations while using the same semiconductor technology. Please see AN19.16 to become familiar with touch detection basics. Once familiar with the basics, the model can be expanded to extended range proximity. 4.1 Physical Antenna Design Essential design elements include the size of the antenna, location of antenna in relation to a ground plane, and specific settings within the RightTouch device. Adhering to a few simple guidelines will allow the unique design of the RightTouch device to detect the approach of a user or the movement of nearby metallic objects. There are three critical physical design elements needed to achieve maximum range detection with high signal strength and low noise. Maximize the distance of the antenna to a ground plane (all layers of printed circuit board (PCB) and nearby metallic objects). Maximize the size of the antenna. Minimize antenna movement in system (double sided tape, adhesive, clips, etc.). Initial design begins by defining both the range and the function. For example, 5cm range for approach detection in a monitor button board, or 25cm for approach detection in a 10 digital photo frame Ground Plane Any ground plane or metal surface directly adjacent to the antenna will decrease the range of proximity detection. Ground planes have two effects on proximity. The first is to act as a shield of the approaching object if placed in the path of the object, effectively reducing the range of the sensing system. The second is to increase base capacitance when placed directly below or adjacent to the proximity antenna. In free space, an antenna can emit/sense freely in all directions with little attenuation. When a ground plane is introduced, signals emitting/sensed from the antenna want to terminate on the ground plane. As the distance between the ground and antenna decreases, the strength of the field radiating decreases. So as a ground plane is placed closer and closer to the antenna, the sensing range is effectively reduced. In addition to decreasing the range of a proximity antenna, a ground plane also increases the capacitance of the antenna vs. an identical antenna in free space. Large antennas close to a ground plane (e.g., 25cm 2 solid pad with a solid ground plane on layer PCB) will exceed the calibration range of SMSC devices. Revision 1.0 ( ) 2 SMSC AN 24.19

3 4.1.2 Antenna Shape Every system design is unique with specific aesthetic goals as well as physical constraints. SMSC recommends loop antenna shapes (large trace with empty center) for large applications (photo frames, keyboards, etc.) and solid pads for smaller button board applications. Loops reduce the overall capacitance that the SMSC device will see and create a larger coverage area. A pad shape is best for small boards where separation from ground is limited and pad area is needed to create the desired range. A loop antenna can have any aspect ratio (i.e., 20cm x 20cm or 5cm x 40cm). The desired function and form factor will guide this decision. Loops as small as 1cm by 1cm create a small degree of proximity. Loops of 30cm x 30cm (30awg wire) will create a large proximity envelope. Larger loops or thicker gauge wire may exceed the calibration range of the SMSC device. SMSC recommends keeping the total base capacitance to 45pF or less to prevent out of range conditions over temperature or other unique user situations such as calibration with debris on the sensor. Base capacitance can be determined from register B1h (sensor CS1). One LSB of the calibration register equals 0.196pF (i.e., a value of 0xE5 indicates 45pF of base capacitance). If a pad is determined to be the best fit, any shape can be used. A long and thin pad of 1cm x 25cm (25cm 2 ) would be well suited for the bottom or side of an LCD monitor. If space is available, a large 5cm x 5cm (25cm 2 ) pad will create a large dome of proximity detection. A circular pad with r = 2.83cm (~25cm 2 ) would provide a similar dome of proximity. If capacitance is too large, the shape could be converted to a loop by removing the center area of the square or circle. Physical shapes are unlimited. Antenna shapes can include circles, ovals, squares, rectangles, or even serpentine around boards. The overall effectiveness of the antenna is not determined by the shape but rather the area of the conductor relative to the user or object entering the proximity zone. Proximity range is directly proportional to antenna size. Larger antennas provide greater proximity detection ranges Antenna Construction Loop antennas can be created with solid copper wire (with/without insulation), flex circuits, or on a PCB. In the case of a wire, solid core or stranded will perform similarly, however solid core is easier to assemble in the manufacturing process. Larger gauge wire will provide increased range due to the increased surface area. The physical design will limit how large of wire can be used. Designs can start with 30AWG and increase until the desired range is achieved, aesthetic design limits are reached, or calibration limits are reached. In the case of a PCB loop antenna, the larger the trace width, the larger the range. A minimum trace width of 7mils (0.18mm) will function as an antenna, but larger traces will produce greater range. Solid PCB pad shapes need to follow the same guidelines, maximize area and keep nearby ground to a minimum. SMSC AN Revision 1.0 ( )

4 4.2 RightTouch Device Configurations for Proximity Once the antenna has been designed and assembled, software must be written to properly configure the SMSC device for proximity. There are two critical software programming elements to achieve maximum range detection with high signal strength and low noise. Maximize gain (Gain and Sensitivity) in the RightTouch device. Minimize noise (Averaging) in the RightTouch device. SMSC has two distinct types of devices for proximity: one uses built-in proximity modes (CAP1114 and CAP1214), while the second relies on user defined modes to enable proximity (CAP1188 Family). Both of these modes are described below. Each assumes default settings with specific changes highlighted to configure the device for proximity CAP1114 (Built-in Proximity Mode) Channel CS1 is the only proximity channel available. Register settings to begin development: // Configure standby configuration for Proximity 0x00, 20 // Enable Sleep mode 0x21, FE // Disable CS1 in Normal mode 0x29, 01 // Enable CS1 in Sleep mode 0x30, 40 // CS1 Threshold Level 0x42, 82 // Sensitivity x32, Avg.# 16, Config. = Average, Mode = Proximity Register 0x42 = 82h Enable Proximity Mode. Sensitivity - default x32. Adjust Proximity Sensitivity to x16 or x64 to meet range goal. Sensitivity of x128 is not recommended for proximity systems. Averaging - default x16 (low noise, quickest response). Adjust for noise/response - x32 (mild noise) or x64 (high noise, slower response). Averaging of x128 will extend sampling to extremely long times and is not recommended. Sample Config. - default Average mode. Summation mode should never be used. Proximity should be set to detect an approach, then disabled while buttons are being selected. A simple way to do this is by using the Sleep and Normal modes. Register 0x00 controls the power state, and Registers 0x21 and 0x29 control the sensors enabled in Normal and Sleep modes respectively. Maximum duration, Register 0x22, can be used to clear a stuck proximity sensor (or normal buttons) by digitally calibrating the sensors after a specified time has been reached. A stuck proximity sensor could occur if a metallic object is placed (and not removed) close enough to the sensor to exceed the sensor threshold. Enabling the Button Max Duration bit will clear the delta counts as well as the status bits. This feature can be enabled for autonomous maintenance or monitored and controlled by a micro-controller. Revision 1.0 ( ) 4 SMSC AN 24.19

5 4.2.2 CAP1188 Family (User Defined Proximity Mode) All channels are capable of proximity with multiple active simultaneously. Register Settings to begin development: // Configure standby configuration for Proximity 0x00, E0 // Gain 8x, Standby mode 0x20, A8 // Default with Enable Max Duration 0x21, FE // Disable CS1 in Normal mode 0x22, 54 // Repeat Rate = 175ms, Max Duration = 5600ms 0x40, 01 // Enable CS1 in Standby mode 0x41, 38 // Cycle Time 70ms, Sample Time 1.28ms, Avg.# 8, Config = Average 0x42, 01 // Sensitivity x64 0x43, 40 // Standby Threshold level Register 0x00 = E0h Set Gain setting to x8. Settings of x4, x2, and x1 will reduce range by a factor of ~2 for each step. Enable Standby Mode. Register 0x20 = A8h Enable Button Maximum Duration Register 0x21 = FEh Disable CS1 in Normal Mode Register 0x22 = 54h Set maximum duration, default of 5.6 sec is recommended. This will recalibrate and clear a stuck detector. System specifications will determine this value. Register 0x40 = 01h Enable CS1 in Standby mode Register 0x41 = 38h Cycle Time - default 70ms. Adjust to 35ms for the most responsive system. Cycle Times of 105 and 140ms can be chosen but will slow response time. Sample Time - default 1.28ms. Sample Times of 640us and 320us will reduce range. Sample Time of 2.56ms is not recommended for proximity applications. Averaging - default - x8 (low noise, fast response). Settings depending on noise level and desired response speed: x16 or x32 (mild noise), and x64 (high noise, slower response). Averaging of x128 will extend sampling to extremely long times and is not recommended. Sample Config. - default Average mode. Summation mode should never be used. Register 0x42 = 01h Sensitivity - x64. Adjust to x32 for less range or x128 for more range. Register 0x43 = 40h Standby Threshold level - default mid-scale. Adjust higher if noise issues, adjust lower if low noise and additional range needed. SMSC AN Revision 1.0 ( )

6 5 Antenna Configuration Examples Three examples of proximity are shown here. The first is a digital photo frame with a very large loop antenna constructed of sold wire. Second is a button board with a long and thin antenna to detect a user and illuminate the functions of the buttons. Lastly is the SMSC EVB-CAP1188 that can demonstrate both a long and thin pad as well as a PCB based loop antenna. 5.1 Wire Loop around Digital Photo Frame Goal of this design: User approaches the frame from any direction. At 25+cm LEDs turn on to indicate position of touch sensors. Configuration: 0x00, 20 // Enable Sleep mode 0x21, FE // Disable CS1 in Normal mode 0x29, 01 // Enable CS1 in Sleep mode 0x30, 40 // CS1 Threshold Level 0x42, 82 // Sensitivity x32, Avg.# 16, Config. = Average, Mode = Proximity Exterior enclosure dimensions: 25cm x 20cm LCD dimensions: 17.5cm x 13.5cm Antenna Dimensions: 24cm x 19cm Proximity wire dimension: 24AWG Calibrated capacitance: 45pF Proximity range: 26-28cm Hand Approaches LEDs Light Figure 5.1 LED off - Hand Approach - LED On Revision 1.0 ( ) 6 SMSC AN 24.19

7 Proximity Antenna Figure 5.2 Internal View of Digital Photo Frame Antenna 5.2 Long Narrow Antenna with Adjacent Buttons on PCB Goal of this design: User approaches the button board from the front or side. At 5+cm LEDs turn on to show icons of functions of touch sensors. Configuration: 0x00, 20 // Enable Sleep mode 0x21, FE // Disable CS1 in Normal mode 0x29, 01 // Enable CS1 in Sleep mode 0x30, 40 // CS1 Threshold Level 0x42, 82 // Sensitivity x32, Avg.# 16, Config. = Average, Mode = Proximity Total board dimensions: 190mm x 15mm Size of proximity antenna: 188mm x 2.6mm Proximity wire dimension: N/A No ground under proximity antenna on any layer. Calibrated capacitance: 15pF Proximity range: 7cm SMSC AN Revision 1.0 ( )

8 Proximity Antenna Touch Pads Cap Sense Lines No Ground Ground No Ground LED Lines No Ground Figure 5.3 Gerber View of Button Board Revision 1.0 ( ) 8 SMSC AN 24.19

9 5.3 Small Loop Antenna on EVB-CAP1188 Goal of this design: Demonstrate proximity with a trace loop antenna. User approaches the button board from the front and at 6+cm an LED turns on to indicate range of proximity. Configuration: 0x00, E0 // Gain 8x, Standby mode 0x20, A8 // Default with Enable Max Duration 0x21, FE // Disable CS1 in Normal mode 0x22, 54 // Repeat Rate = 175ms, Max Duration = 5600ms 0x40, 01 // Enable CS1 in Standby mode 0x41, 38 // Cycle Time 70ms, Sample Time 1.28ms, Avg.# 8, Config = Average 0x42, 01 // Sensitivity x64 0x43, 40 // Standby Threshold level Total board dimensions: 95mm x 50mm Size of proximity loop: 82mm x 10mm (touch pad not connected for this example) Proximity wire dimension: 7mil (0.18mm) trace No ground under proximity antenna on any layer Calibrated capacitance: 44pF (37pF due to long trace from chip to antenna and many vias) Proximity range: 6cm Proximity Antenna Trace Proximity Pad Touch Pad Figure 5.4 Gerber View of EVB-CAP1188 SMSC AN Revision 1.0 ( )

10 6 Additional Design Considerations Following the basic design rules outlined in this note will provide a robust proximity solution. Occasionally a system will have rigorous design requirements beyond the typical. Highlighted below are two concern areas with potential solutions. These are only offered as suggestions and are not guaranteed to solve system level problems. High RF noise environment Add a 5pF to 15pF capacitor to ground near the SMSC device. This provides an RF path before entering the device. Add small amount of ground under proximity pad (thin trace or very thin crosshatch 1%). This provides an RF path before entering the device. Add 100 ohm series resistor (maximum of 500 ohms). This creates a small RC filter to shunt noise before entering the device. Base capacitance greater than 50pF (i.e., out of range) Add a 25pF to 40pF capacitor in series with proximity pad Reduce proximity pad area Reduce ground area under and around proximity pad Revision 1.0 ( ) 10 SMSC AN 24.19

11 7 Application Note Revision History Customer Revision History REVISION LEVEL AND DATE SECTION/FIGURE/ENTRY CORRECTION Rev. 1.0 ( ) Rev. 1.0 ( ) Microchip logo added and modification to company disclaimer. Initial release Copyright 2012 SMSC or its subsidiaries. All rights reserved. Circuit diagrams and other information relating to SMSC products are included as a means of illustrating typical applications. Consequently, complete information sufficient for construction purposes is not necessarily given. Although the information has been checked and is believed to be accurate, no responsibility is assumed for inaccuracies. SMSC reserves the right to make changes to specifications and product descriptions at any time without notice. Contact your local SMSC sales office to obtain the latest specifications before placing your product order. The provision of this information does not convey to the purchaser of the described semiconductor devices any licenses under any patent rights or other intellectual property rights of SMSC or others. All sales are expressly conditional on your agreement to the terms and conditions of the most recently dated version of SMSC's standard Terms of Sale Agreement dated before the date of your order (the "Terms of Sale Agreement"). The product may contain design defects or errors known as anomalies which may cause the product's functions to deviate from published specifications. Anomaly sheets are available upon request. SMSC products are not designed, intended, authorized or warranted for use in any life support or other application where product failure could cause or contribute to personal injury or severe property damage. Any and all such uses without prior written approval of an Officer of SMSC and further testing and/or modification will be fully at the risk of the customer. Copies of this document or other SMSC literature, as well as the Terms of Sale Agreement, may be obtained by visiting SMSC s website at SMSC is a registered trademark of Standard Microsystems Corporation ( SMSC ). Product names and company names are the trademarks of their respective holders. The Microchip name and logo, and the Microchip logo are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. SMSC DISCLAIMS AND EXCLUDES ANY AND ALL WARRANTIES, INCLUDING WITHOUT LIMITATION ANY AND ALL IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, TITLE, AND AGAINST INFRINGEMENT AND THE LIKE, AND ANY AND ALL WARRANTIES ARISING FROM ANY COURSE OF DEALING OR USAGE OF TRADE. IN NO EVENT SHALL SMSC BE LIABLE FOR ANY DIRECT, INCIDENTAL, INDIRECT, SPECIAL, PUNITIVE, OR CONSEQUENTIAL DAMAGES; OR FOR LOST DATA, PROFITS, SAVINGS OR REVENUES OF ANY KIND; REGARDLESS OF THE FORM OF ACTION, WHETHER BASED ON CONTRACT; TORT; NEGLIGENCE OF SMSC OR OTHERS; STRICT LIABILITY; BREACH OF WARRANTY; OR OTHERWISE; WHETHER OR NOT ANY REMEDY OF BUYER IS HELD TO HAVE FAILED OF ITS ESSENTIAL PURPOSE, AND WHETHER OR NOT SMSC HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SMSC AN Revision 1.0 ( )