Design Activity PCB Touch-switch design & build

Transcription

1 Design Activity PCB Touch-switch design & build Aims The aim of this activity is to design & build a simple microprocessor-controlled capacitive-sensing touchswitch keyboard. You will design a PCB to implement your capacitive-sensing circuit. This will teach a variety of skills which will be useful in future project work. The activity includes some assembly code programming which links with the Introduction to Computer Architecture lecture course. You will also learn how to use EAGLE CAD, a PCB design tool. Since all PCB design tools share many common characteristics, this will be a useful practical skill for project work both within your course and in industry. Objectives After doing this activity students will be able to: Equipment Perform schematic capture & board layout for simple printed circuit boards (PCBs) Use CMOS Schmitt trigger oscillators as elements in designs Design a simple circuit in which analog elements interface with a micro-controller Write small sections of embedded assembly code within a larger embedded program running on a micro-controller Evaluate the advantages and disadvantages of using PCBs in a prototype design and build. PC with EAGLE CAD and AVRStudio software Soldering Iron Digital Storage Oscilloscope AVR Micro-controller board w/ 9V DC power supply AVRisp MkII In-System Programmer Signal Generator Timetable The activity consists of two parts. Sessions 5 & 6 will be later in the Term, called PCB2, assessed separately Session 1 - Work out sensor design Session 2 - Work through Dr. Clarke's tutorial on using EAGLE for standard double-sided PCBs with plated through holes Session 3 - Design and optimise your own PCB for the sensor design. Note that this is non-pth for local manufacture of boards Session 4 - Finish PCB sensor design. If you finish early familiarise yourself with the microprocessor programming requirements and start code design. [PCB2] Session 5 - Build sensor board (your PCB will be returned before start of this session), test sensor frequencies. Session 6 - Finish microprocessor code design, test whole system

2 Introduction Mechanical switches can be bought and used in components to control equipment. A popular modern alternative is to use touch switches, activated by pressing a panel with a finger. These have no moving parts and are more robust. The most popular technology for touch-switch implementation is capacitive where the finger forms part of a capacitor which is sensed by an electronic circuit. This is effective through a thick plastic panel (which serves as the capacitor dielectric) therefore allowing robust sealing against the environment (spillages of tea, etc). Integrated circuits are available which perform the whole touch-switch operation. However, in small projects which typically already have an embedded microprocessor, it is more efficient to use this together with some additional analog circuitry to implement the touch switch. The technique we will use is to design RC oscillators where the capacitance, which affects the output frequency, is made up of a touch-pad sensor which changes capacitance when touched by a finger. The oscillator output waveform is passed to a microprocessor input, it can then be monitored by the microprocessor program and used to determine whether the switch is pressed or not. This can be made more concrete as a design problem by specifying the required function. As a demonstration we require four switches, each of which can switch a corresponding LED attached to the microprocessor outputs. The work we need to complete this task is therefore: Top-down decomposition Analysis & circuit design PCB CAD tool familiarisation PCB schematic entry and board layout PCB build & test Microprocessor software development familiarisation Microprocessor code writing & test System integration To make this task easier in the limited time available, and because EEE students have not yet learnt C, the programming language most commonly used in embedded systems, there will be some short-cuts: The microprocessor circuit, with associated indicator LEDs and programming interface, is already built on a PCB you are given. The control software to drive LEDs is mostly pre-written. You will write only the assembly code to interface with the oscillator waveforms Three Appendices are provided to help you complete the tasks: Appendix A. Introduction to capacitive sensors. This handout gives some hints on a suitable design for your capacitive touch sensors Appendix B. PCB design using EAGLE. This handout gives a short tutorial on using EAGLE CAD, the PCB layout software available in the undergraduate labs. Appendix C. Introduction to AVR programming using AVRStudio. This should get you started using the AVRStudio software to assemble and download code to the Atmel ATMega88P processor found on the micro-controller boards we've produced for you.

3 Appendix A. Introduction to Capacitive Sensors Your task is to design, build and test a capacitive touchpad. Your design will plug into a small pre-built PCB available in the lab. The prebuilt PCB contains : A 5V power supply, An Atmel AVR (ATMEGA88PA) micro-controller An in-circuit programming interface Several LEDs A 10 pin expansion header (with 6 Digital I/Os, 5V VCC and GND pins) Your task will be to design a second PCB which contains at least four capacitive sensors. This PCB containing your sensors and a small amount of analogue circuitry will connect to the prebuilt PCB using a 10 pin ribbon cable. Software running on the AVR micro-controller on the pre-built board can then be used to interpret the signals from your PCB and indicate when finger-touches are sensed. Principles of Capacitive Sensors With an electric field established between two parallel plates, The capacitance of the capacitor they form is C = r 0 A / d where C is the A is the area of overlap of the two plates, d is the distance between them and r is the relative permittivity of the material between the plates. A touch-sensitive sensor can be formed when the capacitor is laid out using two adjacent plates as in Figure 1 - a finger placed over these plates behaves like a conducting plate which forms a capacitor with each plate and hence increases the capacitance between the two plates. Possible Circuit Design. The change in capacitance can be sensed with a simple RC oscillator circuit using a schmitt triggered inverter (see figure 1) The capacitive pads will be a metal pattern within the top copper layer of your manufactured PCB (See figure). Changing the capacitance of the sensor (by contact with a finger), changes the time constant of the RC circuit and thus the frequency of oscillation. The Schmitt trigger oscillator gives a square wave output which can be connected directly to the input of a micro-controller. Software running on the micro-controller can measure the interval between oscillations and determine whether a touch has been detected on each sensor or not. Analysis of Schmitt Oscillator Since you have been given the basic design of your Schmitt oscillator and a design for a planar sensing capacitor, the main challenge of circuit design will be selecting appropriate values for R and C in your oscillator circuit to give a appropriate relaxed and sensing oscillator frequency. The capacitance of the sensor without a finger-touch should be assumed small, and with a finger touching it should be assumed to increase by approximately 5pF.

4 Frequency of Oscillation This section requires some thought but has as deliverable the R & C values you will finally use, which do not affect your PCB design. Therefore if you don t have time to finish it you can skip and come back to it after you have a completed PCB design. Calculation. Schmitt trigger inverters have two switching thresholds, VT+ is the positive going threshold and VT- the negative going threshold. You can use these values and the capacitance differential equation I = C dv/dt you can use circuit analysis from your first year course to solve the differential equation and find the exact frequency of oscillation of this oscillator as a function of R & C. Compare this with the approximate answer given in the NXP 74HC014 data sheet. Note the difference in thresholds between 74HC14 and 74HCT14. Which device do you think it would be best to use, and why? Noise Immunity Overhead Fluorescent lights with inductive ballasts introduce noise at low harmonics of the 50Hz mains frequency. You might also expect noise to be present in your circuit from the power supply in your circuit (which will use the normal bench supplies). At these frequencies (which are likely to be substantially lower than the oscillating frequencies you select for your sensors), you can model the noise as a parasitic current source injected onto the sensor. Your choice of RC time constant should be influenced by this, and the need to balance reasonable acquisition times (how many cycles of the oscillator you measure over) and noise immunity. Electronic ballasts in compact fluorescent lighting operate at much higher frequencies (~40Khz) which may be harder to consider in your design, but may be worth considering if your circuit behaves oddly in testing. Marks will be awarded in your overall assessment for design effort which attempts to mitigate noise problems in your design, or measure its impact during testing. However don t worry too much about this the circuit proposed here is fairly robust and likely to work in any case. Trade-offs in component values. The choice of R & C for your circuit is one of the classic engineering trade-offs. Discuss this with demonstrators. A good way to approach this design is via constraints define inequalities which express approximately the sets of values that you think will work well. (Thus, as an example, if you have decided the frequency must be > 1kHz that can be expressed as an inequality on RC). Select the optimum value that satisfies all constraints. Fine touches It's good practice to include decoupling capacitors (normally 0.1uF ceramic) close to the power pins on every digital chip in your circuit, look-up why this is done.

5 Appendix B. PCB Design using EAGLE CAD PCB Design using EAGLE is explained in Dr. Clarke s EAGLE tutorial which is linked from the PCB handout lab web page. Going through this tutorial will equip you well for the simple board you re expected to produce. Before attempting your board, work through the tutorial making a simple op-amp circuit, using the library ee2parts, and the design rules ee_rules, as described in the tutorial. Your design must have the following specification: Board size: exactly 2 X 2 Board connections: via IDC 10 pin connector to microcontroller board see controller schematic for pins. Board power: 5V (Vcc & GND) via connector to microcontroller board (see Figure 1). Board parts: from EE2Lab library Board design rules: ee2_rules_coarse.dru Figure 1 connections from 10 pin IDC on controller schematic The PCB design you are doing has one complication not mentioned in the EAGLE tutorial. Your PCB will be made in-house on a manufacturing process which does NOT use plated through holes (PTH). Eagle assumes connectivity between top and bottom layers of your PCB at each of the through-hole pins on your PCB, so two tracks top and bottom going to a pin are assumed to connect. This you can usually ensure when assembling components on your board by soldering them both to the top and bottom of your board. You can do this for resistors and capacitors on your board but not for IC sockets or IDC connectors. These must be mounted on the top of your board and this makes it difficult/impossible to solder to the top layer of the board. Therefore you (or in fact the EAGLE auto-route program) must route connections from IC sockets and IDC connectors on the bottom-side only of your board. The components in the EE2Lab library have been modified so that to-side routing is not made in these cases. You will notice this after you use auto-route. It is usually necessary also to use vias between layers on your board, but in this case you will have to solder small wires between the top and bottom layers of the board to make electrical connectivity. A good layout will have the minimum number of vias. You can use component wirepad to implement a via this will be a permanent part of your board design and must be connected on the schematic to the circuit node you wish to place it on. This is more work than using the EAGLE via tool but this has the disadvantage that vias easily disappear when you reroute. Once you have placed your components on the board, you can use the auto-router to route most of the connections for you - you may need to iterate through this process several times, changing the layout of components on your board. While the top and bottom layers in Eagle CAD have a direct mapping to the top and bottom layers of your twosided PCB, other layers have more abstract meanings. trestrict and brestrict are top-restrict and bottom-restrict, these are areas on the top and bottom of the board in which routing is prohibited. The auto-router is not allowed to make connections through the trestrict area on the top of the board, or the brestrict area on the bottom of the board. The vrestrict layer allows you to define areas in which vias are prohibited. The 10 way IDC socket component and 14 pin schmitt trigger have these areas defined to prevent auto-routed connections to their pins on the top layer of the board.