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Prototyping and testing complicated electronic circuits can be time consuming. This versatile package lets you throw away the hardware and design and test on a computer screen. REVIEWED BY PETER SMITH Multisim: for advanced circuit design & simulation O PEN ALMOST any piece of electronic equipment these days and chances are you’ll see just one or two ICs, often with hundreds of pins and only a handful of discrete components. Usually, the components are so small it’s difficult if not impossible to identify exactly what they are (resistor, capacitor, inductor, or what?). It’s easy to imagine the control and precision needed to assemble these miniature PC boards. What about the design of the ICs themselves though – how the heck do they design, prototype and test the circuits inside a 300-pin “mega-chip”? And how do they make sure the ICs will work in a real circuit before committing them to manufacture? Computer software, of course, is the big answer. Ingenious software developers have been able to create virtual development environments which allow the entire design and test phase to be carried out without a piece of hardware in sight. Bringing the design elements together in this way has less obvious advantages, too. For example, hardware engineers can work at a level of abstraction above the underlying logic elements, greatly increasing design speed. In this review, we look at Multisim V6 from Electronics Workbench, a collection of state-of-the-art circuit design and simulation tools. Multisim includes all the tools necessary to take a design from inception to finished project and as such, a detailed review would have to cover an enormous amount of ground. We cannot hope to do justice to all aspects of the product in this short review, so we’ve settled on describing some of the main features instead. Schematic capture Designs are drawn in a familiar Windows environment using the Schematic Capture module. As with all other schematic capture programs, Multisim has a database of the most commonly used components (more than 16,000 in the Power-Pro edition) that can be placed and wired immediately. However, Multisim’s database is perhaps unique in that every component has a simulation model attached to it (we look at simulation a little further on). If a part that you want isn’t in the database, Multisim includes a Symbol Editor that allows you to create your own, either from scratch or based on March 2000  33 Truscott’s •  RESELLER FOR MAJOR KIT RETAILERS •  PROTOTYPING EQUIPMENT •  COMPLETE CB RADIO SUPPLY HOUSE •  TV ANTENNA ON SPECIAL (DIGITAL READY) •  LARGE RANGE OF ELECTRONIC COMPONENTS Professional Mail Order Service Truscott’s Come And See In ur New StoO re ELECTRONIC WORLD Pty Ltd ACN 069 935 397 Ph (03) 9723 3860 Fax (03) 9725 9443 27 The Mall, South Croydon, Vic 3136 (Melway Map 50 G7) email: truscott<at>acepia.net.au www.electronicworld.aus.as 34  Silicon Chip Fig.1: schematic entry and editing is a straightforward process. Fonts, colours and label positions can easily be changed for a more professional look. an existing component (or “symbol”). Wiring between components is a simple matter of clicking on the start and end points and Multisim makes the connection automatically. Manual control is possible too, of course. Once wires and components are placed, they can be moved by clicking and dragging. Multisim includes a multi-level undo feature but it performs more like an “undelete” than an “undo”. This means, for example, that deleted symbols and wires can be restored but operations like wire and component movement cannot be undone. Each node in the circuit is automatically assigned a unique node number during the wiring process. Using a feature called Virtual Wiring (“virtual” because no actual interconnections are shown), it is possible to connect nodes together by manually assigning the same node numbers. Typically, the supply rails in a circuit are connected in this way, resulting in less clutter and more readability. Readability is also one of the aims of Multisim’s subcircuit feature. A section or entire page of an existing circuit can be defined as a subcircuit and then used within another circuit. An optional add-on module expands the functionality of subcircuits even further, allowing them to be saved and edited just like any other schematic file. Completed schematics can be exported in variety of formats to suit all Fig.2: if a symbol is not in the database, it can be created from scratch or an existing symbol can be modified using the Symbol Editor. Fig.3: to access simulation model information, it’s just a matter of right-clicking on the component and choosing properties. Models can be created or imported from the model tab. Fig.4: using Model Makers to create a simulation model from the manufacturer’s data sheets. In this example, we have chosen to make a BJT (Bipolar Junction Transis­ tor) model. Model Makers supports many other model “classes”, including diodes, MOSFETs, SCRs, op amps, strip lines, waveguides, etc. major PCB layout software packages. However, the transition to PCB layout is much smoother when using the Electronics Workbench product – Ultiboard. This is because Ultiboard recognises information from Multisim like component footprints and minimum track widths (gleaned during simulation) without modification. Types of simulation As we mentioned earlier, simulation provides a means of examining circuit behaviour without having to physical- Fig.5: view from the drivers seat – the virtual oscilloscope. Fig.6: this spectrum analyser costs a lot less than its real world equivalent! ly construct it. Before we look at how a simulation is performed in Multisim, let’s touch briefly on the technologies involved. Multisim supports three different simulation technologies – SPICE, VHDL and Verilog. SPICE is an analog circuit simulator, the core (or kernel) of which has become an industry standard since its release to the public domain in 1972. A number of companies offer SPICE simulators that expand on the functionality and feature set of the original release. A notable example is XSPICE, which provides extensions for digital logic simulation. Multisim includes support for all of the most popular SPICE extensions. SPICE, by the way, is an acronym for Simulation Program with Integrated Circuit Emphasis! VHDL and Verilog are hardware description languages (HDLs) that are used to both document and design electronic systems. VHDL was born out of a US Defence Department contract and since its release in 1985, has been standardised by the IEEE (Institute of Electrical and Electronics Engineers). Verilog started life as a proprietary hardware modelling language and in 1990, it too was released to the public domain and standardised by the IEEE. VHDL and Verilog provide a means of designing and simulating complex digital logic, especially Complex Programmable Logic Devices (CPLDs) and Field Programmable Gate Arrays (FPGAs). Devices like our imaginary 300-pin “mega-chip” are designed using these languages. It is important to note that VHDL and Verilog are behavioural level languages. They describe what a circuit’s inputs and outputs are, what functions are performed in the middle and how long it all takes to happen. By contrast, when talking about digital logic, SPICE could be described as a transistor/gate level language. Multisim provides simulation engines for all three of these standards and what’s more, they can work together to co-simulate an entire mixed mode analog and digital circuit at the board level. This is a big advance, as separate simulators (often from different companies) were previously needed to simulate mixed mode circuits – and they rarely talked to one another! More about models We mentioned that all components in the database are associated with a simulation model. Simply put, these models “tell” the simulator how com- Fig.7: the logic analyser is another of Multisim’s virtual instruments. Setting up triggers couldn’t be simpler. Fig.8: signal sources are configured from their properties page. Here we set the amplitude and frequency of an AC voltage source. March 2000  35 Fig.9: in this screen shot, we have a virtual potentiometer (VR1) in circuit. The properties page shows that it is increased and decreased with the “a” and “A” keys, with each keystroke varying the value by 5%. ponents work. Multisim supports SPICE, VHDL, and Verilog models. In addition, where a ready-defined model isn’t available, Multisim provides a feature called Model Makers. This feature allows you to build an accurate simulation model (analog or digital) directly from the manufacturer’s data sheets. And if that’s not enough, circuits can be modelled at behavioural level using the C programming language – Multisim calls this Code Modelling. Whew! So, a simulator “knows” about com- ponents in a circuit by interpreting their respective models. But how do we “see” what the simulator is doing? Simulation in action To examine the operation of a prototype circuit we have constructed, we would apply appropriate stimulus to the input and view the results at the output. In a Multisim simulation, we do exactly the same thing, except that all our instruments are “virtual”. Multisim includes a whole host of virtual instruments that function Fig.10: the Bode plotter output from a high pass filter as displayed by the Analysis Grapher. With the aid of moveable cursors, we can see that the cutoff frequency is around 67Hz. Control over any of the available analyses and the way results are finally displayed is entirely customisable. 36  Silicon Chip just like their real-world counterparts. These include an oscilloscope, spectrum analyser, logic analyser, wattmeter, distortion analyser, network analyser, Bode plotter, function generator, word generator and of course a multimeter. Forget hunting for those missing test leads – simply drop your virtual instrument of choice onto the schematic and wire it in! Double-clicking on the instrument icon brings up its display and control panel, with mouse-activated knobs and switches. In addition to the function generator and word generator instruments, Multisim provides other means of applying stimulus to your circuits. A whole class of components called “sources” generate AC and DC currents and voltages, as well as clocks, pulses, one-shots, etc. Specialist AM and FM modulated sources for radio frequency design are also included. The parameters for each source (such as amplitude, frequency, etc) are individually controllable via their property pages. Well, this probably all sounds just too complex if you are a beginner to electronics. Connecting a logic analyser to a 2-chip counter circuit may seem like overkill but Multisim has the bases covered here, too. A class of components called “indicators” provides a voltmeter, ammeter, logic probe, hex display, lamp and bargraph, all of which operate like their real-world cousins. For example, the buzz­er sounds the PC speaker and the hex display segments “light up” in line with their logic inputs. While simulating the high-power audio amplifier circuit published this month, I unexpectedly discovered that Multisim’s fuses actually go open-circuit when their rating is exceeded. As far as I know, Multisim doesn’t include sound effects or burning smells (I don’t miss them)! Virtual components With the circuit complete and instruments and sources connected and configured, it’s then just a matter of hitting the simulate switch to start the simulation running. One of the features I really like here is the ability to change component values in the circuit without even having to stop the simulation. This is achieved by temporarily substituting Fig.11: the Postprocessor can act on results from an analysis using a variety of mathematical operations. The results can then be displayed as a graph or table, or simply exported to Excel or Mathcad. any components you would like to vary with their “virtual” equivalents. Virtual components (resistors, capacitors and inductors) can be increased or decreased in value in real time by hitting certain keys on your keyboard – you decide which. Naturally, the property pages for virtual components allow setting things like initial value, percentage change with each keystroke, etc. Circuit analysis We’ve talked about how Multisim’s circuit simulator can display real-time results on virtual instruments but it is capable of far more. Using the SPICE simulation engine, many different types of analyses can be performed. These include DC operating point, transient, AC frequency sweep, Four­ ier analysis and noise and distortion, to name a few. The results from these analyses are automatically graphed and can be exported to other applications such as Excel or Mathcad. Analyses results can be handed to the Postprocessor module, which performs mathematical wizardry according to your requirements and plots the results on a chart or graph. Types of mathematical operations include arithmetic, trigonometric, exponential, logarithmic, complex, vector, etc. Programmable logic design As the name suggests, programmable logic devices (PLDs) are ICs containing many logic gates (or building blocks) which are connected at programming time to perform the desired functions. Our imaginary “mega-chip” could be one of these. In order to work efficiently with devices of this complexity, designers describe what they want in high level programming languages like VHDL and Verilog. Multisim provides a complete development environment for PLDs. Using the inbuilt editor, the engineer first enters a design using the VHDL or Verilog languages. The result is then passed to the simulator, which is used to examine and debug the design. Finally, an output file is generated for programming into the target PLD. Note that once a PLD design is complete, it can be simulated at the board level just like any other component in Multisim. The engineer would simply create a symbol for the PLD and import the VHDL/Verilog file. Unfortunately, a detailed look at PLD design is beyond the scope of this article. If you would like to know more about VHDL or Verilog, check out the EDA industries web page at www.eda.org Summary Fig.12: simulating and debugging VHDL code. This example was taken from one of the many Multisim sample designs. Multisim really is an outstanding package. It excels in the simulation department, with features that would make it attractive to both professionals and educators. Multisim is available in four editions, being Power Professional, Professional, Personal and Education – we reviewed the Power Professional edition. Not all features are available in all editions, and some tools, such as the Ultiboard PCB layout and the Programmable Logic Synthesis module must be purchased separately. For further information or to order, visit the Emona Instruments website at www.emona.com.au or phone (02) 9519 3933. Extensive information on the Multi­ sim package can also be obtained from the Electronics Workbench website at www.electronicsworkbench.com SC March 2000  37