Schematic Editors:Symbols,Instance Names,Important Elements of Symbols and Electrical Connections.

Symbols

Symbols may represent a lot of different design objects of a circuit. The most important one is the device symbol. A device may be an electronic object that either will be mounted on a PCB by soldering or a cell which will be placed on an integrated chip. Besides such primitive symbols, modules of primitives may be combined to sub- designs with a corresponding block symbol. Both types of symbols are handled in the same way.

Figure 3.3 shows various types of symbols for different design systems. The decision as to which kind of symbol to use depends strongly on the target application.

• The logic symbols for PLD designs represent only the logic behavior. This is because after the optimization process which follows, mapping to the chosen PLD structure will take place.

• For the development of full custom digital or analog IC cells, mainly symbols of single transistors and some other analog elements are needed.

• A design for CBICs (Cell Based Integrated Cir- cuits) must only contain cells from a standard cell library which will be supplied by the chosen semiconductor foundry.

• Symbols for printed circuit boards have to identify the devices and thus specify exactly the shape and size of the target packages. These devices could be standard parts, PLDs or even ASICs (Application Specific Integrated Circuits).

Instance Names

The data of a single symbol are stored in a symbol library. When a symbol is placed in a schematic on the screen the data set of that symbol does not usually get copied into the design. Only a pointer is stored in the design. Since the same symbol may be instanced any number of times in various places in the design, the single placements have to be distinguished by their placement or instance names.

Instance names are a special symbol attribute in the object oriented organization of symbol data (see section 3.2.3). Most editors use an automatic numbering mechanism for the instance names.

For example, the library symbol could use the string ‘U?’ as a generic instance name. When placing the symbol, the ‘?’ gets replaced by a running integer number starting at 1. The user may re- edit the instance name and replace it with a unique character string. Thus a risk of multiple instances with the same name may appear but can easily be detected by an Electrical Rules Checker. The method of distinguishing symbols in hierarchical structures is described in section 3.2.2.

Figure 3.4 shows an IEC symbol, mostly used in Europe, and an American ANSI symbol of NAND3 (7410) which is taken from a 74 library. The most important elements are: Symbol Icon, Symbol Name, Instance Name, Input, Output and Supply Pins, Bounding Box, and Origin.

Symbolic Design Entry-0015

PCB design systems usually call the placement name a ‘reference designator’. These designators also carry the information of which instance shall be used if there are multiple occurrences of the same function in one package; for example, in 74 device families.

Important Elements of Symbols

Owing to their graphical character, the position of the symbol pins is extremely important because only the pins establish the connectivity. In fig. 3.4 the end point of the pins is marked with a little

1) EN 60 617-5

cross (x) to show the point of connection. In the final schematic these crosses will not be visible when they are properly hooked up. The rest of a symbol only has the character of an icon for better visual recognition.

Symbolic Design Entry-0016

The power supply pins PWR and GND will also not be visible in the schematic since they are declared as ‘hidden pins’. The designer does not have to wire them up because they will be connected to global nodes during netlist creation. Figure 3.4 also shows the bounding box, which is a rectangular outline to highlight the symbol when the mouse cursor is clicked inside this box.

This now leads to the subject of standardization. Basically, two internationally accepted standards exist for electronic symbols: IEC/DIN/EN and ANSI/IEEE standards. The latter are usually used for ASIC designs. It depends solely on the de- signer’s experience and taste which symbols lead to better readability. For most of the following ex- amples the IEC type of symbols are used, but they can easily be ‘translated’ using the explanations in Appendix A.

In an editor the replacement of one symbol type by another is only possible when the symbols carry the same device names and if they have the same pin coordinates as well as the same origins. Given that those are the same, it is only a question of the library configuration as to which symbol will be displayed. Since the crosses of the visible pins A, B and C are the same for both symbols of the device 7410 in fig. 3.4, they may be replaced by each other.

Depending on the specific application, further symbols may be necessary which do not always represent electronic devices:

• Symbols for voltage supply and ground unless  they are connected implicitly via hidden pins. They are needed mainly in analog designs and PCB schematics;

• Input and output ports (I/O-ports) for sub- designs which connect the module to other levels in the hierarchy through block symbols;

• Symbols for stimuli which are only used for simulation and not for the implementation;

• Symbols for global signals, as long as they are not generated during netlist conversion (for ex- ample, clock signals);

• Non-electrical information, such as markers to set attributes for a net to appear in the waveform display or to set initial conditions.

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