I recently purchased a development board that uses a 100-pin microcontroller (MCU). The symbol was just a square with 25 pins to a side. The schematic consisted of 15 mini-sheets. Probably two thirds of the pins were named as nets that may or may not have connected to one or more nets of the same name on other sheets. How do you try figure out whether a pin goes to a connector or a LED on the same sheet, different sheets, or both? I don't believe this lack of information is unique to the manufacturer I was using.
Now, if you are working within the CAD package, maybe you could argue that it has tools to highlight the net in total, so that you can easily locate and work with it. But when you print the document, you don't have access to the CAD tool and are therefore SOL (sadly out of luck). Another problem, as we shall see in an example in a moment, is that CAD systems evolve, mutate, and even disappear over time. Even if your CAD system is called the same name as an earlier incarnation, it may not work with the electronic files in the same way. PCAD -- as it evolved through different owners and from DOS to Windows -- provides a prime example. In such a case, a paper copy is your only backup.
Over the years, I have designed my system to avoid disjointed nets on a single sheet and to have as few intersheet connections as possible. For a large IC, I partition it into several parts, and I try to be as intuitive as possible. Where the schematic spreads to several sheets, it is possible to place a block near the associated components. For example, one of the Cypress PSoC4 range is presented in a 48-pin package. I partitioned this as illustrated in Figure 1.
Three individual symbols that comprise a single IC. They can be placed anywhere in the schematic to reduce on-sheet net breaks
and intersheet connections.
(Click here for a larger image.)
When creating symbols, I have found different CAD packages to be quirky and sometimes surprisingly inflexible on things like text orientation and style. I always prefer my text sans serif, but it is not always possible to set up a default font. Take a look at the symbols in Figure 1. Even though everything is default, there is a mix of sans serif and avec serif fonts, along with different text sizes. And try to persuade the vertical text to be horizontal. This is from Altium as it currently stands. Funnily enough, the older PCAD could do this, but it couldn't tell if a font had already been loaded, so it would load a new font for every symbol, bulking up the file size. Sigh.
One of the problems associated with using your own symbols comes up when the software goes through an upgrade. When PCAD was discontinued and subsequently wouldn't run happily on Windows 7, my company moved to Altium, which had taken over PCAD. Promised compatibility is not always as smooth as you would hope. Figure 2 shows a monostable flip flop as it was in PCAD, while Figure 3 is the translation as seen in Altium.
This symbol originally looked quite good in PCAD.
This translated/imported version of the previous symbol will require substantial adjustment to use it in Altium Designer.
I know I am old school, but I prefer it when my work looks good, and I feel it adds to the understanding of the circuit. How do you approach your symbols? Does your company have standards with which the symbols must comply?
This article was originally published on EBN's sister publication EE Times.