Altium
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The ability to reuse existing sections of designs is like the quest for the holy grail in our design team – something we value highly and dream of finding, but have yet to discover. And if you were to push me to take a position, I used to doubt it would ever be found. So I’d have to say that I was skeptical when I read statements about design reuse in Altium Designer – “I’ll believe that when I see it” were my exact words.
Everyone in the team was already practicing design re-use, we were copying and pasting sections of existing designs – like standard comms or power supply sub-circuits – into our current projects. But there was no integrity in the copied design, because any number of mistakes could be made during the copy/paste process, and the engineer could and would modify the circuit to suit their idea of a good circuit. That meant it had to be subject to the standard review and triple-check sign-off process.
The challenges of design reuse
For design reuse to work, there are a couple of fundamental challenges that must be solved.
- First, the schematics must be available for everyone to use, but secure from any editing. We want everyone in the team to be able to easily explore and use all our proven circuits, with all additions and changes controlled and managed by our formal review process.
- Secondly, since this central resource cannot be edited, the design environment must support the ability to update the component designators on each new design, without affecting the assignments on the master copy (as that would break rule 1). This has always been the deal-breaker in the past – how to maintain unique component designations in the new design, without resorting to some complex and convoluted naming scheme, like R1_Power_1v2, in the master re-use copy.
Well, it’s time for this skeptic to put his money where his mouth is. My current design has been prototyped on the Desktop NanoBoard, and it’s time to move it to a custom board. I can save loads of time and accelerate that process if I re-use circuitry directly from the Desktop NanoBoard, so it’s time to brave the waters of design reuse.
An elegant approach to design reuse
Like any engineer, I’m always curious to see how things work. So when my snazzy new NanoBoard arrived I had the schematic and board files open in Altium Designer as soon as the board itself had been plugged in and powered up.
One of the first things that caught my eye on the top schematic sheet for the NB2 was the recycling symbol inside some of the sheet symbols. 
Pressing F1 as I hovered the mouse over the symbol, I discovered that this was a Device Sheet Symbol (instead of a standard sheet symbol), which points to a Device Sheet, and that Device Sheets are reusable blocks of circuitry.
I now knew that a reusable chunk of circuitry was called a Device Sheet, and it was accessed by placing a Device Sheet Symbol. And by clicking the link in the summary in the Knowledge Center panel, I had a PDF called Using Device Sheets open, ready to guide me.
I drilled down into one of the Device Sheets on the NanoBoard schematic, the USB in fact, because my design would use the same USB device. The Device Sheet had interesting watermarks on it, the recycling symbol was there again, and also the text, Read Only. I printed the sheet to check that the watermarks were not included, but just the circuitry printed as I’d hoped. They were useful on-screen, but I definitely did not want them included in the client’s PDF!
Press F1 for information about the object under the cursor
I then checked the help PDF to learn more. I was happy to read that a Device Sheet is actually just a standard schematic sheet, which means it will be straightforward to take sheets from our existing designs and reuse them as Device Sheets. I also learnt that while a Device Sheet lets me reuse a chunk of circuitry, I could also build a hierarchy of Device Sheets if needed. This is perfect for those situations where there is a larger section to be reused.
Device Sheets are standard schematics, but because they are placed as a Device Sheet
Altium Designer can be configured to show them as read only, and recycled.
Reusing existing circuitry
To let Altium Designer know where my reusable schematics, or Device Sheets, are stored, I just needed to reference the location in the Schematic – Device Sheets page of the Preferences dialog. I noticed that the supplied Altium Device Sheets were already hooked in, so I added another link to a suitable location on our network, where I could start copying schematics of our own reuseable sections of designs.
In no time at all I had added USB and touchscreen hardware into my design, using the Place Device Sheet Symbol command and placing the appropriate Device Sheet Symbols from the Select Device Sheet dialog. Signal Harnesses are an excellent partner for Device Sheets as well. They bundle nets and buses together so the structural schematic sheets can be kept clean and simple.
Reusing existing circuitry is as simple as placing a Device Sheet Symbol.
So far I was impressed. The Device Sheet mechanism meant it was straightforward to exploit our existing proven sections of circuitry, and as an added bonus I had easy access to the circuitry on the Altium Desktop NanoBoard.
But the big question that remained was how Altium Designer would handle the designators. Although I only had a portion of the hardware captured, I decided to look into it straight away.
Managing the component designators
Back to skimming the Using Device Sheets PDF file. I learnt that the secret to annotating a design with Device Sheets is called Board Level Annotation. It’s called Board Level Annotation because it allows me to re-annotate my design, as it is destined for the board, without re-annotating the original schematics.
I ran the Board Level Annotate command, and discovered in the Options that there is a flat naming scheme, called $ComponentPrefix$GlobalIndex. This meant I could still have the traditional R1 to Rn numbering across the entire design, which was just what I was after. After running the Board Level Annotation process, I explored the results by clicking the Tabs at the bottom of the schematic editor workspace.
Original component annotations are shown on the left, with the re-annotated version on the right.
Note that the superscripts are optional, and can be turned off in the Preferences.
I went back to the PDF to see what other neat features were available. It showed that the Annotate Compiled Sheets command is used to renumber all the sheets in the design, including the Device Sheets. Like BLA (board level annotation), this let me modify the sheet numbering in this project without affecting the original schematic.
A new Device Sheet and BLA convert
I have to admit that I seriously doubted the design reuse claim when I first read about it. But now, basking in the success of my rapid hardware development – my hardware design capture was half-complete already and it wasn’t even lunch time – I realized how wrong I’d been.
Not only did Altium Designer provide a simple yet elegant method of letting me reuse existing, proven circuitry, it also gave me direct access to all of the hardware in my prototype design that was running on my Desktop NanoBoard. And all of these Altium Designer features were fully functional right out of the box.
The custom board nearing completion
