Proper Routing Makes For Many Happy Return Paths

March 18, 2024 by Dan Maloney 18 Comments

Here’s a question for you: when your PCB has a ground plane layer, where do return signals flow? It seems like a trick question, but as [Kristof Mulier] explains, there’s more to return path routing (alternate link in case you run into a paywall) than just doing a copper pour and calling it a day.

Like so many other things in life, the answer to the above question is “it depends,” and as [Kristof] ably demonstrates in this concise article, the return path for a signal largely depends on its frequency. He begins by explaining current loop areas and how they factor into the tendency for a circuit to both emit and be susceptible to electromagnetic noise. The bigger the loop area, the worse things can get from a noise perspective. At low frequencies, return signals will tend to take the shortest possible path, which can result in large current loop areas if you’re not careful. At higher frequencies, though, signals will tend to follow the path of minimal energy instead, which generally ends up being similar to the signal trace, even if it has a huge ground plane to flow through.

Since high-frequency signals naturally follow a path through the ground plane that minimizes the current loop, that means the problem takes care of itself, right? It would, except that we have a habit of putting all kinds of gaps in the way, from ground plane vias to isolation slots. [Kristof] argues that this can result in return paths that wiggle around these features, increasing the current loop area to the point where problems creep in. His solution? Route all your signal return paths. Even if you know that the return traces are going to get incorporated into a pour, the act of intentionally routing them will help minimize the current loop area. It’s brilliantly counterintuitive.

This is the first time we’ve seen the topic of high-frequency return paths tackled. This succinct demonstration shows exactly how return path obstructions can cause unexpected results.

Thanks to [Marius Heier] for the tip.

PCIe For Hackers: Our M.2 Card Is Done

July 25, 2023 by Arya Voronova 17 Comments

We’ve started designing a PCIe card last week, an adapter from M.2 E-key to E-key, that adds an extra link to the E-key slot it carries – useful for fully utilizing a few rare but fancy E-key cards. By now, the schematic is done, the component placement has been figured out, and we only need to route the differential pairs – should be simple, right? Buckle up.

Getting Diffpairs Done

PCIe needs TX pairs connected to RX on another end, like UART – and this is non-negotiable. Connectors will use host-side naming, and vice-versa. As the diagram demonstrates, we connect the socket’s TX to chip’s RX and vice-versa; if we ever get confused, the laptop schematic is there to help us make things clear. To sum up, we only need to flip the names on the link coming to the PCIe switch, since the PCIe switch acts as a device on the card; the two links from the switch go to the E-key socket, and for that socket’s purposes, the PCIe switch acts as a host.

While initially routing this board, I absolutely forgot about one more important thing for PCIe – series capacitors on every data pair, on the host TX side of the link. We need three capacitor pairs here – on TX of the PCIe switch uplink, and two pairs on TX side of the switch – again, naming is host-side. I only remembered this after having finished routing all the diffpairs, and, after a bit of deliberation, I decided that this is my chance to try 0201 capacitors. For that, I took the footprints from [Christoph]‘s wonderful project, called “Effect of moon phase on tombstoning” – with such a name, these footprints have got to be good.

We’ve talked about differential pair calculations before in one of the PCIe articles, and there was a demo video too! That said, let’s repeat the calculations on this one – I’ll show how to get from “PCB fab website information” to “proper width and clearance diffpairs”, with a few fun shortcuts. Our setup is, once again, having signals on outer layers, referenced to the ground layer right below them. I, sadly, don’t yet understand how to calculate differential impedance for signal layers sandwiched between two ground planes, which is to say – if there’s any commenters willing to share this knowledge, I’d appreciate your input tremendously! For now, I don’t see that there’d be a tangible benefit to such an arrangement, anyway.

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Machining Wood Inlays, No CNC Required

October 17, 2021 by Dan Maloney 13 Comments

It’s almost hard to remember a time when the obvious answer to most questions about manufacturing wasn’t “Throw it on the CNC.” CNC machines have become so entrenched that the acronym has become a verb; few people would misunderstand a statement like “Let’s just CNC that.”

But before CNC machines became so ubiquitous, there were plenty of clever tricks for cutting material in a controlled fashion, as [Pask] shows us with this tool to machine wood for inlays. The tool is called a parser (or passer) drill, and is designed for use in conjunction with a steel template. [Pask]’s version seems pretty easy to make; a pair of mild steel bars are forged flat into spade shapes before having a cutting surface ground into them. The two halves of the drill are welded together and ground down to fit in the chuck of a hand drill, a modern nod to the fact that few people will want to use the traditional bow and breastplate that drove the original parser drills.

In use, a steel template that determines the shape of the inlay is affixed to the workpiece. The cutting edges of the bits are plunged into the template cutout to machine out the wood; the overhangs of the bits act as depth stop and guide. It only takes a few seconds to make a neat, CNC-free inlay. The video below shows the tool being made and in action.

It’s nice to see what can be accomplished without the need for fancy CNC machines. Not that we have anything against them, of course, but when the same results can be had with some scraps of steel and a little ingenuity, it’s pretty impressive. Looking for something between manual tools and CNC for woodworking? The pantorouter might be just your speed.

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Get Over Your Fears

October 24, 2020 by Elliot Williams 23 Comments

Some projects are just too complex, that’s for sure. But I’d be willing to bet that some things you think are too difficult actually aren’t, and it may be that all you need to get over your personal hurdle is a good demonstration. Here come three cases in point.

I was looking at the new Raspberry Pi Compute Module last weekend. They have a whole bunch of high-speed traces: things like Gigabit Ethernet, HDMI, and those crazy-fast SDI serial camera interfaces. I have no experience in high-speed design and layout at all, and frankly it gives me the willies. But the Raspberries also shipped me an IO demo board, and concomitant KiCAD design files, with the review board. Looking at it, they were just wires — maybe pairwise length-matched and impedance controlled — but also just wires. Opening up the KiCAD board file and clicking on the traces just like I do with my own designs, I’m a lot less scared. That was a revelation for me.

In a great writeup of his experience building ten different Linux single-board-computers from scratch, Jay Carlson had a similar effect on me. I would never have considered breaking out the hotplate for some CPU-and-DRAM action, and I’ve never had to lay out a PCB with a high density BGA chip before either. I’m not quite into Dunning-Kruger territory yet; I still have a healthy respect for the layout intricacies in fanning out a tight BGA CPU into a DRAM. But Jay’s frank assessments of what is easy and what is hard make it all seem within the realm of the doable.

As Mike and I were talking on the podcast about Jay’s work, Mike came clean about his fear of BGAs. I’ve done enough reflow-plate soldering, with parts that have a lead pitch that’s a factor of two finer than the 0.8 mm pitch BGAs in question, so it doesn’t seem implausible to me. And I’m 100% sure Mike could pull it off too, but he is in need of a BGA guru. Any good hobbyist videos out there?

Being a nerdy type, I’m much more focused on the knowledge and the inspiration, but maybe the courage is equally important — at least I think I undervalue it. I don’t need to lay out HDMI lines, or build a from-scratch Linux box, but I am no longer afraid that I couldn’t, and that’s because I’ve seen detailed examples of fellow hackers who’ve done the same. I might not get it right on the first shot, but I’m not afraid to try, and I wouldn’t have said the same before looking over other folks’ shoulders. Forza e corragio!

Extreme Pi Overclocking With Mineral Oil

November 30, 2018 by Bryan Cockfield 31 Comments

Liquid cooling is a popular way to get a bit of extra performance out of your computer. Usually this is done in desktops, where a special heat sink with copper tubing is glued to the CPU, and the copper tubes are plumbed to a radiator. If you want dive deeper into the world of liquid cooling, you can alternatively submerge your entire computer in a bath of mineral oil like [Timm] has done.

The computer in question here is a Raspberry Pi, and it’s being housed in a purpose-built laser cut acrylic case full of mineral oil. As a SoC, it’s easier to submerge the entire computer than it is to get a tiny liquid-cooled heat sink for the processor. While we’ve seen other builds like this before, [Timm] has taken a different approach to accessing the GPIO, USB, and other connectors through the oil bath. The ports are desoldered from the board and a purpose-built header is soldered on. From there, the wires can be routed out of the liquid and sealed off.

One other detail used here that we haven’t seen in builds like this before was the practice of “rounding” the flat ribbon cable typically used for GPIO. Back in the days of IDE cables, it was common to cut the individual wires apart and re-bundle them into a cylindrical shape. Now that SATA is more popular this practice has been largely forgotten, but in this build [Timm] uses it to improve the mineral oil circulation and make the build easier to manage.

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DIY Bookshelf Is More Than Meets The Eye

January 14, 2018 by Tom Nardi 28 Comments

It might surprise you, Dear Reader, that not every project featured on Hackaday needs to pulsate with LEDs, or update the world about its goings-on over Twitter. They don’t even, contrary to what you may have heard, need to have an Arduino inside. No, sometimes you can pull off a pretty neat hack with nothing more than some wood, a couple of tools, and a unique idea which repurposes something that would otherwise be in a landfill.

Such is the case with the latest project from [Keith Decent], which uses plywood and the spines of old books to create a secret compartment “bookshelf”. The concept is probably best described as a roll-top desk on its side, and while the action does appear a little stiff, it scores extra points for how easy it looks to replicate.

Using a router, [Keith] cuts a channel into the top and bottom sheets of plywood, which the “books” will eventually ride in. This channel goes around the entire perimeter of the shelf, and it’s important to make it as straight as possible so nothing binds up. To make sure things move through as smoothly as possible, some sandpaper is used to clean-up the inside edges.

The next step is to rip some books apart and salvage their spines. Used books can be purchased for next to nothing at flea markets, so even if you don’t have a home library filled with vintage tomes to eviscerate, it should be easy enough to get your hands on some if you want to build your own version. For sanity’s sake it would seem that books with the same size spines are ideal, so keep an eye out for old sets of encyclopedias and the like.

When the spines are removed from the books, they get glued to individual wooden slats. These slats then have holes drilled in the top and bottom, and standard wood screws driven in to act as “rollers”. Real rollers would undoubtedly make for smoother action, but you can’t beat his method if you’re trying to get it done cheaply and quickly.

The slats are then glued onto a piece of fabric, creating what is referred to as a tambour. The fabric backing links all the slats together and makes it so that pushing and pulling one slat will move them all together as one. The book spine tambour is then inserted in the routed channel, and the back panel of the shelf can be installed to lock it all together.

At this point the project is essentially done, but [Keith] does take it the extra mile by sealing all the book spines and doing some finish work on the shelf to make it look more like a real vintage piece of furniture instead of some scrap plywood screwed together.

If this exercise in woodworking has gotten you interested in the wonderful world of dead trees, you’re in luck. We’ve covered several woodworking projects from the hacker perspective, so you won’t be completely lost.

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Becoming Your Own ISP, Just For Fun

November 18, 2017 by Lewin Day 46 Comments

When moving into a new house, it’s important to arrange for the connection of basic utilities. Electricity, water, and gas are simple enough, and then it’s generally fairly easy to set up a connection to an ISP for your internet connection. A router plugs into a phone line, or maybe a fiber connection and lovely packets start flowing out of the wall. But if you’re connected to the internet through an ISP, how is the ISP connected? [Kenneth] answers this in the form of an amusing tale.

It was during the purchase of data centre rack space that [Kenneth]’s challenge was laid down by a friend. Rather then simply rely on the connection provided by the data centre, they would instead rely on forging their own connection to the ‘net, essentially becoming their own Internet Service Provider.

This is known as creating an Autonomous System. To do this involves several challenges, the first of which is understanding just how things work at this level of networking. [Kenneth] explains the vagaries of the Border Gateway Protocol, and why its neccessary to secure your own address space. There’s also an amusing discussion on the routing hardware required for such a feat and why [Kenneth]’s setup may fall over within the next two years or so.

It’s not for the faint hearted, and takes a fair bit of paperwork, but [Kenneth] has provided an excellent guide to the process if you really, really just need to own your own corner of the internet. That said, there are other networking tricks to cut your teeth on if you’d like a simpler challenge, like tunneling IP over ICMP.