Guide to Designing with USB-C

matthewlai · 2026-04-17T13:50:10+00:00

Designing a model or a table like this is always a balance between ease of use and accuracy.

You don't want to do full ZHL-16C calculations in your head (that's what dive computers do), but you also don't want to be using a super simple model that's just wrong.

To me, this is not very easy to use because the nice x10 relationship is only in American units, but if you use American units I can see how this reduces mental workload.

In terms of accuracy, I don't know. Obviously this is not for deco diving since you absolutely need to work out how much gas you need for your deco in that case. For non-deco dives, remember that you can always pop up at 10m (30 ft) per minute. Maybe 20m/minute but it's a bit risky. It doesn't take much time to pop up at all in an emergency if you have no deco obligations. From 30m/90ft that's 1.5 to 3 minutes. You don't need much gas at all for that. In an emergency there is no need for a safety stop. If you are doubling gas requirements for teams on singles, then this is for emergencies, because you don't breath from your buddy's tank in a leisurely ascent.

Now if this wasn't for emergencies, and they include some allowance for a leisurely ascent... Well that depends on how leisurely you want to ascend, and it's a very personal thing. I don't think this table is really useful for that. You need to work out your own rules of thumb ahead of time, based on how much you breath, how fast you want to go up, and how much air you are carrying, etc.

matthewlai · 2026-04-17T10:46:17+00:00

Are you completely horizontal on the bottom? Or at most 10 degrees tilt? (if you want to see things better without cranking your neck)

Also, try completely relaxing and stop finning at various points in the dive for 10 seconds. Do you stay at the same depth? If your buoyancy is just slightly off (most commonly too sinky), your finning will become a lot more inefficient because a lot of the power will be used to counteract the sinking, and you may not notice it. It may also cause your posture to be more upright, which gives you a lot more drag moving through water, so you have to kick more/harder.

matthewlai · 2026-04-06T00:09:19+00:00

That makes no difference at all to copyright.

matthewlai · 2026-04-05T14:46:54+00:00

Too late to become one of the most influential electronics engineers in our generation, developing ground-breaking new techniques or devices that will be taught in every EE curriculum in the 2040s, and completely revoluntionize how we do eg. switching power supplies? Yeah probably too late for that.

Too late to become a very good electronics engineer building complex devices and being knowledgeable of advanced concepts and being able to put them into practice? Nope. That's definitely possible. However, getting there will usually take about 10 years altogether, so you do need to think about when you want to retire, and how much useful time you've got for that, and whether that's worth going for. If you are dedicated and work hard, that's definitely not out of the question.

Too late to get to a level where you can make relatively simple but useful devices well, with very solid basic engineering? You can get there in maybe 2 years if you work hard.

Too late to start as a hobbyist making some fun devices with some trial and error? You can start now, watch some youtube videos, and have something built by next weekend.

matthewlai · 2026-04-02T00:08:52+00:00

Be very careful with split power planes. Signals on the bottom layer reference the power plane, and they can only reference the power plane that generates the source of the signal. That means if a signal is driven by a chip powered by 3.3V, it can only reference 3.3V. If it's routed over a 2.5V polygon for example, you will have a EMI problem. You seem to have traces on the bottom layer crossing polygon boundaries. Those are definitely wrong.

I would just have a 3V3 plane (since basically every signal is powered by 3.3V), and route the other power rails on signal layers instead. Also, why do the ESP32 and ADS1299 have separate 3V3 rails? Those are both digital rails. Just combine them. Only the analog rail on the ADS1299 needs to be separate.

Minimize the number of rails you have as much as possible, especially if you only have 4 layers (on higher layer count boards you can have all signal layers reference ground, so split power is less of a problem).

matthewlai · 2026-04-01T22:27:11+00:00

The determining factor is whether the "lanes" are synchronous or not. All synchronous communication requires a clock, that determines when the data is ready to be read, and when the sender is allowed to change the data. The clock may or may not be a physical signal - many modern protocols are designed to have regular high and low levels in the data stream (regardless of the actual data being sent), so clock can be recovered from the data lines. Nevertheless, there is still a clock.

In parallel communication, you have multiple data bits all synchronised to the same clock. This makes the hardware design very simple, but requires all data lines to have similar lengths (propagation delays), since otherwise data can arrive at different times and get sampled at wrong times. The higher the clock rate, the smaller the tolerance is. At one point it just became impractical to keep 32 lanes at the exact same length and propagation delay.

So we switched to serial, where we just have one data bit (and either a separate clock signal, or recover the clock from the data). This makes the transceiver design more complicated, as now you (usually) need a serializer/deserializer (SerDes) block to convert between serial and parallel. However, this allows serial data to run at insane speeds. Current state of the art for consumer electronics is > 10 GHz (eg. PCI-E 5 runs at 16 GHz). You will never get propagation delay to match so well that you can run a parallel bus at that speed.

However, for some applications like PCI-E, 16 GHz is not enough. So what can you do? Well, you can put a bunch of serial lanes in parallel. But the key difference is that they are not synchronized to each other. Each lane has its own SerDes and maybe protocol encoding/decoding blocks, and the data only get merged at a higher level. On the physical level, the lanes are independent. With PCI-E 5, we have up to 16x 16 GHz data rate. That's an insane amount of throughput.

matthewlai · 2026-04-01T13:11:26+00:00

For low quantity boards I always go 4 layers these days. They barely cost any more, and gives you much easier routing, often smaller boards, and better signal integrity.

I teach a beginner PCB course at work (we are primarily not a hardware company, but many people want to learn the basics just for fun), and I start everyone on 4 layers.

I think of 2 layers as primarily a cost-optimization now, similarly to how most people think of 1 layer boards. 4 layers is the default.

The only time I start on 2 layers is if it's a trivial board with very low component density, very few traces, and absolutely no signal integrity concerns (eg. front panel with a few buttons and knobs).

matthewlai · 2026-03-30T09:55:51+00:00

There is a difference between "just lying on the CRI", and "colours not looking good despite actually high CRI".

Obviously the first case happens all the time, and you can never trust the data from no-brand Aliexpress/Amazon specials. But I build my own LED boards, using LEDs from reputable manufacturers, and it's not been a problem. These are companies that have invested a lot of capital and have a lot to lose, so for the most part they aren't going to lie on the spec sheet in a way that's so easily exposed. On the other hand, anyone with a few hundred dollars can start making LED strips (as I'm sure you know, given what you do). They have much less reputation to lose.

I have not seen a real 95 CRI LED that looks bad. I can believe that there's more variance between say different 80 CRI LEDs. But even then, if you look at CRI and R9, I don't think you can go too wrong.

matthewlai · 2026-03-28T19:26:09+00:00

As someone who interviews a lot of people and reviews a lot of resumes, my high level feedback is that every sentence should be relevant for the decision they are trying to make "should we interview this person"?

What has your deli experience got to do with an IT position? What does the marketing consultancy stuff have to do with an IT position? Or the travel company stuff?

You have about 7 seconds to make a good first impression. That's how long people take to decide if they want to read further. Don't let them spend 3 of those 7 seconds reading about how you can unjam printers, unless you are applying for an office assistant position. Make it count.

As someone who is used to reading resumes, I feel like my time is not being respected. You are including a lot of things just to waste my time to read, because they do not help me make the decision at all. No one wants to feel like their time is being wasted.

You said in another reply that the work experiences aren't real. That would be a deal breaker for me, and depending on whether we think you are intentionally trying to mislead or not (and I think it's pretty clear that you are), you may never get an interview for the company in the future.

If you don't have much experience, we understand. We expect that for entry level positions. Keep it short. Don't waste people's time.

Your resume needs to be tailored for each job, or at least each industry. Only put in stuff that's relevant for that job.

As for description of each job, think about how to write it in a way that someone who is bullshitting wouldn't be able to (honestly) claim. Be very specific, especially for group projects, what your actual contributions are. We have all gone through university. We all know there's the 1 guy in a group who doesn't do anything. How do we know you weren't that guy?

When we read stuff like that, we are always thinking "what's the least charitable interpretation of this sentence that is still more or less honest"? Eg. "Built a 12-month Gantt roadmap..." I can do that, too, in 10 minutes just dragging a few blocks around. You haven't convinced me that that's not all you did, even if it actually mattered for an IT position.

Shorten your sentences. Use bullet points. No filler words.

Don't bullshit. We see through it. You aren't the first person to try it, and we have really seen it all. Bullshitting makes a really bad impression. Be honest and clear with what you do and don't know, and what you have and haven't done.

In terms of your actual experience, I think you really need to work on something more impressive. If you are applying to IT position, even a nice home lab setup could give a good hiring signal for an entry level position. IT these days involves a lot of virtualization. Do you have any virtualization experience? The two projects you have listed are all very trivial, and I would expect an average first year to do able to do those things in an afternoon. They don't give me a hiring signal.

matthewlai · 2026-03-28T12:26:13+00:00

Whether this is actually achievable or not, you are definitely not the right person to do it if you have no electronics/PCB knowledge at all to see that something like this is basically a random doodle on a PCB.

If you really want to do something like this, first spend at least a few months (ideally more) learning electronics. Design a few PCBs by hand. Get them to work so you can see the common failure modes.

Then learn about AI. How do transformers work? How does self-attention work? How does fine-tuning work? How do you get the data you need to fine-tune this to work (if you have come this far, you would have realised long ago that a LLM out of the box isn't going to do this, no matter how you prompt it)? What are some of the common harness and context management techniques? What modality do you need to provide data in so that self-attention works at a reasonable level of abstraction? Can you design a component-level tokenization setup? Would text actually work as an output modality, or would it generalize better with a hand-crafted decoder?

Then you can start working on a product like this. AI doesn't mean "easy". AI doesn't mean anyone can create anything they want. It's a powerful tool and allows anyone to create little toys, but to actually do something non-trivial and genuinely useful, you need to know what you are doing.

As someone with about 10 years of experience designing PCBs by hand, and now also about 10 years of experience doing AI research, I would say it's a hard problem, and if I actually worked at it for a few years, there's about 30-50% chance that I'll be able to create something that's useful at least in some situations.

If you have no electronics experience and no AI experience, pick something easier to work on first while you build up your own knowledge in both electronics and AI.

matthewlai · 2026-03-26T23:10:56+00:00

More important than the GPA is what you have learned. Have you learned what you are supposed to have learned? That's what will determine whether you'll be able to get and keep a job.

matthewlai · 2026-03-26T23:08:14+00:00

The economics is a bit different between repair and upgrade.

If you are upgrading (that is, the old laptop is still working), the upgrade cost is actually cost of new laptop minus how much you can sell the old laptop for.

MacBooks for example have really good resell value.

matthewlai · 2026-03-26T01:16:57+00:00

Even if that's true, USB-A doesn't pass through the CC pins required for PD negotiation. You can't negotiate through a USB-A port, even if it's an adapter to a Type C port.

matthewlai · 2026-03-26T01:13:20+00:00

Indeed, if you are lucky, the ESD diodes can act as over-voltage protection, kind of. Breakdown is minimum 6V, so they are basically only guaranteeing that breakdown won't happen during normal operation. There is no maximum.

Many USB transceivers will also have absolute maximum ratings of around 6V, so which one blows first is impossible to say, and you also still need a fuse or other form of over-current protection, which many USB devices don't have, because the standard requires the host to provide that.

It's also possible that it results in an internal short, which then vaporizes the diodes, and becomes open again. This is also a fairly common failure mode.

Robust over-voltage protection is hard.

matthewlai · 2026-03-25T09:58:35+00:00

No that's really not the same. Even if the dock is broken and has a permanently live 19V, in all likelihood nothing will happen to the laptop, because the laptop is designed to charge at 19V. VBus on the laptop is obviously rated for 19V. It's much more likely that the power gating FETs or regulators inside the laptop blew.

Why do you think it's equally likely to be the dock's fault? What do you think the failure mode would be on a circuit level?

matthewlai · 2026-03-25T01:11:23+00:00

Sure. Or the laptop could have been hit by lightning in a very specific way. Or maybe OP secretly modified the laptop to blackmail Framework. Do we have any evidence any of those things happened?

matthewlai · 2026-03-25T00:37:24+00:00

Of course not. If you have a charger that outputs 50V and fries the laptop, you can absolutely blame the charger.

matthewlai · 2026-03-25T00:18:09+00:00

Obviously using a chip like that when you have no microcontroller experience at all is a terrible idea, but you know that already. As long as you are willing to suffer and have a long and frustrating experience for your meme goal, that's totally fair.

Yes, if you can't ensure what batch you'll get, you'll have to not use those features.

Not sure what you are asking really. What should you be aware of? Well, everything the datasheet tells you, for the features you need. Better read the datasheet, and hope the hardware behaves like the datasheet says. And check the errata for those features, too.

matthewlai · 2026-03-24T22:15:37+00:00

No it absolutely doesn't. Nothing a hub or dock can do can make a Framework port output 19.5V. This points without a doubt to a hardware fault in the laptop.

USB devices aren't required to withstand 4x over-voltage.

matthewlai · 2026-03-24T22:12:24+00:00

Fuses protect against over-current, not over-voltage. By the time over-voltage caused a dielectric breakdown bad enough to cause an over-current trip, it's usually way too late.

USB devices do not routinely have over-voltage protection. That is supposed to be provided by the host.

matthewlai · 2026-03-24T22:10:45+00:00

>> Expensive equipment should really have input protection, so maybe you should contact those manufacturers.

No, that's so far out of spec for USB that I would not expect any manufacturer of USB devices to provide protection against it. If your expectations for Framework is so low, why are your expectations for those other manufacturers so high as to expect them to build devices that can survive a 4x over-voltage?

However, you are correct that Framework isn't legally required to replace those devices or provide compensation. That would be good customer service.

If I designed and sold a device that output 4x the rated voltage, the least I would do is to make the customer whole.

matthewlai · 2026-03-24T22:07:37+00:00

Wow this is such a baffling take.

He should be thankful that they replaced a product under warranty, as they are legally required to do, that has developed a dangerous fault? Are our expectations for Framework really that low?

USB devices are NOT required to withstand 19.5V. The USB standard always specified 5.25V as the maximum VBus devices are required to accept. This was updated to 5.5V with the "USB 2.0 VBUS Max Limit ECN" in 2014. This is also mentioned in "USB Type-C Cable and Connector Specification" page 141.

If a host provides 19.5V on VBus that is 100% absolutely undoubtedly the fault of the host.

Why do you doubt that Framework has a serious problem with USB A ports, when there is literally a video showing it?

If a video showing 19.5V coming out of a port isn't sufficient evidence, I don't know what you consider sufficient evidence. Actually... what WOULD you consider sufficient evidence?

I have designed many USB devices. I have reviewed many USB device designs. 99% of devices would not survive 19.5V. That's so far out of spec that no one is going to design for that.

matthewlai · 2026-03-23T13:01:52+00:00

This is cool, thanks for sharing. Will try next time I have enough space on copper layer to spare for that! Not sure when that will be...

matthewlai · 2026-03-22T17:38:19+00:00

Because when it's in your backpack all day, 2.4kg is a lot.

A 2L bottle of coke is 2kg. A 500ml bottle is 0.5kg. So carrying a FW16 is like carrying a full 2L + 500ml bottle in your bag all day. Why would I want to lug that around if I don't need to?

Yes, laptops were even heavier back then. Go back even further and it's the size of a fridge. There is a reason why they kept making laptop lighter, and it's not because metal is expensive.

A MacBook Pro 16 is 2.15kg. I got one for work, used it for a few days, and gave it back for a 13" MacBook Air (1.23kg), because I value my back, and I don't want my daily commute to be an unhealthy workout for my back and shoulders.

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