Cool project. Thanks for doing a write-up that includes so much detail.
I do a lot of PCB review for students and makers, so if you're interested I wrote up some notes. Consider this an incomplete review because I only had so much free time, but hopefully this information can be helpful for your next rev or next project.
Design:
- Add ESD protection to all pins exposed by connectors and around the buttons. A simple TVS will work. However, the capacitance of the TVS needs to be small enough to minimize burden on your higher speed lines like USB and VGA. I'd use a cheap TVS for the low speed pins and then maybe look at some dedicated USB-HS protection parts and then just re-use those on the VGA lines. The USB3300 part you're using looks to have some minimal built-in ESD protection but as you observed it's probably not enough for real world use.
- Consider a buffer between your resistor DAC and the output. This could be an op-amp or a dedicated buffer chip. This will let the resistor DAC operate without significant load, isolated from the cable. You can then put a 75 ohm resistor in series with the buffer output to get optimal impedance matching with the 75-ohm VGA cable and load. You can get away with resistors connected straight to the VGA cable for demo purposes, but having it properly buffered and impedance matched will clean up the edges even further.
- Look into the design of an R-2R DAC as an alternative to the weighted-summing configuration you've used. You would want 0.1% resistors at 8-bit depth but you'd only have to buy and manage 2 resistor values (R and 2R) which is helpful when doing hand assembly.
- Better yet: This project is a good place to use a real DAC chip. Resistor DACs are cool to play with and useful for ultra-cheap demo boards where manufacturing cost is a high priority (Like the RP2040 VGA demo) but for a hand-assembled project like this a good DAC chip is great choice. It would shrink your PCB, reduce assembly time, and allow you to choose a physically smaller MCU package because you don't need 24 IO pins for the resistors.
Schematic:
- When using hierarchical schematics it's a good practice to avoid putting ICs and components in the root sheet. I suggest adding additional sheets for the MCU and other parts, then connect the blocks in the root. Treat the root page like a block diagram.
- KiCAD's bus functionality could help reduce a lot of the pins going into your blocks, like USB_D[0:7] and VGA_R[0:7]
- Breaking up that big STM symbol into a couple parts would be a good way to learn how to use KiCAD's symbol editor and would allow everything to fit on standard A4 sheets
- Don't hesitate to add notes to the schematic that might be helpful during debugging, assembly, or for your future self when you return to the project.
- I like to do a cleanup pass on schematics to make sure all labels are readable and there's enough space to comfortably see everything. It's a good practice to get into and pays off when you're tired from a long debugging session and trying to read a schematic. I suggest using the empty space on some of your pages to space components out more so no text is overlapping or hard to read.
PCB:
- You used 0.15mm (5.9 mil) traces almost everywhere. It's best to avoid using small traces unless you have to. You want to stay away from the minimum trace/space of your PCB manufacturer by default and reserve those for only the places where you need it. It will improve yields and make rework easier. Many of your traces could be 0.25mm or 0.5mm without any problem.
- Likewise, avoid letting traces run too close to pads unless you really need to. It's okay when you need to squeeze a trace into a narrow spot, but by default you should try to give as much room between pads and traces and between adjacent traces as you can. The tighter the spacing, the easier it becomes to accidentally short something out during assembly.
- Similar story with vias: Try not to use the absolute smallest vias everywhere when you have space.
- A lot of your vias are grouped together in a way that creates large slots in your ground plane. This doesn't matter too much at the low speeds you're working with, but it's good practice to avoid creating large cutouts in ground planes. The return current has to flow somewhere and those big slots will force it to take a longer path, which increases emissions and distorts high speed signals.
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