HW5-OTP: A super small single-system HW5 variant.

[Ancyker]

Overall, the concept is simple: A HW5 variant that only has a single cartridge slot. Sure, there are single-system slot-adapter PCBs, but I don't think they are that popular. Likely because they work with the main PCB and frame, so they are huge relative to how small they could be. You could get a six-slot adapter and only solder the slot for the system you want, then you could add the others later if you change your mind, or don't, but doing it that way doesn't change the cost so why not?

Well, it didn't, until now! Enter the HW5-OTP, a super small remix of HW5. How small? Just a little taller than the LCD module, that's how small! It's all of HW5 on a single PCB. It's designed in such a way that you are supposed to solder the MEGA MINI to the back directly -- without any headers -- just having it sit flush against the board. You can do the same with the clock gen for systems that need that. You then plug it into the LCD module directly -- no wires. The result should be an OSCR that is just a little taller than the LCD + slot. For most systems, the thickness should be < 15 mm without a case.

Here are the renders:

As you can see, this is pretty much a full-featured HW5 other than the number of slots (there's also no reason someone couldn't make it taller and add more than 1 slot). In fact, firmware-wise this is still HW5, hence the name. There are no HW5-OTP-specific settings that need to be toggled or changed, you flash it just the same as HW5. The only flags you need to worry about are which systems you want on the menu, and the ones for if the clock gen, VSELECT, and/or the RTC are installed.

The more observant among you may have noticed there is a place for a clock gen yet this is a GBA board. That's because overall the board is meant to be semi-universal. The idea is you can delete the GBA slot, add whatever slot you want to use, route the traces, and off you go. There are a few gotchas, however:

• The 3.3V regulator is needed even if the system you are making it for doesn't need 3.3V.
• VSELECT is not optional if you want to support multiple voltages (of which the only single system that applies to is the GBA as far as I know).
• If you are only working with a single voltage you do not need to install VSELECT. There are solder jumpers on the back you can bridge that will supply the specified voltage instead of needing to use VSELECT. Note that some MEGAs don't like being flashed at 3.3V so you may have issues flashing via the USB C port without VSELECT. If that happens you can flash the MEGA using its own USB port instead. Just don't plug something into both ports at the same time.
• As you can see, in order to make it this small the use of SMT components was unavoidable. I even had to use SMT LCD headers so the mega could sit flush with the PCB.
• This is a 4-layer PCB. This wasn't needed for the GBA and probably not for other systems either, but it makes the routing easier to do and also lets the components be closer together. A 4-layer PCB costs almost the same as a 2-layer PCB, and a single 4-layer PCB from JLCPCB is cheaper than the 2+ 2-layer PCBs HW5 normally needs.

With that said, I still think it's a nifty little take on the standard HW5. I'm also using it to prototype my idea for avoiding the VBUS jumper wire on HW5 by bypassing the MEGA's USB port altogether. This is done by reimplementing the CH340 on the PCB. On HW5-OTP I did this as the intended way of using it and no alternative is natively provided, but with standard HW5 it will be optional. You can do the jumper wire as we've been doing or use this new method. A big upside is you get a USB-C port (that is properly implemented) regardless of what your MEGA has on it.

Here's the USB port schematic:

A lot of that is just USB protection stuff as well as a proper USB Type-C implementation (it's not as simple as just swapping the ports -- if you have ever had issues where a USB C charger/adapter wouldn't power a "USB C" device, this is why). One thing I'm still not sure of is if the method to auto-reset when flashing will work, which is why I added a jumper that can be cut if it messes anything up. That's part of why I needed to prototype it 😅

As for the rest of the schematic, I tried to add lots of notes that explain what stuff is doing/why it's there/etc to make it easier for someone to make their own version for whatever system they want even if they aren't super familiar with everything going on...

After I confirm it works I'll push it to a new folder called main_pcb_hw5-otp. It will only be for the GBA/GBx, but if people want I could make and push the ones from the six-slot adapter as well. Now that it's all done changing the slot is pretty easy/quick, 5-15 minutes. Also, it can even be made to work directly with the systems that currently need adapters, like the WonderSwan. Someone could also remix it to support a few systems with a specific theme, like only having SEGA systems, or only having portables, etc.

If it works, all that's left is for someone to make a little case for it. Ideally, it would leave the top part above the LCD exposed so it would work with any system rather than needing system-specific ones. At least a case that covers the back of the PCB would be best, as there are exposed components on the back (mostly the mega, but also a few capacitors for the USB-C port).