Designing a Ext. Prototyping Breakout (With Raspberry Pi Hat Compatibility)

[DustinWoods]

This thread is for discussing designing an Ext. breakout module for prototyping and potentially connecting raspberry pi hats.

Some Design Considerations

• Cheap
• Usable as just a PCB (no extra components)
• 2.54mm spacing with through-hole connectors
• Compatible (or at least safe) with raspberry pi hats

Compatibility
I’ve been spending some time getting an iteration worked out. Below are some schematics, layouts, and pin assignments. The main image I’d like to draw attention to is the pin assignment sheet. You’ll notice that this design allows UART, I2C, PCM, SPI, and PWM functions to exist in the same locations. However, with limitations, this leaves several general purpose pins unconnected. Which is why this breakout wouldn’t be guaranteed to work with all hats. However, with with pin functions, 5V, 3V3 and GND all in the same respective locations, this should make hats safe to plug in.

Another requirement to get a hat working on the DevTerm would be remapping pins in the driver /software.

Cables
With the 2.54mm spacing, this design would be perfect to solder an IDE ribbon cable, exiting the DevTerm printer door. If you already own a Pi Cobbler, you could plug the cable right into this!

USB, Speakers, and Camera
For compatibility, the 40-pin breakout would not include the camera pins, speaker pins, or USB pins. These are special non-gpio pins. To not completely remove their functionality, I think adding solder pads might be helpful. If this design had a larger scope, it would be possible to re-incorporate the USB ports, camera connector, and speaker pads. But to keep this design low-cost and simple, I think simple pads for these should be fine.

Feedback
If anyone else is interested in this design, please feel free to jump in on this conversation. At the very least if nobody is interested, this thread will serve as documentation.

Pin Guide

Current Progress

Feel free to email me if you are interested in one of these boards! dustin@whatup.email
I’ve got a handful of extras All gone for now!.
The next production will be a revision. I’ll also get around to posting the gerber files for anyone who wants to order these PCBs themselves.

Update 06/10:
Here are the gerber files you can use to order the proto board and the ext board. It’s important to use a PCB thickness of 1mm for the ext breakout! This will ensure it fits in the connector.

• Ext Breakout v1
• Proto Board v1

[Metonym]

just like what @qqMajiKpp said, someone get this guy a cape

[qqMajiKpp]

make it L shaped so it sticks out the bottom of the case, then it wont matter how much space there is for a header

[DustinWoods]

Here we go! All traces connected, and things labeled. I managed to squeeze speaker pins in there too.

More to come: Looks like the printer area has lots of clearance, I’m working on a proto board for that area. Would be perfect to house soldered components.

[DustinWoods]

Quick render to for context

[DustinWoods]

Last follow up for the day.
Here are some quick mess mockups of a proto board used with the breakout board. The pin alignment would allow the ribbon cable to attach to the proto board, but also extend out of the enclosure for bread board prototyping. (I have barely yet put thought into the proto board layout)

[qqMajiKpp]

i still think bending it 90 degrees and extending it out the bottom to fit a full header would be nice… soldering a ribbon cables seems like a huge pain to me. Awesome work regardless!!!

[DustinWoods]

How about if the ribbon cable has a header to solder on? I wouldn’t want to solder on individual wires either (unless I was hacking something together).

I think your idea is clever! My only complaint would be that you couldn’t close the enclosure all the way (unless I’m not thinking about it correctly?)

(FYI: I’m thinking about throwing a kit together that includes the PCBs and the ribbon cable.)

[qqMajiKpp]

you know whats up and can picture it in your mind. you are wholly correct. my idea requires either a modification to the shell design and then print at home or to actually modify the case (full disclosure, im prone to the latter since i cant print… anything) i paid primo on shipping to get 5 sheets with designed templates with battery labels…

[Godzil]

PCB cost is by the surface, the bigger the more expensive.

As expensive ribbon cables are, they are still cheaper (and either to replace) than extending the PCB to connect both sides.

The only potential issue with the ribbon cable is signal integrity especially with high frequency stuff if not done properly.

As for connecting the ribbon, don’t use an IDC one for internal, just a flex type, but even, I’m pretty sure there is enough space to have a proper pin header ont eh PCB and use IDC cable with already put connector on them (like a good old IDE/SCSI cable)

But for the board to board inside, use a flex instead there are plenty with enough pin, you can always ahve the pinheader on the protoboard to be able to connect a more robust ribbon cable using a proper connector

In fact I’m pretty sure thare are extender cable for that PCIe connecor they use that would fit and allow to no need the small PCB and not have to bother with the connection to the protoboard as all you would need here is the edge connector

[qqMajiKpp]

the main board already uses an fpc connector… if i had a reflow station, i’d just put an fpc connector on the ext board if i could… i could still convert that to generic breakout, no need to modify the case, form factor and uniform. but if im sending a a file for print, i might pay the extra for a bigger board.

[DustinWoods]

@Godzil

The only potential issue with the ribbon cable is signal integrity especially with high frequency stuff if not done properly.

I was wondering about trace length too. In some PCB designs, high speed data lines need equal trace lengths. This is a little beyond my knowledge though.

@Godzil

I’m pretty sure thare are extender cable for that PCIe connecor…

This was the first thing I looked into! I found a couple cables online, but it was difficult to find (specifically the mini pci-e). It then occurred to me that these extensions wouldn’t have any support to keep them plugged in - something that’s solved with a custom PCB and the holes for the standoffs.

@Godzil

As for connecting the ribbon, don’t use an IDC one for internal, just a flex type

I’m pretty keen on using the ribbon cable internally. Ribbon cables can be spliced and soldered to things. Much more versatile for prototyping. For example: Connecting to the proto board is not the only intended use. I’ll post some more images soon for all the use-cases I’m thinking, but imagine wanting to do breadboarding externally or even have the proto board external from the DevTerm while working on it. A ribbon cable (with a few inches extra length) would help with these different use cases.

[DustinWoods]

I’m still working ouut some details for orientation of pins and such. But this is my current concept for the proto board:

Note: Even with the design emphasis on being pi compatible this board doesn’t need to be used as a pi-hat adapter. It could be used for standard prototyping. Again: versatile!

[qqMajiKpp]

when i think about hanging a ribbion out of my devterm… it’s not for raspberry or Arduino type prototyping… im picturing lil john o’conner at the atm in terminator2

[DustinWoods]

Quick update: I got a response from help@clockworkpi.com… we now have the official Ext board outline specs! I’ll be making some revisions to my breakout design soon.

Here’s the DXF they sent me: Ext_Module.dxf - Google Drive

But to save you some time, here are screenshots from that file:

[DustinWoods]

Something I’m confused about is the PCI-e connector. The specs they sent don’t seem to align with a photo of the actual PCB on instagram. Gonna try to confirm things…

[DustinWoods]

@yong - could you help us out with the Ext board specifications? Just trying to figure out if this DXF file is outdated, or if the photo on instagram is for a outdated prototype. Thanks!

EDIT: looks like maybe a 2mm discrepancy? (the spacing between the Ext and Main board)

[yong]

The DXF file is up-to-date. Thanks.

[DustinWoods]

Thanks!

Another update on the breakout board: I’ve got the footprint updated with the official Ext specs.
I ran into an issue with the through-hole design on this board, as I scouted for ribbon cable assemblies, I the shortest I could find was 7.8mm tall, but we’ve only got 5mm clearance. So I had to rethink how to connect a ribbon cable with this clearance. Came up with something clever using pads and a 2x20 header! Here’s the new revision:

[qqMajiKpp]

i imagine the case will need modification to fit that… not that im complaining or anything… maybe a lil surprised someone actually threw caution to the wind and said to heck wit the case. not many folks seem to feel the way i do about modding the chassis