Breaking out the surface mount pads on the bottom of the BX


#1

How best might one go about breaking out the inner surface mount pads on the bottom of the BX board?

I figure 2.54mm SMT headers would do nicely, but then I wonder if soldering the outside (long) main DIP pins upside-down would be preferable then (for breadboarding accessibility, though it means the LEDs and reset button would be facing the board), or if there’s some other even-more-useful configuration?


#2

I ordered one of these things myself. Might socket it.

https://www.tindie.com/products/loglow/tinyfpga-bx-breakout-revision-a/


#3

That breakout board is the way to go if you want to have access to all the bottom pins and use it in a breadboard. The outer two rows of that breakout board can snap off as well to make it skinnier.


#4

I appreciate the tip and will give it some thought. I’m not sure I need all the extra pins broken out on both, but I wanted to do it on at least one of them for experimenting purposes. There’s a 15% discount if you sign up for their newsletter so that’s a plus.

What are the GBIN pins all about? What would you use the SPI flash pins for other than what they are already doing? I see there’s another thread here on the latter, though I’m not sure how that all works in practice yet.

Edit: Actually, the outer two rows appear to be wired as two bus bars each so yeah, they aren’t terribly useful unless you’re screwing it into some enclosure.

Edit 2: I did some extrapolating from the photos, and (without those bus bars) it adds three pins (0.300") of width to the BX’s already 0.700" wide footprint. On a standard breadboard that 1" total width on the carrier board leaves one exposed outer hole per pin to easily work with (if you have two standard breadboards with a single bus bar in between them, you should get three outer holes on each side by straddling the bus bar). If it were any wider that’d become annoying, but as it is it would be just right for even a single breadboard so that’s a plus in my book.


#5

Hi there,

I make the breakout board kits that were mentioned earlier in this thread.

Re: the two bus bars (“carrier rails”): There’s no soldermask on the bottom side of them, so if you need to separate the bussed holes it’s really easy with a razor blade.

These bus rails make socketed connections (often used in robotics) really easy when you need a 3-wire header that has signal, voltage, and ground.

Let me know if I can answer any questions about the breakout!

Assembled boards will be available soon also.

Best,
Dan


#6

That makes sense!

My order just arrived - now I just need to figure out how to best solder it all together this weekend… :smiley:

Edit: Interestingly, the 2x3 SMT header is a bit taller than the 2x8 header, so much so that I wonder if the 1v2 pogo pin will actually make contact (the others should with some tilting - or a different header if I can find one to replace the 2x3 ones that matches the 2x8’s slimness).


#7

Just so it’s clear what I’m talking about. The difference is a bit less than 1mm, but the pogo pins aren’t very forgiving on height (perhaps I can solder the pogo a bit higher to compensate). It looks huge here but it’s magnified of course (I kept even pressure on it and neither the breakout nor the BX has any curvature to note).

The second image shows better the difference in construction between the two. I did try pressing down firmly on just the plastic part of the 2x3 to try and level it, but it won’t budge (it’s well-made, it’s just not quite the same design as the 2x8 - the different plating being the obvious clue).

I’ll figure it out, just thought you should know.


#8

Followup as I solder - one kit has the too-tall 2x3, the other one is the correct height for the job. I don’t have spares but will hold off on soldering the kit with the wonky header til we’ve talked.


#9

Wow, that was a surprisingly intricate little project! I ended up breaking out only one of the two BXs, and just put a standard set of headers on the other one for now. Maybe down the road I’ll tackle the other one or something - at least I’ll have an extra kit handy for it.


#10

Hey MartyMacGyver, thanks for the excelent photos in your posts!

I do sometimes source the double-row SMD headers from different suppliers, hence the slight differences in height that you noticed here.

The key is this: After you solder them onto the BX board, it shouldn’t matter much if there ends up being a little bit of space between one of the header’s insulation and the breakout, especially since the header will end up being soldered solidly to both boards. There will definitely be some space between the insulation on the outer rows of through-hole headers. Does this make sense?

In any case, I’m always happy to send replacements if any of the components are missing, broken, or wonky. Just let me know :slight_smile:

Finally, I’m planning to update the datasheet with specific assembly instructions soon. In the meantime, it might be helpful to check out the assembly instructions for one of my Teensy breakout kits, since the concepts there are extremely similar, and the BX kit assembly is overall simpler.

Best,
Dan


#11

Also, someone mentioned socketing the BX earlier…

I do have parts for socket kits available, but I’m not sure yet if I’m going to sell them as an actual product or not. If anyone wants them on a case-by-case basis, just contact me and we can work something out. You can also source them all pretty easily. Samtec is pricy (but generous with samples) and the longer pogo pins can be sourced from Digi-Key:

  • 2x Samtec SLW-114-01-F-S
  • 1x Samtec SLW-108-01-F-D
  • 1x Samtec SLW-103-01-F-D
  • 2x Samtec MTSW-114-07-F-S-170
  • 1x Samtec TSM-108-04-L-DV
  • 1x Samtec TSM-103-04-L-DV
  • 3x Mill-Max 0908-6-15-20-75-14-11-0

Assembly is pretty straightforward, just contact me if anything’s unclear. It’s actually faster too, since you don’t need to trim any of the (expensive, custom-length) pin headers.

Why do some of them need to be custom lengths? Because the surface-mount headers and the through-hole headers sit at different heights from the board, so the pins need to be different lengths to end up mating well with the uniform-height low-profile socket strips.


#12

The gap on the pins on the perimeter of the BX was expected and present on the one I made up (with the matched SMT headers on the inside). Those SMT ones set the limit on that height, which is why the unusually tall 2x3 was worrisome - it’s just about the same height as the pogo pin next to it, so there would be less pressure (and less contact) with that pogo pin.

(If not for the pogo pins this would all be moot, but they just don’t allow much tolerance. As it is I’m surprised I got it all together by hand, particularly getting the pogo pins to stand perpendicular to the board. Definitely an interesting project.)


#13

Cool, I’m very glad to hear that everything worked out ok! :smile:

Most of the double-row headers should be of the slightly lower-profile type.

Thanks again for your great photos too.

Once TinyFPGA has the BXs back in stock, I’ll be able to offer assembled breakouts as well…