4/9/2014— Jia Huang
About 2 days ago we put Tessel in production. We just got back the time estimates from our manufacturer– it’ll take about 1.5 months for production to finish, and if we’re lucky, 2 months till ship.
One year ago Jon, Tim, and I set out to build what would become Tessel. Now that we’ve put Tessel in production (for real this time), I think it’s a good time to go through all the different revisions we made along the way.
Version 1, April 2013
Jon mainly designed this board for a class on electronics prototyping. I believe it was supposed to stream music. The main MCU was a Freescale chip and there’s a breakout for Arduino shields. There’s a population option for a second chip (probably as a programmer of some sort). The Wifi chip is already present.
Jon pretended that the LEDs could blink.
Version 2, May 2013
I started working on the PCB and electrical design at this point. The board is completely redesigned with the chips we are still using (although in different footprints). For this revision we were just trying to get the main LPC chip working on our own PCB. I messed up on the JTAG breakout that’s used for programming though, so I ended up having to splice on some wires in order to get the right pin connections.
We got the LEDs actually blinking.
Version 3, June 2013
This is basically the same as the previous version except we fixed all the previous mistakes. All the main components are populated at this point though I think we still had some problems with WiFi.
It was taking me about a day to assemble one of these things even with a stencil. Jon and I would sit side by side and make two of them, of which one would actually work. I think we only ever had two or three of these things working.
Around this time I started getting really scared that WiFi would never work so I made all these WiFi breakouts. They look pretty similar but I was playing around with trace spacing and top and bottom copper.
Version 4, July 2013
We had Eric contracting for us at this point so I got lucky and didn’t have to lay out PCBs anymore. Obviously Eric is way better at it than me because this revision had us going from chips with visible pins (and solderable by hand) to Ball Grid Arrays. Worthington Assembly helped us make this batch.
You can see the beginnings of our 10-pin header modules start to form along with the final formfactor.
Wifi actually worked at this point. We could probably run the current Tessel firmware on these boards if we really wanted to.
Version 5, August 2013
Eric got fancy and rotated the chip which gave us room to add the GPIO bank at the back. We lost some room for the mounting holes though and ended up only having 2 in the middle. There’s also a WiFi debug port so we could debug that easier.
We had some USB issues with this revision, but were able to run it fully. This is the model shown in our crowdfunding videos.
Version 6, October 2013
We added some additional power regulation features on this board so that it could be powered externally. There’s also a breakout for the WiFi antenna so that a bigger external antenna can be attached if the chip antenna does not provide a good signal.
The boot selector pins (pins that determine if Tessel boots from USB or from internal Flash) are exposed so that we could revert bad firmware versions easily.
We still ran into USB issues with this version though, so we had to do another prototyping round.
Version 7, November 2013
We finally fixed the USB issue with this revision. We thought this was the version we were going to use for production, until we hit that reset issue.
We ended up ordering a bunch of PCBs for this revision and then scrapping them when we decided to push back production.
Version 8, March 2014
In order to fix the reset issue we added a chip that holds our MCU in reset until after our external Flash boots up. Since we had to do this revision anyway, we thought that we might as well add on another feature (bad idea). The extra feature involved changing around the power circuitry (all the chips on the bottom left) so that Tessel can be powered off of more than 5.5V externally.
We also removed the external boot selection pins because they were on our high speed RAM lines and messing with them would sometimes crash Tessel or prevent it from booting correctly. In its place we wrote a custom bootloader to recover from bad firmware.
We ended up having problems with the power chip though, so we did another revision to fix it…
Version 9, April 2014
Here we’re approaching maximum utilization of all available space. The power circuitry (bottom left) gets even more crowded.
This is the version we actually put into production about 2 days ago.
Originally Tim, Jon, and I thought that we would have this thing completed in 3 months over the summer, so we tripled the time (like everyone said we should) and gave ourselves an additional 6 months for the crowdfunding campaign to ship.
Well, it turns out it was harder than we thought, so now 3 months after that deadline we’re almost there. We’re still patching up our firmware and related tools, along with trying to provide everyone with a superb first run experience.