Breaking the silence

Ok, so we know we’ve been quiet for a little while now but that’s only because we’ve made epic progress.

Last we said was that we’d laid out the PCBs ready for ordering them. Well, we have ordered them but it all underwent a full re-work and re-plan before submitting the orders finally because we learnt a few important things.

V0.1 of the TwitALU PCB layout

V0.1 of the TwitALU PCB layout

So, as you can see, everything has gotten much smaller and all appeared on one PCB. This has happened for a number of reasons, the main one being cost. We got some quotes from PCBTrain with our original sizes and layout which were just a crazy number; nigh on £150 for one of the registers. We then shrunk them all down and tried again, this got us to about £200 for the 4 boards on a 15 day turn around time. This is where the board above steps in. We reckoned that if we turned them all into one board and then formed the tabs between boards, that can be seen in the green outline, we can just separate them ourselves then it would be cheaper and surprise, surprise, it was. The board above cost us in the region of £130 on a similar turn around time!

Apart from the obvious lesson in our experience we also found out about the express service PCBTrain offer, admittedly after we’d already placed an order but ho hum. The express service is a 1-2 day turn around service that costs about the same as the 15 day service we chose but comes with limited routing and no silk screen or solder resist on the board. So it’s perfect for proto-typing since the first PCB will never be perfect.

On the theme of the first PCB not being perfect we have also discovered one or two slight errors in the Gerber files we submitted for production. One of the connections for routing carry out from the shift right logic back to the ALU control port expander was missed during a session of renaming buses and bus wires in the schematic. While it’s definitely non-ideal it’s only one connection to make with a jump wire and it’s on the reverse of the board so it’ll be pretty invisible. The second error we found was was the “Register B” label somehow got deleted from the silk screen layer. All in all, they aren’t major problems but it does mean the production of some corrected, ‘neat’ boards will have to happen at some point.

There’s still plenty more work been going on so expect more updates soon!


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