Monday, February 23, 2015

Designing SAC Interface custom PCBs


I carefully went over the specifications for my new ZTEX board and the Spartan 6 FPGA onboard, to ensure that my interface logic will not damage it in any way. After a review and any tweaks that may be necessary, I will lay out the printed circuit boards for the driver and receiver interface circuits, then send it out to a production house to be fabricated. Finally, I will need to order the right surface mount parts to complete the new board.

The layout of the board was extraordinarily tedious, with more than 250 parts that are located on a 6.5 by 3 inch 4 layer board (175 x 75mm, very approximately). It took me more than seven hours to work with the PCB layout software trying to decent quality design, which I could submit to the fab house to build it. I will be able to get ten boards for $120, not counting shipping, a very reasonable price considering it included automated testing and solder mask.

I chose four layer boards so that ground and the CMOS 3.3V power could be routed on the interior planes, simplifying layout because each connection to those levels is simply a via hole up to the closest point to the component pad. It  should have left me plenty of room to squeeze in all the SMT components and to build the board with excessive power filtering. I had a large tantalum capacitor for each voltage level for every two signals. A single board had 12 3.3 V supply capacitors and 6 3V ones.

Since the new fpga board has a different arrangement of connections and fpga pin assignments, I need to work though my documentation and plan documents for the interface, work out the new assignment of signals to the board, and think through the physical changes to make in the interface box to fit the new board and wiring. I designed my new PCBs to match the existing board shapes, allowing me to mount them with the same mechanism.

The PCBs should be shipping in 4-5 business days after receipt of order, with the factory reopening tomorrow after the Chinese New Year holiday. That should fit in with the delivery time of the ZTEX fpga board and plenty of time to stock all the surface mount parts needed.

I am curently fighting to get everything routed. For some reason, the program can't route the ground signal fully - even with a ground plane inside the board. I ran out of time today but will continue with this to complete the board with safe manufacturing and soldering margins, then place the order. I estimate this will take another four or five hours due to the density of the circuitry. 

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