Designing the mainboard, using Eagle

Point-to-point wiring, using pre-drilled boards is too time-consuming, hard to reproduce and ugly… More, producing a picture like this one, representing the actual board, costs a lot (since no  software can provide this, AFAIK). It’s time to switch to a better environment.


So the idea is to produce PCB boards. But, before, I need to design the board. Eagle is a great software, available under Linux. It’s free for non-profit applications. It can be used to draw schematics, using a huge library of components. When done, Eagle is able to produce the PCB layout (using autorouting). Drawback is it’s quite tough to learn.Sparkfun has great tutorials which helps me a lot: drawing the schematics, building the PCB layout, and evencreating new parts.


First step is to draw the schematic. I’ve started from the previous mainboard and added modified it, according tothis ticket. Power supply is now provided by several connectors. They provides either +5V, either the unregulated power supply (used as input to 7805). The big 2×13 HE-10 connector is not splitted into two smaller 2×5 HE-10, one for PORTA, one for PORTB. +5V and ground are also available for convenience. Xtal quartz is now connected as a small board, so 16F88 can be configured to run with its internal oscillator (more, using different Xtal frequencies involves different caps, so Xtal and caps are dependent). Finally, a push button on MCLR can be used to reset the PIC without switching the power supply off (a little straight, but it works…).

The schematic is ready, it’s time to get the board. Eagle is able to build the board from this schematic… but it needs to be configured. The actual board will be built using special PCB pen and a copper board, so tracks and space between them shouldn’t be too narrow. After several tries, it appears 32mil is a good value (in “Design Rules”, “Clearance” tab and “Sizes” tab, “minimum width”). While configure “Autorouting”, I disabled the “top” layer, since this is a single side copper board. Autorouting the PCB gave 79% done, the last must be done manually. I define straps on the top side, with vias. I don’t if this is the best method in Eagle… Here the final result:

Once done, I print the layout in a postscript file (so scale is 1:1, whatever the resolution is), checking “Mirror” and “Upside down” options. The layout is ready to be reproduced on the copper side. Here’s a link to the postscript file.


Known bugs:

  • pin RA6 is not connected the PORTA HE-10 connector: (bottom right pin in the connector is a orphan)… Can’t really know why, since it’s connected in the schematic. Probably weird problems while connecting nets together. It’s not that important, since this pin is reserved for Xtal (when used), thus can’t be used in any daugther board.
  • the power supply polarity is inverted: ground is in center, + is around, while it’s often the invert. I’ve been fooled by the schematic of the power supply jack. Note a diode will prevent any polarity problems if not connected correctly. If the LED won’t light, the problem may come from this.
Once we have the PCB layout, it’s time to actually build the PCB. To be continued…


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