I’m trying assemble a Mar2012 1.2u opensprinkler board and I had some questions. I’ve assembled the board using the schematic and did notice some differences from 1.1 and 1.2(apr2012) version. I assume the 8mhz crystal on x1 is needed but what about cx1 & cx2 ?I am missing something. Should the resistors for RD+/- be 47ohm or 68ohm? I was also unsure which resistor networks to use. The schematic says 220×4 but neither 1.1 or 1.2u use those. I’ve uploaded pics of what I have done so far if you have any input. The 24vac to 5v conversion circuit is working although I now need a new power switch.
1) the 8MHz crystal is optional, although I recommend you to solder it because if you happen to set the mcu’s fuse bits wrong, you can still re-program it (otherwise, if you set the mcu’s fuse bits to use external crystal and if that crystal is not present, it will not be able to start and hence you can’t program it). The spacing of the pins is much narrower than a standard 8MHz crystal, so try to pinch the pins together and fit them in. You actually don’t really need cx1 and cx2: those are there to stabilize the crystal but it usually works fine without cx1 and cx2.
2) RD+/- can be either 47ohm or 68ohm, but I would recommend 47ohm because that’s a standard value recommended by the V-USB design. That being said, 68ohm works just fine.
3) RN2 and RN3 can be either 220×4, or 330×4. RN1 can be almost any value: 220×4, 330×4, 1Kx4, 10Kx4 (basically it forms a voltage divider so the absolute value does not matter).