ATMega32u4 Schematic review

Thread Starter

gelliott

Joined Apr 19, 2019
13
Hi,
I have previously designed a schematic, board, and had a PCB printed out and assembled, only to find I made a couple significant mistakes in the schematic (https://forum.allaboutcircuits.com/threads/short-circuit-in-assembled-design-why.158928/) and I want to get some eyes on my revised schematic before wasting time laying out the board, and wasting money getting failed PCBs made up.
My previous mistake in the schematic was a misunderstanding of how decoupling caps should be used, which I hope I've rectified. I also took some other helpful advice to make the schematic a bit more polished.

Many thanks in advance for any assistance and guidance you can provide
 

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BobaMosfet

Joined Jul 1, 2009
936
Some tips--
  • If your pcb software support linking your schematic so that you can verify net connections match between schematic and PCB, that will help you.
  • Don't get a PCB made right away- Get the schematic done, and then take (literally) 3 or 4 days, reviewing the schematic and the PCB drawing to make sure that a) the schematic is right, and b) the PCB faithfully reproduced the schematic.
  • Always make sure you have no accidentally connection your ground plane to your positive plane on your PCB (or your - rail to your + rail), by a via, or a capacitor or something.

The capacitor symbols you're using in your schematic are _polarized_. The flat side is always the positive side, and the curved side is always the negative/ground side. Some people think those symbols are fine for any capacitor, but that is not so. Not all caps are polarized. Sometimes you don't want them to be. Other times you do. I say this, because I see your symbols are being used backwards in some cases.

A non-polarized capacitor is just two parallel lines, perpendicular to the conductor.

Just one 0.1uF cap is all that is needed between your +5VDC (aka Vcc) and Ground pins, close to the MCU, normally. That value may be different based on the regulator you use, so check your regulator datasheet, if you use one for their suggestion. It's primary purpose is to fill up, and to provide immediately available current to help feed the MCU faster than the power-supply can. It does not decouple, bypass, or anything else in this circumstance because it is not being used as a filter, and it is not being used in an AC environment. Not really. You also need the same 'bypass' cap between your AVCC and GND pins on the other side of the MCU. AREF should be tied to Vcc.

I'm not sure you have your ISP jumper wired right. You will need a 4K7 resistor as a pull-up on your reset pin, to keep it high, but still allowing an ISP programmer to pull it down for reset.
 
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