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Topic: USB to ATmega16U circuitry (Read 100 times) previous topic - next topic

overholt

Hello,

I am trying to figure out if some of the circuitry between the USB and the ATmega16U on the Arduino Uno Rev3 is mainly for circuit protection (If the input USB is rated at too high of a voltage or if for some reason the input current is high as well). The specific components I am curious about are the varistors, the inductor, the fuse, and the 22Ohm resistors.

I feel like all of those are mainly for circuit protection, but I've also read the 22Ohm resistors are necessary for input signals from an external USB. Please let me know what exactly these components are explicitly for!

Thank you.

vaj4088

The 22 ohm resistors may be to limit the output current from the ATMega 16U2 in case the outputs are shorted to each other or to ground.  I am not sure but then I don't know much about the ATMega 16U2.

The fuse is protection for the Uno in case something is wrong with the Uno.  Otherwise, a dangerous amount of current could be supplied from the USB bus to the Uno.

The other components provide overvoltage protection.  Think static electricity applied to an exposed cable or connector.

DrAzzy

And yet, the 16u2 on the uno still burns out if you look at it and imagine voltage spikes on the power rail. We get at least one person per day posting about trashing their official uno's 16u2, and a couple posts a year from people who've managed to do the same to their clones with different serial adapters, despite the huge popularity of clones.

IMO, better to use a ch340g if rolling your own serial adapter. Easy to solder package, and you can copy the circuit from the datasheet and don't have to reprogram it. Costs like 25 cents each shipped on ebay for the bare chip too.

Or for that matter, just use an external serial board with the 6-pin header unless it really has to be integrated on one board (search ebay: ch340g 6pin, black pcb with the small voltage switch, or green one with micro usb connector and small voltage switch are the good ones. The ones that show a jumper on the row of 6 pins are no good, they don't break out DTR so can't use them to program without modifying the board).
ATTinyCore for x4/x5/x61/x7/x8/x41/1634/828/x313 megaTinyCore for the megaavr ATtinies - Board Manager:
http://drazzy.com/package_drazzy.com_index.json
ATtiny breakouts, mosfets, awesome prototyping board in my store http://tindie.com/stores/DrAzzy

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