What's the purpose of 68ohm resistors on D+ and D- lines?

[luke]

Hi.

I couldn't find any information about them on the web. I bet they are needed since I've seen them in every single project using USB. I'd like to understand everything before I start soldering and it's the only mysterious part of the schematic.

Some pointers or short explanation would be great.

Thanks in advance.

[christian]

These resistors have several functions:

* Limit the current if zener diodes are used for voltage limiting on D+ and D-.
* Limit the current in case of short circuit or if higher voltage is applied from extern.
* To some degree impedance matching to the cable when sending data.
* Low pass filter to avoid ringing when level is switched.

Having that said: You may omit these resistors if you have no cable and don't use zener diodes.

[luke]

Now everything's clear. Thank you.

[MaxxxPower]

For some reason I ordered 82 ohm resistors instead of 68 ohm. I can't figure out why I thought it was 82, since every schematic I've checked says 68.

I'm guessing it shouldn't make a difference?

[christian]

Should work as well.

[Guest]

tested, works fine.

[Guest]

For a low-speed USB device are the values of ther sries resistors at D+ and D- not very important. Values from (0)47 Ohm to 330(1000) Ohm are working.