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What's the purpose of 68ohm resistors on D+ and D- lines?
Posted: Tue Apr 22, 2008 12:15 pm
by 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.
Posted: Wed Apr 23, 2008 4:42 pm
by 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.
Posted: Thu Apr 24, 2008 6:07 pm
by luke
Now everything's clear. Thank you.
82 ohm?
Posted: Tue May 20, 2008 3:19 pm
by 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?
Posted: Tue May 20, 2008 7:34 pm
by christian
Should work as well.
Posted: Wed May 21, 2008 4:32 am
by Guest
tested, works fine.
Posted: Wed Jun 18, 2008 8:21 am
by 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.