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

General discussions about V-USB, our firmware-only implementation of a low speed USB device on Atmel's AVR microcontrollers
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luke
Posts: 3
Joined: Tue Apr 22, 2008 11:33 am

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

Post by luke » Tue Apr 22, 2008 12:15 pm

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
Objective Development
Objective Development
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Joined: Thu Nov 09, 2006 11:46 am

Post by christian » Wed Apr 23, 2008 4:42 pm

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
Posts: 3
Joined: Tue Apr 22, 2008 11:33 am

Post by luke » Thu Apr 24, 2008 6:07 pm

Now everything's clear. Thank you.

MaxxxPower

82 ohm?

Post by MaxxxPower » Tue May 20, 2008 3:19 pm

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
Objective Development
Objective Development
Posts: 1443
Joined: Thu Nov 09, 2006 11:46 am

Post by christian » Tue May 20, 2008 7:34 pm

Should work as well.

Guest

Post by Guest » Wed May 21, 2008 4:32 am

tested, works fine.

Guest

Post by Guest » Wed Jun 18, 2008 8:21 am

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.

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