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