Why USB Data Series Resistors?

[Gytis]

Hello All,

I have seen many USB-interfaced application PCBs, but one thing I never understood: why are there series resistors for the data lines: I see many At(x)mega applications use 22ohm, i've seen other applications use 33ohm, even 68ohm, or just connected into the IC with no resistors.
 I don't find anything explanation in Atmel datasheets, and I didn't find any thread similar to this around here.

What are these resistors doing? Does it have to do with transmission line theory?
I am designing a small PCB interfacing through USB, (Atxmega32A4U). Do I need 22Ohm resistors?
Can I just leave them with no series resistance?

Thanks!

[Dago]

Hello All,

I have seen many USB-interfaced application PCBs, but one thing I never understood: why are there series resistors for the data lines: I see many At(x)mega applications use 22ohm, i've seen other applications use 33ohm, even 68ohm, or just connected into the IC with no resistors.
 I don't find anything explanation in Atmel datasheets, and I didn't find any thread similar to this around here.

What are these resistors doing? Does it have to do with transmission line theory?
I am designing a small PCB interfacing through USB, (Atxmega32A4U). Do I need 22Ohm resistors?
Can I just leave them with no series resistance?

Thanks!

They are series termination resistors. The USB standard specifies that the signal-to-ground impedance is 30 ohms and differential impedance (between data lines) is 90 ohms. Also technically your traces should also be matched to these impedances. Matching is important to avoid problems with signal integrity (reflections and so forth).

http://www.usb.org/developers/usbfaq#sig6

[Araho]

Read the datasheet for your part. In the USB section, it specifies 22 ohm resistors in series with the datalines. As noted above, they are termination resistors, used so that the total impedance of the line, including internal circuitry in the part, matches that of the USB specification. In addition, series resistors limit the edge-speed of pulses, which means less EMI-generation. 22 ohms probably won't have much of an effect here, though.

That said, your circuit would probably work without the resistors, but you're risking problems with signal integrity. At least put the footprints for the resistors in, and then add a jumper or solder bridge if it turns out you don't need them for your required level of performance, at least while prototyping.