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I am using the Atmel AVR32 UC3C AT32UC3C2512C for a project and the datasheet doesn't say anything about terminating the USB connection, but the AVR32768 Application note, clearly points out a 39 ohm termination resistor (page 13).

Is it a problem if I left the PCB with no termination resistors at all?

I will connect this USB to another and make an extension so I don't know how this will influence the impedance of the traces and deal with this resistor. (I am new to Signal Integrity and High Speed Signals, but I know the basic theory already).

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On a prototype I built I made the same mistake. I got away with it with a short USB cable (0.5m) but when I tried a long one (3m) I could not get it to work reliably.

Needless to say the production units have these resistors and work reliably with both long and short cables.

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  • \$\begingroup\$ Good to know it! Which MCU did you use to make the USB connection? And did you use 39ohm resistors as well? \$\endgroup\$ – mFeinstein Aug 22 '13 at 3:27
  • \$\begingroup\$ @mFeinstein I was using an ARM processor with USB built in: LPC2148. Note: the impedance includes the output impedance of the USB chip (in my case processor); Use the value recommended by the manufacturer. In my case it was 39 ohm and yes I have them fitted. \$\endgroup\$ – Warren Hill Aug 22 '13 at 5:45

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