USB and IPAM layout rules

The following is a contribution by one of our partners.

We had an issue with the USB interface to the IP-AM300. Our problem has come about when we mount the IP-AM300 into our system we have a long wiring connection  (about 20 cm) to the USB socket. We found that USB playback has been very unreliable, and tracked this down to corruption of the data coming from the USB memory stick. When we measure the signals on the USB line we see a lot of ringing, overshoot, and undershoot. This is significantly improved by placing 56 ohm series resistors in the USB signal lines - either close to the USB stick end, or close to the IP-AM300 end.

We would like to suggest that IPAM development spec / product sheets are revised for PCB layout to include:

  • Good PCB layout practice is required for USB signal runs of more than a few cm
  • The USB signal lines should use both tracks the same electrical length, and the same track width for their entire length
  • The USB signal lines include provision for termination resistors. The exact value of the terminations may need to be checked or confirmed by a designer, but in the range of about 30 to 100 ohms is most likely. These terminations should be close to either the USB socket or the IP-AM unit
  • A more complex (capacitive) termination may be needed under some circumstances.
  • Designers should use techniques suited for high speed bi-direction data bus design when the USB signal run is more than a few cm in length
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