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Question about USB spec

Hi,
I don't understand the timing spec (18ns) for low speed USB. For a low
speed line, the timing delay should be longer. Why it is 18ns, smaller
than 26ns. Could you explain it to me? Thank you very much.





1.   Why are there cable length limits, and what are they?

A:   The cable length was limited by a cable delay spec of 26ns to
allow for reflections to settle at the transmitter before the next bit
was sent. Since USB uses source termination and voltage-mode drivers,
this has to be the case, otherwise reflections can pile up and blow
the driver. This does not mean the line voltage has fully settled by
the end of the bit; with worst-case undertermination. However, there's
been enough damping by the end of the bit that the reflection
amplitude has been reduced to manageable levels. The low speed cable
length was limited to 18ns to keep transmission line effects from
impacting low speed signals.