From: Matt (boomer) Russell ([email protected])
Date: Tue Oct 26 1999 - 09:04:23 PDT
"D. C. Sessions" wrote:
> "Yehuda D. Yizraeli" wrote:
> > Hi,
> > Just wondered if someone knows the nature of the requirement for
> > matching of the rise and fall slope time in the USB spec.
> Shalom Yehuda!
> The rise and fall times, as well as the crossover point, need to match
> for two reasons:
> 1) Receiver performance. Mismatch would take the receiver out of its
> best common-mode range and cause increased jitter. Thanks to SE0
> conditions USB can't require large receiver common-mode ranges.
the usb spec actually has 3 receivers,
1 differential where it is required to have a large common mode range,
0-vdd for all intensive purposes, though the gain can go down at the extremes.
2 single ended receivers to monitor for the SE0 case.
The core logic needs to be able to filter out differential SE0 glitches as it
stands so I don't think that this is really a reason for the edge rate
> 2) EMI. Mismatches between the rise and fall, esp. in unshielded
> cables, result in large common-mode energies that radiate like crazy
This is probably the main reason.....
> D. C. Sessions
> [email protected]
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