Re: [SI-LIST] : USB Slope matching and RFI/EMI

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From: Matt (boomer) Russell (boomer@lsil.com)
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.

hi,

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
matching spec.

> 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
> dc.sessions@vlsi.com
>
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