On Mon, 26 Feb 96 16:23:13 MST (Gary Peterson wrote:
> > I am trying to get a feel for how one might mix 3-volt level signaling and
> > 5-volt level signaling PCI bus driver/receivers on a common PCI bus.
>
On Mon, 26 Feb 96 22:03:34 EST Andy Ingraham wrote:
> Perhaps you didn't mean to say "signaling"? 3-volt signaling and 5-volt
> signaling -- as in sections 4.2.1 and 4.2.2 of the PCI spec -- are mutually
> exclusive environments that can never mix. (Like mixing TTL and ECL.)
>
> What you can do, however, is use a device that is powered by 3.3V, and
> outputs 0/3-volt output levels, on a PCI bus designated as a 5V
> signaling environment. This is what you referred to as "3-volt level
> signaling and '5-volt compliant.'"
>
> This arrangement is permitted because the output voltage specs of the
> 5V signaling environment are wide enough to encompass what you might
> call 3V switching levels, even though they are indeed "5V signaling
> environment" PCI levels.
>
> And by the way, the output levels of these 3.3V-powered, 5V signaling
> devices is rather similar to good old TTL, which, despite having 5V
> power, might drive only as high as 2.4V in the high state. The 5V PCI
> switching levels are essentially TTL levels.
>
> There are other differences besides the lack of clamping diodes that
> differentiate 3.3V signaling devices from 5V signaling devices, but that
> is one of the biggies.
>
In a 5 volt signaling environment with 3.3V voltage level devices, you may want
to be careful about low to high switching of the 3.3 V devices on the bus. 5
volt devices will overshoot far enough to turn on the high clamp diodes,
supressing the ringback component to reasonable levels. But with 3.3 volt
drivers, all of the overshoot energy will ringback and can enter the threshold
region of receivers on the PCI bus. With small geometry, fast devices, there
can be an extreme amount of overshoot.
--scott
Scott McMorrow
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Subject: Re: Wanted...PCI bus 3V driver/receiver models
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