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RE: Is is possible to receive a cheep SubVendorID?
> Does the CPLD have to clock the bus?
No, but to be PCI 2.1 clock-compliant means following
the PCI bus clock faithfully, with a very tightly
defined maximum skew. The PCI bus clock can vary
from DC to 33 MHz on a cycle-by-cycle basis, so
various re-clocking schemes almost invariably fail
in systems that, for instance, do so-called 'spread-
spectrum' clocking. In spread-spectrum clocking,
the idea is to spread the clock frequency over a
broader sprectrum, to meet FCC requirements for
RF radiation limits. This is done by varying
the clock frequency with a pseudo-random sequence on
a per-cycle basis. PLL re-clocking circuits cannot
follow this kind of behavior. Also, dynamic clock
circuits will fail if the system goes into 'green'
mode, where the clock may be set at 100 KHz or so.
Note that the system does not need to be functional
at that clock frequency, but it should be operational
the instant the clock is turned back up to full speed,
so loss of state is not allowed.
> As I mentioned earler, my usage of PCI is non-standard so
> I won't claim (nor am I after) 100% compliance. At the very
> least, my connectors are different.
In embedded designs, all limits are off. If you don't intend
to extend what you are doing beyond your own covers, you can
do pretty much whatever you want (including requiring a nice,
stable, high-speed PCI bus clock). Even if your bus contains
connectors, if they are mechanically non-standard, then you
can rest assured that no one will attempt to plug a 3rd party
PCI desktop form-factor board into that connector. At that
point, it is your private playground, so have fun. 8-)
> So far I have an MPC8240 processor, a display controller and
> a bridge to an older system on my bus; a DSP with a coolrunner
> CPLD between its HPI and the PCI will come soon. I'll put some
> sample designs on our www site along with the logic compiler
I'm sure that other embedded designers will be interested in
what you are doing. I'm looking forward to seeing what you've
done (personally, of course 8-).