Re: What is real lowest speed of PCI?

[Patrick Maupin <pmaupin@speakeasy.net>]

Henry:

A compliant PCI device needs to not cause system
problems if the bus speed is reduced to DC.  Obviously,
if you are capturing video, it won't get the necessary
bandwidth at really low speeds, but the device should
still follow PCI protocols at any speed below 33MHz.

One thing to bear in mind is that, for power savings,
a system could "burst" cycles, e.g. run the bus at
a really low speed until a /REQ is asserted, then run
it at top speed until the bus is idle and no device
has /REQ asserted.  If, for example, the bus was run
at 10KHz during idle periods, that would add at most
100us latency to the first request.

Also, for debugging, low speed is great.  I once ran
a PCI system at 16KHz so that an attached Quickturn
box could emulate a PCI adapter.

Finally, by "old spec", you must mean "REALLY old spec."
I don't think 16MHz has been the minimum since 1.0,
and even then operation down to DC was recommended.

Regards,
Pat




Henry Gong wrote:
> 
> Hi,
> 
> According to PCI specifications v2.2, any 33MHz PCI device must support
> bus speed from DC to 33MHz. What a bizarre requirement! Can any PCI
> device really work in DC? But if you don't, your device is not PCI
> compliant! Old spec clearly defined the lowest speed, 16MHz. But what is
> the lowest speed you are "supposed" to support for v2.2?
> 
> Most mother-boards on the market just implement 33MHz PCI buses.
> However, lower speed PCI slots do exist. What is the lowest speed on the
> market?
> 
> Nobody seems to be interested in extremely low speed. People simply piss
> off the spec and still claim PCI compliant, or my understanding in
> incorrect?

-- 
Patrick Maupin
5216 Crooked Oak Cove
Austin, Texas 78749
512 891 6037