Re: 66 MHz Capability bit in AGP

["Monish Shah" <monish@hpfcmss.fc.hp.com>]

On Nov 4,  5:46pm, Peter N. Glaskowsky wrote:
> Subject: Re: 66 MHz Capability bit in AGP
> > I am facing a dilemma in how to set the PCI Config Register 66 MHz
> > capability bit for an AGP compliant device. We plan to be compliant
> > with the 66 MHz timing requirements of the AGP bus, but not the
> > more stringent 66 MHz timing requirements of the proper PCI bus. Also,
> > we intend for our chip be able to go on a PCI-33 board as well as an AGP
> > board.
>
> I'd set the bit to 1 to indicate that your part is capable of 66-MHz
> operation. When it's installed on a conventional PCI card, ground the M66EN
> pin. This will prevent the card from operating at 66 MHz. ....

This sounded like a clever solution, for a moment.  However, it is
non-compliant and is likely to cause problems.

If you PCI (not AGP) card grounds the M66EN pin, it had better also make
the 66 MHz capable bit false.  The host is free to look at either one to
decide which frequency it should run at.

If you really want to build silicon to work either as AGP or as PCI, you
should either just design to 66 MHz PCI timing or burn a pin on the chip to
tell you whether you're on a PCI board or an AGP board and set the 66 MHz
capable flag accordingly.

> .                    png

Monish Shah
Hewlett Packard
ó4
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