linux-kernel - Re: Is the PCI clock within the spec?

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Date:	Tue, 4 Dec 2007 10:51:33 -0500
From:	"linux-os (Dick Johnson)" <linux-os@...logic.com>
To:	"John Sigler" <linux.kernel@...e.fr>
Cc:	"Linux kernel" <linux-kernel@...r.kernel.org>,
	<linux-pci@...ey.karlin.mff.cuni.cz>
Subject: Re: Is the PCI clock within the spec?

On Tue, 4 Dec 2007, John Sigler wrote:

> Dick Johnson wrote:
>
>> You can't just touch a scope-probe to the PCI
>> clock pin and clip the scope-probe grounding
>> lead to a convenient "ground" to make these
>> measurements! You need a special fixture that
>> will make a low-inductance connection to the
>> PCI bus in the same manner as the interface chip.
>
> (This is waaay over my head.)
>
> Why do you think the two plots (at least the second one)
> were not obtained as you describe?
>

They didn't describe the test setup, simply supplied a
"go away and don't bother me" picture of something
that means nothing.

>
> Why would the system manufacturer botch the measurements
> when I asked them to show me evidence that their system
> was compliant?
>

I think they just sent you a picture, thinking it would
make you go away. It is a common ploy.

>> A scope probe will allow you to see if there is
>> a clock signal. That's all. You can't determine
>> its quality. A 4-inch ground lead on the scope
>> probe will result in 10-20% overshoot and undershoot
>> being observed.
>
> I don't understand this 10-20% figure.
> (0V + 10-20% is still 0V.)
>

There is a definition of overshoot and undershoot. 10 percent
undershoot will show a 0->3.3 volt signal going 0.33 volts
below "ground," i.e., -0.33 V on peaks. 10 percent overshoot
will show a 0->3.3 volt signal going to 3.63 volts above ground
on peaks, i.e., +3.63 volts.

> AFAIU, the nominal peak-to-peak voltage is 3.3V. The observed
> peak-to-peak voltage is 6.08V (3.3V + 84%).
>

This may be a "5 volt" bus. 3.3 volt devices are supposedly
5 volt tolerant. For instance, the PLX, PCI 9656BA, probably
the most common PCI interface chip in use, will handle those
voltages fine. The actual logic-level switch occurs at about
1.5 volts. Since the PCI bus is clocked, it is unlikely
that a "lockup" you describe is caused by the bus, more
likely a hung DMA operation caused by a failure to handle
errors (hardware or software, cause unknown). With the
chip I describe, it can be programmed so that it will not
hold the bus "forever," with a hung DMA operation. However,
some cooperation with the stuff in your FPGA as well as
the software in the driver is necessary.

> Regards.
>

Cheers,
Dick Johnson
Penguin : Linux version 2.6.22.1 on an i686 machine (5588.27 BogoMips).
My book : http://www.AbominableFirebug.com/
_

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