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

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Date:	Tue, 04 Dec 2007 14:27:02 +0100
From:	John Sigler <linux.kernel@...e.fr>
To:	Sebastien Dugue <sebastien.dugue@...l.net>
CC:	linux-kernel@...r.kernel.org, linux-pci@...ey.karlin.mff.cuni.cz
Subject: Re: Is the PCI clock within the spec?

Hello Sébastien,

Sébastien Dugué wrote:

> John Sigler wrote:
> 
>> I have an x86 system, running Linux 2.6.22.1-rt9, in which I plug one
>> or two PCI I/O boards. I had been experiencing complete system lock-ups
>> until I sent the system to the board manufacturer, and he fixed the
>> problem. However, he told me that the PCI clock seemed out of spec,
>> as far as voltage is concerned.
>>
>> (Disclaimer: my knowledge of PCI is 0.)
>>
>> The board manufacturer sent me the plot of (what appears to be) voltage
>> versus time for the PCI clock.
>>
>> http://linux.kernel.free.fr/plot1.jpg
>>
>> The system manufacturer sent me a similar plot.
>>
>> http://linux.kernel.free.fr/plot2.jpg
> 
> Why did they send you those plots? What was their point?

The board manufacturer originally thought that the voltage under-
and overshot might be responsible for the system lock-ups we were
experiencing. They sent us the first plot to document the problem.
(In the end, the lock-up was linked to a bug in their DMA engine.)
I asked the system manufacturer whether they could reproduce the
voltage issue, and they sent me the second plot.

>> As far as my understanding goes, the signal should alternate between
>> 0 V and 3.3 V (??).
> 
>   Yep, that's the idealized 3.3V signaling case. However, it looks like
> the signal is overshooting a bit (-0.8V below 0 and +0.8V over 3.3V from looking
> at the 1st plot) which may be due to incorrect impedance matching on the bus,
> probes artifacts, ...
> 
>> In the second plot, it looks like Vmax ~ 4.6V
>> and Vmin ~ -1.4V (Pk-Pk(C1)=6.08V might mean peak-to-peak voltage?)
> 
>   This one looks a bit high (if they measured the same voltages I wonder
> where they got their scopes calibrated ;-) )

The first plot was obtained on my system. The second plot was obtained
on a different system, presumably identical to mine, but I don't know
for sure.

>> 0) What is this C1 both plots mention?
> 
>   Scope Channel 1
> 
>> 1) Am I reading the plot correctly?
> 
>   Yep
> 
>> 2) Is -1.4V in DC even possible?
> 
>   Why not!

Errr... I need to think about it :-)

>> 3) 4.6V is 1.3V above 3.3V and -1.4V is -1.4V below 0. (Assuming I read 
>> the numbers correctly) Are these values within the PCI spec? Or are 
>> these voltages dangerous and / or might cause some problems with some 
>> PCI boards?
> 
>   Well it depends on which of the plot is lying. Looking at the PCI spec
> (4.2.2.1) the Vih max for a device is Vcc-max+0.5 = 3.6 + 0.5 = 4.1V
> the Vil min is -0.5V so in this case it looks a bit high. But I would not
> worry too much, those are only the overshoots, and the circuits have
> clamping diodes on their inputs.
> 
>   The test waveform voltages for the maximum ratings (4.2.2.3) against which
> every PCI device should be qualified are higher than what you have here: 7.1V
> peak-to-peak.

OK. I suppose I should not worry then :-)

>   Hope this helps.

Very much! Thanks.

Regards.


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