[coreboot] Question about PCIe separate reference clock

Zheng Bao fishbaoz at hotmail.com
Thu Jan 12 15:26:28 CET 2017


Q: do you use local xtal attached to   Si52111-B5 to generate local PCIe 25MHz clock?

Zheng: yes


Q: If you dothis, my next question is how you synchronize these two clocks: Local
PCIe 25 MHz and common reference clock from CPU?

Zheng: We do NOT sync these two.

Q: Since these two clocks, as I understand above scenario, are
asynchronous to each other?!

Zheng: yes. Asynchronous.


The VPX connector does not have a PCIe ref clock signal, so we can not send CPU PCIe ref clock to

device.  The PCIe spec says if the separate refclk on devices should be 100MHz ± 300PPM, with SSC

off.  We believe our board meet this requirement. So we doubt the problem lies in PCI configration space.


Zheng


________________________________
From: Zoran Stojsavljevic <zoran.stojsavljevic at gmail.com>
Sent: Thursday, January 12, 2017 9:22 AM
To: Zheng Bao; Predrag Vidic
Cc: coreboot at coreboot.org
Subject: Re: [coreboot] Question about PCIe separate reference clock

Hello Zheng,

For decades, I've been FW/SW engineer, but I do understand a little
bit of a HW. I have looked into the Si52111-B5 data sheet for
clarification.

My problem here is to understand, your use case: do you use local xtal
attached to   Si52111-B5 to generate local PCIe 25MHz clock? If you do
this, my next question is how you synchronize these two clocks: Local
PCIe 25 MHz and common reference clock from CPU?

Since these two clocks, as I understand above scenario, are
asynchronous to each other?!

Please, clarify for us your use case.

Thank you,
Zoran
_______

On 1/12/17, Zheng Bao <fishbaoz at hotmail.com> wrote:
> Our VPX design uses separate reference clock source, which is Si52111-B5 (No
> spread), instead of common ref clock from CPU.
> Now The system is unstable. Reading PCIE configuration space is unstable
> too. (If we add some fly wire to make it work with common ref clock, the
> system becomes stable.)
>
> (abstracted from PCIe spec: 12 Slot Clock Configuration - This bit indicates
> that the
> component uses the same physical reference clock that the
> platform provides on the connector. If the device uses an
> independent clock irrespective of the presence of a reference
> clock on the connector, this bit must be clear.
> For a multi-Function device, each Function must report the
> same value for this bit.)
>
> Based on my understanding, the BIOS need to read bit "Slot Clock
> Configurationclear" to see if
> separate ref clock is used.  BIOS then write bit "Common Clock
> Configuration".
>
> On our board, the bit "Slot Clock Configuration" is always 1, which I assume
> should be 0.
>
> My question is, how the hardware affect the bit "Slot Clock Configuration"?
> How do we need to design our board to make the bit "Slot Clock
> Configuration" be 0?
>
> Thanks.
>
> Zheng
>
>
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