[MinnowBoard] System power/reset control architecture with PCIe

[Leigh Gawne]

Hi all,

 

I am in the process of designing a custom expansion board for the
MinnowboardMax (really liking my experience with this board so far BTW :-)
and have a question regarding PCIe power supplies. 

 

One of the interfaces I am implementing on the expansion board is a Mini
PCIe card slot. On my expansion board, I have all the required power
suppliers for card slot, and hence don't need to rely on any power coming
from the MinnowboardMax expansion connector.

 

There is a requirement in the Atom e38xx documentation (actually a PCI
Express 2.0 spec requirement) that all power rails associated with PCIe
should be valid for ~100ms prior to PMC_CORE_PWROK assertion. 

 

>From what I can see in the MinnowboardMax schematic, there is an RC which
controls the assertion of SYS_PWRGD providing 100ms delay as part of the
power sequencing. My understanding is that as long as I ensure that my PCIe
supplies are up before providing 5V to the MinnowboardMax, then I should be
meeting this requirement. So a couple of questions just for clarification:

 

1)      Is this an acceptable approach to take given the MinnowboardMax
power supply architecture? I have a seen there is a Lure expansion board
that includes mini PCIe and I presume this derives the required supplies
from those provided on the expansion header (perhaps the +5VSB supply, which
should also mean this constraint is met).

2)      Any other recommended design approach? Backfeeding is always one
area of concern I have which I need to manage carefully - just want to make
sure there is nothing else I am overlooking/am not aware of.

3)      I am intending on using #PMC_PLTRST_R_V1P8 as the system reset
signal for PCIe card slot and some other devices (with appropriate buffers)
- any thoughts here?

 

Many thanks for any information/clarification!

 

Regards,

 

Leigh

 

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