05-23-2012 01:13 AM
We are designing a x8 PCIe card and the required power is 30W. The slot available is x16 capable. Can we draw 30W of power from x16 slot using x8 PCIe card? (we dont have access to any other power sources)
05-23-2012 08:31 AM
Can you guarantee that the board won't get plugged into a x8 slot?
It's generally not a good idea to start designing a board with the intent to disobey
the rules of the bus it plugs into. The normal approach to higher powered boards
is to add the 6-pin Molex auxilliary connector to pick up additional 12V power. If
you add the connector and someone blows up their system because they didn't
plug the cable into it, at least it wasn't your fault.
05-23-2012 08:47 PM
Thanks for clarification,
Yes, we can design the card for x16 lines with only x8 routed on board. So essentially its a x8 Card but only x16 pinout to make sure that it fits only into a x16 slot.
Then can we draw more than 25W?
Does the PCIe root port limit the current as it detects only x8 lanes?
05-24-2012 05:08 AM
At least for standard PC's there is no electronics to limit the power draw on any particular
slot like you might have, for example in power over Ethernet. However the standard was
based on the current carrying capacity of the card-edge connectors (more connector pins
allow more current) and the power budget for the overall supply (can the supply power all
the components you can plug in). It's likely that you can draw more than 25W without
getting into trouble, in most cases. Certainly if you have control over what else gets plugged
into the motherboard, you could take additional power up to the current-carrying capacity
of the connector. The 6-pin Molex for added current allows much more power, and limits
the board population to the number of 6-pin supply connectors on the system power
supply. Is there some reason you don't want to add the additional power connector?
It would seem to allow the board to fit more systems if you kept the edge connector
at the 8-slot length (although most motherboards only have 1 4 or 16 lane slots).
05-24-2012 09:00 PM
Thanks for your detailed reply.
In response to your query on reluctance to add another connector is we cannot increase card width by adding connector and the cable associated. Since we know that x16 slot can support 75W of power, we want to utilize the same. Our motherboard is customized for our application and we can make sure there are only x16 slots on it. We can also make sure the main SMPS can supply this extra power. In that case I am hoping that we can draw more power.
05-25-2012 06:00 AM
I took a look at the PCIe spec again. Extending the footprint to x16 would guarantee that
your board can only go into a x16 slot, however it does not add power pins - only more
grounds. So it's unlikely to be much help for power draw. That being said, if the x16
power budget is good enough without the auxilliary power connector, then your
x8 card should work fine. You could use a x16 connector footprint to get the additional
ground pins as well as prevent the board from plugging into a smaller slot. However you
should still wire the presence detect for x8 so the lane detection of the motherboard
starts from the correct size.