PCIe connection to Computer MB
Added by Keith Murison 4 days ago
Hello,
We are working on testing the PCIe interface on the Mighty-Mini Agilex-5E dev kit.
The PCIe x4 connector is a socket (Root Port).in order to connect this to a standard computer mother board, also RP root port, we are going to try the following cable
https://www.digikey.ca/en/products/detail/3m/8KJ2-0743-0500/5128346However, the PCIe REF_CLK may be sourced from the Critical Link SoM ... do we need to modify the SoM board to receive the external REF_CLK instead of sending one ?Thank you,
Keith.
Replies (7)
RE: PCIe connection to Computer MB - Added by Keith Murison 4 days ago
okay I see there are some 0 ohm resistors to feedback the REFCLK+/- (PCIe_CLK) in end point mode... R81 and R83 on carrier board...
is this all that is needed to make the connection into standard motherboard ?
RE: PCIe connection to Computer MB - Added by Keith Murison 4 days ago
however I think the Tx and Rx lines need to be swapped w.r.t. the connector between EP and RP ... we cannot change the direction of the GTS signals on the FPGA.
can you please confirm this point ?
RE: PCIe connection to Computer MB - Added by Michael Williamson 4 days ago
Hi Keith,
Your concern is valid. You will need some sort of "cross-over" cable to swap Tx/Rx lines, etc.
I am looking around, I thought maybe Samtec might have something you could use.
Our Standard A5 SOM (larger, coming out in the next quarter) will have an endpoint / slot PCIe connection. But that won't help you today.
If I find something I will update this thread.
-Mike
RE: PCIe connection to Computer MB - Added by Keith Murison 3 days ago
Hello,
Samtec does offer Cross-Over type of cable in their Gen 4 and Gen 5 cables.
https://www.samtec.com/products/pciec-g5We should order one and try it out soonthanks,
Keith.
RE: PCIe connection to Computer MB - Added by Keith Murison 3 days ago
Hi,
Actually, there is another issue, in EP mode, the Presecence signals, PRSNT2_*X1*# and PRSNT2_*X4*# need to be pulled-down by the Target/EP device,
however, these signals are pulled-up on the SoM, for the PCIE_X4_PRSNT2n_3V3 signal, we have a 10K pull-up, so it is okay for the FPGA to drive this to logic "0".however for the PCIE_X1_PRSNT2n_3V3 signal, there is a 75 ohm and 2.1V 20mA LED: https://www.digikey.ca/en/products/detail/dialight/5988070107F/1291269
the maximum HVIO current for Agilex 5E is +/- 12mA .. so it may not be strong enough to pull-down this net with the LED diode in series...
worst case.. I think we can add a 100 ohm pull-down resistor to GND .. do you agree ?Thanks,
Keith.
RE: PCIe connection to Computer MB - Added by Michael Williamson 2 days ago
Hi Keith,
I don't think you can drive it in the FPGA (PCIe needs to be up fairly quickly and may need to be there before the FPGA fully configures).
I would just tie or (use on a 100 ohm as you suggest) both lines to ground and ensure those pins are configured as tri-state or no input.
-Mike
RE: PCIe connection to Computer MB - Added by Keith Murison 2 days ago
Hello,
yes, actually it is worse than that.. the FPGA can only tolerate about 10mA, and must be less than 1.2V during power up ... so best to add 100 ohm pull-down to both these signals.thanks,
Keith.
| PCIe_RP_EP_convert4.jpg (283 KB) PCIe_RP_EP_convert4.jpg | page showing Iout pin current | ||
| ds-813918-813919 (3).pdf (1.28 MB) ds-813918-813919 (3).pdf |
