Required mPCIe data signals for LimeSDR XTRX

Projects LimeSDR
1

Hello everyone,

Is it enough to have 1 PCIe lane wired to the connector, or is USB/SMBUS/I2C required as well?

The question originates from looking for a SoM for a custom carrier board, but could also become very handy when looking for a mPCIe adapters and devkits. Some of them do not route SMBUS, some of them do not route USB, possibly same goes to i2c.

P.S. Congratulations to the Lime team for the XTRX launch!

2

You can find the mPCIe connector pinout in the documentation:

https://limesdr-xtrx.myriadrf.org/documentation/board-description.html#mini-pcie-edge-connector

3

Thanks for confirming the pinout.

I am still a bit confused, though. Does it mean that for proper operation of the XTRX the carrier board must route USB and SMBUS to the connector?
Or, maybe put it this way, for demonstration purposes. Is it possible to use, say, Raspberry Pi CM4IO or the Rock 5 for prototyping with XTRX? Neither of these boards route SMBUS to the connector, and, to my memory, only Rock 5 provides USB.

Hopefully the question makes sense.

P.S. I was looking into specifications of the XCOM, and it does not have USB on the mPCIe connector. So I guess it is optional, although I understand LimeSDR XTRX might have different requirements.

4

I don’t think so, but tagging @Zack as he could say for certain.

5

Hi,
I am aware this is an old thread, but I am trying my luck here.
I am currently evaluating the LimeSDR XTRX for one of my projects. Just want to understand better the mPCIe interface on this board.

  1. Is this mPCIe technically connected to the USB2.0 transceiver and in fact it is actually a USB2.0 interface converted to mPCIe?
  2. Is the mPCIe only support for control / configure of the board, rather than used for high speed data (IQ sample from LM7002) transfer? Would I be able to use it for IQ sample from LM7002 transfer to a PC connected to LimeSDR XTRX via this mPCIe interface?

Apologise if these questions are obvious ones, but I have just started evaluating the feasibility of this board, and any help will be highly appreciated.

Thanks in advance.

6

The XTRX is connected via PCIe lanes which are used for both configuration and streaming. AFAIK XTRX has 2 lanes wired, so to get maximum performance you’d need to pay a bit of attention to possible adapter boards you’d be using.

The documentation mentions USB controller via both micro-B port and mPCIe connector but it says its only for power and programming. I didn’t see any USB devices listed, so not sure if its something to be implemented later, or whether it is time for me to update the gateware (I didn’t update it in a while, as things works fine for me).

Hope it helps :slight_smile:

7

@sergey Thank you very much for your reply. Had a look at the mPCIe edge connector pin definition in Table 16, it seems to be a standard 1x lane mPCIe pin arrangement. Do I miss anything here? Could you please help me on the 2 lanes wiring aspect?
I am considering connecting it with a computer board with a mPCIe interface for I Q sample streaming at around 120MB/s. Is there any tricky aspect I need to pay attention to?
Thanks for your help.

8

Thank you very much for your reply. Had a look at the mPCIe edge connector pin definition in Table 16, it seems to be a standard 1x lane mPCIe pin arrangement. Do I miss anything here? Could you please help me on the 2 lanes wiring aspect?

There are PCI_TX1_N, PCI_TX1_P, PCI_RX1_N, and PCI_RX1_P on pins 39, 41, 47, and 49. I think those are not very standard, but are used on some of the embedded systems.

I am considering connecting it with a computer board with a mPCIe interface for I Q sample streaming at around 120MB/s. Is there any tricky aspect I need to pay attention to?

I think you should be fine with a single PCIe lane for 120MB/s, as a single gen2 lane supports 500MB/s (in each direction).

There are some numbers Ricardas mentioned in this post: Some tech questions about the LimeSDR XTRX ecosystem - #3 by ricardas

9

Thank you. Much appreciated!

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