Share clock between two LimeSDR Mini v2.2

I am thinking this is probably a dumb question, but I have 2 LimeSDR mini’s v2.2. They are both plugged into the same usb hub, are right next to each other, and I want them to share one clock so they are synchronized in time. I can easily connect the OUT for one clock to the IN for the other, but it appears that maybe I need to do something else to at least one of the boards? If anyone can quickly set me on the right path, I appreciate it.

Thanks in advance.

Tom P.

I think you would just need to tell one of them to use an external clock and switching would be automatic. With LimeSDR USB (the original 2x2 board) and srsRAN you configure an external clock by adding to the device arguments extref=10e6 (for a 10MHz GPSDO reference).

In any case, tagging @Zack as he can give you a definitive answer.

Hi @tkp,

If you are using v2.2 version (you can find version printed on silkscreen next to USB connector) you have to solder some resistors to connect REF_CLK_IN (J8) to clock buffer. To be exact you have to remove R63 and solder R66 0 ohm resistor.


You can find more info here in “Clock distribution” section:

Later board versions v2.3 and v2.4 have onboard switch and clock source can be selected by modifying FPGA registers 0x00CF(0):
0 - onboard VCTCXO oscillator (default)
1 - External clock source (REF_CLK_IN, J8 connector)

Not sure if there are higher level functions on software to control this. Maybe @ricardas can comment.

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Thank you so much for this information - turns out they are both v2.4; I thought they were 2.2 as that is what it said when I purchased them, but on the boards it says 2.4, so I guess I can set that FPGA register (using LimeSuiteGUI presumably, or the appropriate LimeSuite function).
I would switch the register on the device I want to use the other clock, but what about on the device whose clock I am using? Do I need to do anything there, or can I just connect to the clock out?

Yes, you can set it trough LimeSuiteGUI.

REF_CLK_OUT is directly connected to J7 U.FL connector, so just connect to the clock out.

Also check gateware version, it should be at least 2.5 in order to control clock source switch, otherwise you should update to lates one.

Thanks again for this information - I really appreciate it!

I have a quick question about this incredibly useful information: I placed a jumper between the clocks of the two boards, and was able to switch the one board to use the external clock by setting the FPGA register indicated (0x00CF) - I was able to do it with LimeSuiteGUI, but it is easier for me to write a short c++ program to do it since it needs to be done every time the board has its power cycled.
My question regards the source of the FPGA register info: I don’t see this FPGA register listed as being used in the gateware description: LimeSDR-Mini-v2_GW/LimeSDR-Mini_lms7_trx/doc/LimeSDR-Mini_Gateware_Description_V01r00.pdf at main · myriadrf/LimeSDR-Mini-v2_GW · GitHub
Is there a more updated document somewhere?