Issue Enabling External 40 MHz Clock on LimeSDR Mini v2.4

Projects LimeSDR
1

Hi,
I am trying to use an external 40 MHz reference clock with my LimeSDR Mini v2.4.

From this Discussion : LimeSDR-Mini v2.4 - external clock - #8 by VytautasB

it is mentioned that the external clock source can be selected via the onboard switch (IC20) by modifying the FPGA register:

Address: 0x00CF[0]

  • 0 → Internal VCTCXO
  • 1 → External REF_CLK_IN

Following this, I attempted to:

  • Go to: Modules → SPI → Board SPI
  • Write value 0x0001 to register 0x00CF

However, I am facing the following issue:

  • The written value does not retain (reads back as 0x0000)
  • The board continues to use the internal clock source

So the question is, how can you use an external clock in LimeSDR mini v2.4?

2

Hi @Prajwal,

which gateware version are you using?

3

Gateware 3.5

4

How are you testing that board continues to use internal clock source?

5

Hi @VytautasB,

I am verifying the clock source by observing the signal at the J7 U.FL connector using an oscilloscope.
I am continuously observing the output from the J7 connector, and even after changing the value at register 0x00CF, there is no change in the waveform on the oscilloscope. This indicates that the clock source is not switching.

6

Hi @VytautasB ,

I got the problem resolved
The register 0x00CF is not mapped correctly , instead register 0x0013 is mapped to the IC20(for Gateware 3.5).
When checked the default value of 0x0013 it showed 006B. So when changed the value to 007B , the IC20 selects External clock.

7

Thanks for sharing your findings. I will investigate it further.

Can you double check if your board is v2.4 (check it on a silkscreen) and also read 0x0003 FPGA registers and post the results.

8

Hi ,
I checked the board , it is v2.4(on the silkscreen)
And the register value of 0x0003 is 0007.