SMB Clock output in presence of XB-200 nay?/yea but..? how?

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SMB Clock output in presence of XB-200 nay?/yea but..? how?

Post by CormacG » Fri Nov 10, 2017 9:18 am


Question about the SMB clock (output) on the bladeRF (my setup; x40 with firmware v2.0, FPGA v0.5 and using XB-200 transverter board. Running on linux, using libbladeRF version 1.8.0).

In the 1.7.2 API documentation (most recent I've found online) relating to SMB clock port control ( ... o_c_k.html), it states
Warning. Do not use these functions when operating an expansion board. A different clock configuration is required for the XB devices which cannot be used simultaneously with the SMB clock port
Sorry to be a pedant but this is kinda important for what I'm trying to do; I just want to confirm does the above warning mean either (a) I cannot use the SMB clock output at all i.e. it's in effect unavailable when I've the XB-200 connected OR does something different have to be done (different clock configuration) to set & get an smb clock frequency from the bladeRF + XB-200 combo? If so, a pointer to info on that different clock configuration would be appreciated.

Many thanks.
Cormac (EI4HQ)

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Re: SMB Clock output in presence of XB-200 nay?/yea but..? how?

Post by rtucker » Mon Nov 13, 2017 1:01 pm


Using both the XB200 and the SMB port is officially unsupported because they both use the same clock driver.

Referencing page 12 (CLOCKS) of the bladeRF schematic:

On the Si5338 clock generator (U68), the CLK3 driver is shared between the SMB port (A) and the expansion port (B). Both xb200_attach() (in xb.c) and smb_mode_output() (in smb_clock.c) assume they have exclusive control of CLK3 when setting things up.

That said... I believe you should be able to get a 38.4 MHz output on the SMB with the XB200 enabled. Indeed, if you "set smb_mode output" before "xb 200 enable", it looks like it very well does that, but I haven't actually verified this yet. I'm not as certain about using the SMB as a clock input because I'm not sure how it will handle back-feeding into a clock driver port that's "off" (the datasheet doesn't seem to define what "off" is...). Using frequencies other than 38.4 MHz is going to either be impossible or very painful, however.

What is your use case?
Rey Tucker (she/her)
Systems Engineer, Nuand LLC
Rochester, NY, USA

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