Re: [Discuss-gnuradio] Minimum external clock for FPGA?

[Matt Ettus]

On 01/17/2010 07:01 PM, John Orlando wrote:
>     It is my understanding that the current FPGA image used for OpenBTS
>     will fail on the transmit side if the FPGA clock is slower than the
>     USB clock.  The USB clock is 48 MHz.
>
> If you (or anyone else involved) can provide the details of what the
> issue is here, I may have some time to dive in and potentially come up
> with a fix.  Let me know if interested.
>
>
>     Also, the GSM symbol clock is derived from 13 MHz, so if you use a
>     multiple-of-13 clock, you can simplify a lot of the decimation.
>
>     These two facts together led us to choose 52 MHz for the current
>     OpenBTS USRP clock.
>
> Gotcha.  Thanks for the quick response David.
>
> Anyone else know of any other issues that would prevent the FPGA from
> being clocked at lower speeds?


The 9862 is only spec'ed down to 10 MHz.

Matt