Hi Yuri
supported frequencies are described in i.MX8QXP Datasheet
sect.4.10.12 FlexSPI timing parameters:
The FlexSPI interface can work in SDR or DDR modes. It can operate
up to 60 MHz at 3.3 V, 166 MHz at 1.8 V SDR mode
https://www.nxp.com/docs/en/data-sheet/IMX8QXPAEC.pdf
There is no min.frequency limit, as for "2.5MHz" - one can consider
various parents clocks with dividers in Chapter 8 Clocking i.MX8DQXP Reference Manual
https://www.nxp.com/docs/en/reference-manual/IMX8DQXPRM.pdf
Best regards
igor
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