imx rt: Which PLL clock?

bp1979 — Fri, 02 Feb 2024 11:21:33 GMT

Dear NXP,

I might have a silly question, I hope you can still help me out. I am struggling to understand which clock sources I can set for flexspi.

According to the clock tool, I can choose 4 different clock sources (SEMC, PLL2 PFD2, PLL3_SW, PLL3_PDF0)

In fsl_clock I can set the pfdFrac of a clock. E.g. with "CLOCK_InitSysPfd"

But the argument to the function is only clock_pfd_t. So I can't choose PLL2 or PLL3.

Does that mean that "SysPfd" is either PLL2 or PLL3?

And if that's the case, which one is PLL3? Usb1Pfd?

Re: imx rt: Which PLL clock?

EdwinHz — Wed, 07 Feb 2024 22:12:21 GMT

There is no such thing as a silly question, just silly people that don't ask questions.

PLL2 is System PLL, and PLL3 is USB1PLL.

Therefore, when selecting PLL2 PFD2, you are selecting the clock output of the System PLL passed through its third Phase Fractional Divider (PFD2):

When selecting PLL3 PFD0, you are selecting the clock output of the USB1PLL passed through its first Phase Fractional Divider (PFD0):

And finally, when selecting PLL3 SW, you are selecting the clock output of the USB1PLL directly:

CLOCK_InitSysPfd() initializes the System PLL's PFDs, while CLOCK_InitUsb1Pfd initializes the USB1PLL's PFDs.