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Chris Rice
Associate III
December 10, 2019
Question

48MHz clock source selection, not necessarily 48MHz?

  • December 10, 2019
  • 3 replies
  • 752 views

I have (another) dumb question... but I learn so much from my dumb questions, so bear with me.

On our STM32F7, we are using a couple of peripherals that use the PLL48CLK. I see from [RM0410, Figure 13 page 153/1954] that this is selectable as the output of PLL or PLLSAI via the DKCFGR2.CK48SEL bit.

Upstream of those, are configurable parameters, namely the Q divisor in the PLL and the P divisor in the PLLSAI.

This is a little confusing to me, since this signal is called throughout the datasheet, the "48MHz clock". Well, it's not necessarily 48MHz, I suppose, if I modify those P and Q values.

So which is the case, is it (a) I *need* to configure P and/or Q (and upstream settings) so that the result is a 48MHz clock, or things won't work? or (b) is "48MHz clock"/"PLL48CLK" just a nickname of sorts, a guideline, and I can set this to anything I want in actuality and the downstream peripherals will still work, although faster or slower?

Or, sadly, (c) I completely misunderstand everything.

Thanks very much.

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3 replies

Tesla DeLorean
Guru
December 10, 2019

Can be other things.

RND CRYPT​ USB may have specific expectations, SDMMC not so much.

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Chris Rice
Associate III
December 10, 2019

Thanks. If we are seeing USB misbehavior, would it behoove us to configure this as close to 48MHz as possible?

Chris Rice
Associate III
December 10, 2019

put another way, would those "expectations" pretty much be 48mhz?