STM32L4 series PLLs?

turboscrew · ‎2017-09-04

Posted on September 04, 2017 at 20:27

There seems to be something I don't quite get.

In the reference manual it says 'The PLLs input frequency must be between 4 and 16 MHz.' and 'The selected clock source is divided by a programmable factor PLLM from 1 to 8 to provide a clock frequency in the requested input range'.

Also all other PLL parameters are described to be dividers.

How do you get 80 MHz?

waclawek.jan · ‎2017-09-04

Posted on September 04, 2017 at 20:36

PLL is a high frequency oscillator conditioned by comparing its divided output to a reference clock.

You may want to look it up in some basic reference.

JW

turboscrew · ‎2017-09-04

Posted on September 04, 2017 at 20:52

Ah, there's 'N' that's the multiplier. It was just not shown in the clock trees...

Brian TIDAL · ‎2017-09-04

Posted on September 04, 2017 at 21:54

Hi,

http://www.st.com/en/development-tools/stm32cubemx.html

will help you to better understand the clocks and PLL configuration and will help you in configuring the clock tree.

You'll achieve a 80MHz configuration with the following settings:

- HSE 8MHz PLLM=1, N=20, R=2

- or MSI 4MHz, PLLM=1, N=40, R=2

- ...

Regards

Bruno

In order to give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.

waclawek.jan · ‎2017-09-05

Posted on September 05, 2017 at 09:49

Hi Bruno,

the block schematics in the RMs are perfectly understandable, except that those in the L4 RM miss the PLL ''multiplication'' element itself.

That was what turbo was talking about initially (and I misunderstood his question).

Please, re-instate the ''xN'' (even if the multiplication sign is factully incorrect there), as is in other families' RM (e.g. in RM0090):

Thanks,

Jan

turboscrew · ‎2017-09-05

Posted on September 05, 2017 at 19:36

Exactly, Jan. Thanks. Now that makes sense.

waclawek.jan · ‎2017-09-05

Posted on September 05, 2017 at 21:09

It's missing in RM0390 ('F446) too

ST, please, correct these RMs.

JW

Nesrine M_O · ‎2017-09-07

Posted on September 07, 2017 at 11:13

Hi,

You are right!

Thank you for bringing this issue to our attention.It is noted and will be fixed in coming release of the documents

-Nesrine-