STM32L552 Nucleo board: HSE error

[krook]

Hi, I am trying to program a STM32L552ZEQ to use HSE instead of the default clock, but I am doing something wrong.

I attach the reference manual, as well as the user manual for my specific board. In the user manual, on page 24, it is stated that there is a HSE with 16 MHz clock frequency. I try to enable this by setting the HSEBYP and HSEON bits in `RCC->CR`, like so

```

RCC->CR |= RCC_CR_HSEBYP_Msk | RCC_CR_HSEON_Msk;

while(!(RCC->CR & RCC_CR_HSERDY_Msk));

```

Using GDB I observe the change to the register value happening as expected. After setting it, I am waiting for the hardware to set the HSERDY bit, indicating that everything went OK. But this bit is never set. I never seem to get out of this while-loop. I observe the bit not being set using e.g. GDB.

I looked at the reference manual and saw that MCO is fed to OSC_IN, which seems correct.

I then thought that it appears as if the MCO can be connected to several different sources, one of which being the HSE. I look at the reference manual, and find this

I modify my code to configure MCOSEL to have the value `0100`, to indicate that we should output the HSE to the MCO pin. I am using the CMSIS header files to access the correct addresses, and it seems as if the define for MCOSEL_2 carries the right value (4).

```

// the define is in the CMSIS header-file, but I paste it here for clarity

#define RCC_CFGR_MCOSEL_2 (0x4UL << RCC_CFGR_MCOSEL_Pos) /*!< 0x04000000 */

RCC->CFGR |= RCC_CFGR_MCOSEL_2;

```

This seems to make no difference at all, and I am now a bit unsure of how to progress.

 

Thanks in advance for any tips that might make me able to figure this out.

[krook]

I should add that these clock configurations are the first things that are happening inside my main function.

[Andrew Neil]

I suggest that you start by doing this with CubeMX.

Even if you don't want to go forward with CubeMX, it should give you a starting point to work from ...

[krook]

I am not sure how to do it with CubeMX either, and I've failed to install CubeMX when I've tried. I'd rather understand why my code doesn't work. I don't think there is a lot missing, I've just misunderstood some minor detail.

[Andrew Neil]

---

@krook wrote:

In the user manual, on page 24, it is stated that there is a HSE with 16 MHz clock frequency. .

---

It also states that, by default, this is not implemented:

 

So have you made the necessary hardware changes?

[krook]

I am not sure that I have. I've ever made modifications like this, where can I read more about that? I thought I could implement it in software.

[Chris21]

I would connect the solder bridges to use the 8 MHz MCO from the ST-LINK.

[Andrew Neil]

I think the default hardware configuration should be that the solder bridges are set for the 8MHz MCO from the ST-Link?

Correction: UM says it isn't (credit @Chris21 )

[Chris21]

 

[Chris21]

It looks like you have done the correct things with the hardware and software.

I wonder about R19 on the ST-LINK:

It looks like that would need to be present.

 

[krook]

@Chris21, that does indeed seem relevant. May I ask where you found this diagram? For my own future reference in trying to figure these things out.

 

When I look at my board, there is indeed what I interpret as an OFF solder bridge there. I believe that I should put another 0-Ohm resistor there to make the current be able to pass. This was less obvious to me as R19 was not mentioned in the two documents I looked in. It makes sense that this detail should be found in the ST-LINK documentation, but I only considered that my STM-board was misconfigured.

Thanks,

Robert

[krook]

I misclicked and accepted an answer while I was trying to figure out how to edit my comment above (let's hope that R19 is the only missing piece of this puzzle, and that that answer is indeed the correct one!).

 

I was going to add a question, what does 100R mean? It is stated next to R19 in the diagram @Chris21 

[Chris21]

The schematic for the board is on ST's web site: https://www.st.com/en/evaluation-tools/nucleo-l552ze-q.html, click on the CAD resources tab.

100R denotes a 100 ohm resistor, but 0 ohms (or a bit of solder) should work.

Did you check with an oscilloscope to see if there's a clock signal at SB143?

 

 

[krook]

I will have to return to this thread when I've gotten my hands on those components. Thanks for your help so far!

[mƎALLEm]

Hello @krook ,

Just to remind you that even with STLINK-V3 (the version on STM32L552ZEQ Nucleo board), the MCO is configurable and the default config is generated from HSI (HSI/2 = 8MHz) not the HSE crystal of the STLINK.

So if you are looking for an accurate clock source, you need to configure your STLINK. Please read this article.