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.

[Chris21]

ST does not connect the MCO on some Nucleos such as L4 and L5, likely to allow lowest possible power consumption.

[krook]

I am sure that I can find a colleague at work to help me solder. I can see the empty hole for PH0, e.g., but it is not clear to me from reading the user manual from where I can fetch ST-LINK MCO. What physical modifications do I need to do? I would love to learn how I can decode this from the two documents.

Thanks,

Robert

[Andrew Neil]

---

@krook wrote:

 it is not clear to me from reading the user manual from where I can fetch ST-LINK MCO. What physical modifications do I need to do?

---

From the extract @Chris21 posted, these are the settings you need:

"Off" means that there is no connection across the solder bridge - the bridge is open;

"On" means that there is a connection across the solder bridge - the bridge is closed.

(look like they forgot that this is just a solder bridge and not a jumper link - where the jumper would be literally on the pins, or off them)

PS:

https://community.st.com/t5/stm32-mcus-products/docmentation-of-solder-bridges/m-p/689362

[Chris21]

Where ST's default SB is On, you will find a zero ohm resistor across the bridge; these can be a little annoying to remove unless you have good soldering equipment / skills. 

Here they say to remove SB142, but it looks like that may not really be necessary.