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.
