STM32WB55 Outputting a 40Mhz clock out signal

[kullum]

I have an IC on a board I'm designing that needs a 40Mhz clock in signal. Due to a variety of design and regulatory requirements, I would like to avoid having an external oscillator and instead use the clock out signal of the STM32WB55 on the board. I have written firmware that can output a 32Mhz signal, but I'm struggling with outputting 40Mhz:

 

volatile unsigned int *RCC_AHB2ENR = (unsigned int *)0x5800004C;

volatile unsigned int *RCC_CFGR = (unsigned int *)0x58000008;

volatile unsigned int *GPIOA_MODER = (unsigned int *)0x48000000;

volatile unsigned int *GPIOA_AFRH = (unsigned int *)(0x48000000 + 0x24);

#define RCC_AHB2ENR_GPIOAEN (1 << 0)

#define RCC_CFGR_MCOSEL_Pos (24)

#define RCC_CFGR_MCOSEL_SYSCLK (0x1 << RCC_CFGR_MCOSEL_Pos)

#define RCC_CFGR_MCOPRE_Pos (28)

#define RCC_CFGR_MCOPRE_DIV2 (0x1 << RCC_CFGR_MCOPRE_Pos)

void configure_clock_out() {

// Enable the clock for GPIOA

*RCC_AHB2ENR |= RCC_AHB2ENR_GPIOAEN;

// Configure pin A8 as alternate function (AF0)

*GPIOA_MODER &= ~(0x3 << (16)); // Reset the bits for pin A8

*GPIOA_MODER |= (0x2 << (16)); // Set the bits for pin A8 to alternate function mode

*GPIOA_AFRH &= ~0xF; // Reset the alternate function bits for pin A8

// Configure the MCO output to 32 MHz

*RCC_CFGR &= ~(RCC_CFGR_MCOSEL_SYSCLK); // Reset the MCO configuration bits

*RCC_CFGR |= (RCC_CFGR_MCOSEL_SYSCLK); // Set the MCO configuration to 32 MHz

}

[Issamos]

Hello @kullum 

I suggest you to debug step by step the code and track the variation of clock values in every step. Else, I think you should share the full project here to give the opportunity to our community members and experts to find the issue.

Best regards.

II

[kullum]

Hi Issamos,

This is the full code. This will do what I expect it to do, set A8 to 32Mhz. What I want it to do, that I don't know how to do, is set it to 40Mhz.

[Issamos]

Than you need to use HSI PLL clock and configure the PLL parameters(M,...) to have a 40MHz SYSCLK.

Best regards.

II