Electronic – RCC clock freq in STM32F103

OK I figured it out. There is a mistake in the STM32F4xx_spi.c Peripheral Driver file that doesn't allow the use of the HSI(internal clock) value needed for calculating the appropriate divider and odd values for the I2SPR register. (see the last line) I changed it to:

  i2sclk = (uint32_t)(((HSI_VALUE / pllm) * plln) / pllr);

and now it works. Hours wasted as usual!! The clock configuration tool is also wrong as it creates a message in the system_stm32F4xx.c file that states

*        To achieve the following I2S config:   |
*         - Master clock output (MCKO): ON      |
*         - Frame wide                : 16bit   |
*         - Audio sampling freq (KHz) : 48      |
*         - Error %                   : 0.0186  |
*         - Prescaler Odd factor (ODD): 0       |
*         - Linear prescaler (DIV)    : 2       |

which conflicts with the programmers reference that ODD=1 and DIV=3 for 48kHz sample rate, which is correct.

/* Get the PLLI2SN value */
plln = (uint32_t)(((RCC->PLLI2SCFGR & RCC_PLLI2SCFGR_PLLI2SN) >> 6) & \
                  (RCC_PLLI2SCFGR_PLLI2SN >> 6));

/* Get the PLLI2SR value */
pllr = (uint32_t)(((RCC->PLLI2SCFGR & RCC_PLLI2SCFGR_PLLI2SR) >> 28) & \
                  (RCC_PLLI2SCFGR_PLLI2SR >> 28));

/* Get the PLLM value */
pllm = (uint32_t)(RCC->PLLCFGR & RCC_PLLCFGR_PLLM);      

/* Get the I2S source clock value */
i2sclk = (uint32_t)(((HSE_VALUE / pllm) * plln) / pllr);

Try a 10x (not 1x) probe on the OSC2 pin. You should be able to see the clock. The frequency will be slightly changed by the probe loading.

I don't think your part number is correct, but most 16F chips are similar.

Edit: Scope probe loading is explained in this Tektronix paper.

Crystal pullability is explained in this application note. Note that the typical loading from a 10x scope probe would only change the frequency by perhaps 100ppm (0.01%), which is unimportant in many applications other than timekeeping.