STMicroelectronics STM32WL5 Series Reference Manual page 1310

Serial peripheral interface / integrated interchip sound (SPI/I2S)
Bits 7:0 I2SDIV[7:0]: I2S linear prescaler
I2SDIV [7:0] = 0 or I2SDIV [7:0] = 1 are forbidden values.
Refer to
Note: These bits should be configured when the I2S is disabled. They are used only when the I2S is
in master mode.
They are not used in SPI mode.
1310/1450
Section 37.7.3 on page 1289.
RM0453 Rev 5
RM0453