ST STM32F405 Reference Manual page 204

Reset and clock control for STM32F42xxx and STM32F43xxx (RCC)
Bit 31 Reserved, must be kept at reset value.
Bits 30:28 PLLI2SR: PLLI2S division factor for I2S clocks
Caution: The I2Ss requires a frequency lower than or equal to 192 MHz to work correctly.
Bits 27:24 PLLI2SQ: PLLI2S division factor for SAI1 clock
204/1749
These bits are set and cleared by software to control the I2S clock frequency. These bits
should be written only if the PLLI2S is disabled. The factor must be chosen in accordance
with the prescaler values inside the I2S peripherals, to reach 0.3% error when using
standard crystals and 0% error with audio crystals. For more information about I2S clock
frequency and precision, refer to
I2S clock frequency = VCO frequency / PLLR with 2 ≤ PLLR ≤ 7
000: PLLR = 0, wrong configuration
001: PLLR = 1, wrong configuration
010: PLLR = 2
...
111: PLLR = 7
These bits are set and cleared by software to control the SAI1 clock frequency.
They should be written when the PLLI2S is disabled.
SAI1 clock frequency = VCO frequency / PLLI2SQ with 2 <= PLLI2SIQ <= 15
0000: PLLI2SQ = 0, wrong configuration
0001: PLLI2SQ = 1, wrong configuration
0010: PLLI2SQ = 2
0011: PLLI2SQ = 3
0100: PLLI2SQ = 4
0101: PLLI2SQ = 5
...
1111: PLLI2SQ = 15
Section 28.4.4: Clock generator
RM0090 Rev 18
RM0090
in the I2S chapter.