ST STM32F405 Reference Manual page 208

Reset and clock control for STM32F42xxx and STM32F43xxx (RCC)
Bits 31:25 Reserved, must be kept at reset value.
Bit 24 TIMPRE: Timers clocks prescalers selection
Bits 23:22 SAI1BSRC: SAI1-B clock source selection
Bits 21:20 SAI1ASRC: SAI1-A clock source selection
Bits 19: 18 Reserved, must be kept at reset value.
Bits 17:16 PLLSAIDIVR: division factor for LCD_CLK
Bits 15: 13 Reserved, must be kept at reset value.
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This bit is set and reset by software to control the clock frequency of all the timers connected
to APB1 and APB2 domain.
0: If the APB prescaler (PPRE1, PPRE2 in the RCC_CFGR register) is configured to a
division factor of 1, TIMxCLK = PCLKx. Otherwise, the timer clock frequencies are set to
twice to the frequency of the APB domain to which the timers are connected:
TIMxCLK = 2xPCLKx.
1:If the APB prescaler (PPRE1, PPRE2 in the RCC_CFGR register) is configured to a
division factor of 1, 2 or 4, TIMxCLK = HCLK. Otherwise, the timer clock frequencies are set
to four times to the frequency of the APB domain to which the timers are connected:
TIMxCLK = 4xPCLKx.
These bits are set and cleared by software to control the SAI1-B clock frequency.
They should be written when the PLLSAI and PLLI2S are disabled.
00: SAI1-B clock frequency = f(PLLSAI_Q) / PLLSAIDIVQ
01: SAI1-B clock frequency = f(PLLI2S_Q) / PLLI2SDIVQ
10: SAI1-B clock frequency = Alternate function input frequency
11: wrong configuration
These bits are set and cleared by software to control the SAI1-A clock frequency.
They should be written when the PLLSAI and PLLI2S are disabled.
00: SAI1-A clock frequency = f(PLLSAI_Q) / PLLSAIDIVQ
01: SAI1-A clock frequency = f(PLLI2S_Q) / PLLI2SDIVQ
10: SAI1-A clock frequency = Alternate function input frequency
11: wrong configuration
These bits are set and cleared by software to control the frequency of LCD_CLK.
They should be written only if PLLSAI is disabled.
LCD_CLK frequency = f(PLLSAI_R) / PLLSAIDIVR with 2 ≤ PLLSAIDIVR ≤ 16
00: PLLSAIDIVR = /2
01: PLLSAIDIVR = /4
10: PLLSAIDIVR = /8
11: PLLSAIDIVR = /16
RM0090 Rev 18
RM0090