ST STM32F40 Series Reference Manual page 115

Reset and clock control for (RCC)
The clock controller provides a high degree of flexibility to the application in the choice of the
external crystal or the oscillator to run the core and peripherals at the highest frequency
and, guarantee the appropriate frequency for peripherals that need a specific clock like
Ethernet, USB OTG FS and HS, I2S and SDIO.
Several prescalers are used to configure the AHB frequency, the high-speed APB (APB2)
and the low-speed APB (APB1) domains. The maximum frequency of the AHB domain is
168 MHz. The maximum allowed frequency of the high-speed APB2 domain is 84 MHz. The
maximum allowed frequency of the low-speed APB1 domain is 42 MHz
All peripheral clocks are derived from the system clock (SYSCLK) except for:
●
The USB OTG FS clock (48 MHz), the random analog generator (RNG) clock
(≤ 48 MHz) and the SDIO clock (≤ 48 MHz) which are coming from a specific output of
PLL (PLL48CLK)
●
The I2S clock
To achieve high-quality audio performance, the I2S clock can be derived either from a
specific PLL (PLLI2S) or from an external clock mapped on the I2S_CKIN pin. For
more information about I2S clock frequency and precision, refer to
Clock
●
The USB OTG HS (60 MHz) clock which is provided from the external PHY
●
The Ethernet MAC clocks (TX, RX and RMII) which are provided from the external
PHY. For further information on the Ethernet configuration, please refer to
Section 29.4.4: MII/RMII selection
Ethernet is used, the AHB clock frequency must be at least 25 MHz.
The RCC feeds the external clock of the Cortex System Timer (SysTick) with the AHB clock
(HCLK) divided by 8. The SysTick can work either with this clock or with the Cortex clock
(HCLK), configurable in the SysTick control and status register.
The timer clock frequencies for STM32F405xx/07xx and STM32F415xx/17xx are
automatically set by hardware. There are two cases:
1. If the APB prescaler is 1, the timer clock frequencies are set to the same frequency as
that of the APB domain to which the timers are connected.
2. Otherwise, they are set to twice (×2) the frequency of the APB domain to which the
timers are connected.
The timer clock frequencies for STM32F42xxx and STM32F43xxx are automatically set by
hardware. There are two cases depending on the value of TIMPRE bit in RCC_CFGR
register:
● If TIMPRE bit in RCC_DKCFGR register is reset:
If the APB prescaler is configured to a division factor of 1, the timer clock frequencies
(TIMxCLK) are set to PCLKx. Otherwise, the timer clock frequencies are twice the
frequency of the APB domain to which the timers are connected: TIMxCLK = 2xPCLKx.
●
If TIMPRE bit in RCC_DKCFGR register is set:
If the APB prescaler is configured to a division factor of 1, 2, or 4, the timer clock
frequencies (TIMxCLK) are set to HCLK. Otherwise, the timer clock frequencies is four
times the frequency of the APB domain to which the timers are connected: TIMxCLK =
4xPCLKx.
FCLK acts as Cortex™-M4F free-running clock. For more details, refer to the Cortex™-M4F
technical reference manual.
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generator.
Doc ID 018909 Rev 4
in the Ethernet peripheral description. When the
RM0090
Section 27.4.4: