Reset and clock control (RCC) RM0351
210/1830 DocID024597 Rev 5
can use the USB SOF signal, the LSE or an external signal to automatically and quickly
adjust the oscillator frequency on-fly. It is disabled as soon as the system enters Stop or
Standby mode. When the CRS is not used, the HSI48 RC oscillator runs on its default
frequency which is subject to manufacturing process variations.
For more details on how to configure and use the CRS peripheral please refer to Section 7:
Clock recovery system (CRS) (only valid for STM32L496xx/4A6xx devices).
The HSI48RDY flag in the Clock recovery RC register (RCC_CRRCR) indicates whether the
HSI48 RC oscillator is stable or not. At startup, the HSI48 RC oscillator output clock is not
released until this bit is set by hardware.
The HSI48 can be switched on and off using the HSI48ON bit in the Clock recovery RC
register (RCC_CRRCR).
6.2.5 PLL
The device embeds 3 PLLs: PLL, PLLSAI1, PLLSAI2. Each PLL provides up to three
independent outputs. The internal PLLs can be used to multiply the HSI16, HSE or MSI
output clock frequency. The PLLs input frequency must be between 4 and 16 MHz. The
selected clock source is divided by a programmable factor PLLM from 1 to 8 to provide a
clock frequency in the requested input range. Refer to Figure 15: Clock tree (for
STM32L475xx/476xx/486xx devices) and Figure 16: Clock tree (for STM32L496xx/4A6xx
devices) and PLL configuration register (RCC_PLLCFGR).
The PLLs configuration (selection of the input clock and multiplication factor) must be done
before enabling the PLL. Once the PLL is enabled, these parameters cannot be changed.
To modify the PLL configuration, proceed as follows:
1. Disable the PLL by setting PLLON to 0 in Clock control register (RCC_CR).
2. Wait until PLLRDY is cleared. The PLL is now fully stopped.
3. Change the desired parameter.
4. Enable the PLL again by setting PLLON to 1.
5. Enable the desired PLL outputs by configuring PLLPEN, PLLQEN, PLLREN in PLL
configuration register (RCC_PLLCFGR).
An interrupt can be generated when the PLL is ready, if enabled in the Clock interrupt
enable register (RCC_CIER).
The same procedure is applied for changing the configuration of the PLLSAI1 or PLLSAI2:
1. Disable the PLLSAI1/PLLSAI2 by setting PLLSAI1ON/PLLSAI2ON to 0 in Clock control
register (RCC_CR).
2. Wait until PLLSAI1RDY/PLLSAI2RDY is cleared. The PLLSAI1/PLLSAI2 is now fully
stopped.
3. Change the desired parameter.
4. Enable the PLLSAI1/PLLSAI2 again by setting PLLSAI1ON/PLLSAI2ON to 1.
5. Enable the desired PLL outputs by configuring PLLSAI1PEN/PLLSAI2PEN,
PLLSAI1QEN/PLLSAI2QEN, PLLSAI1REN/PLLSAI2REN in PLLSAI1 configuration
register (RCC_PLLSAI1CFGR) and PLLSAI2 configuration register
(RCC_PLLSAI2CFGR).
The PLL output frequency must not exceed 80 MHz.
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