STM32U535NET3QTR datasheet(45/278 Pages) STMICROELECTRONICS

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DS14217 Rev 1

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STM32U535xx

Functional overview

3.11

Reset and clock controller (RCC)

The RCC (reset and clock control) manages the different reset types, and generates all

clocks for the bus and peripherals.

The RCC distributes the clocks coming from the different oscillators to the core and to

the peripherals. It also manages the clock gating for low-power modes and ensures the

clock robustness. It features:

•

Clock prescaler: in order to get the best trade-off between speed and current

consumption, the clock frequency to the CPU and peripherals can be adjusted by

a programmable prescaler.

•

Clock security system: clock sources can be changed safely on-the-fly in Run mode

through a configuration register.

•

Clock management: in order to reduce the power consumption, the clock controller

can stop the clock to the core, individual peripherals or memory.

•

System clock source: four different clock sources can be used to drive the master

clock SYSCLK:

–

HSE (4 to 50 MHz high-speed external crystal or ceramic resonator) that can

supply a PLL. The HSE can also be configured in bypass mode for an external

clock.

–

HSI16 (16 MHz high-speed internal RC oscillator) trimmable by software, that can

supply a PLL.

–

MSI (multispeed internal RC oscillator) trimmable by software, that can generate

16 frequencies from 100 kHz to 48 MHz. When a 32.768 kHz clock source is

available in the system (LSE), the MSI frequency can be automatically trimmed by

hardware to reach better than ±0.25% accuracy. In this mode the MSI can feed the

USB device, saving the need of an external high-speed crystal (HSE). The MSI

can supply a PLL.

–

System PLL that can be fed by HSE, HSI16 or MSI, with a maximum frequency

at 160 MHz.

•

HSI48 (RC48 with clock recovery system) internal 48 MHz clock source that can be

used to drive the USB, the SDMMC or the RNG peripherals. This clock can be output

on the MCO.

•

Auxiliary clock source: two ultra-low-power clock sources that can be used to drive

the real-time clock:

–

LSE (32.768 kHz low-speed external crystal), supporting three drive capability

modes. The LSE can also be configured in bypass mode for an external clock.

–

LSI (32 kHz low-speed internal RC), also used to drive the independent watchdog.

The LSI clock accuracy is ±5% accuracy. The LSI clock can be divided by 128 to

output a 250 Hz as source clock.

•

Peripheral clock sources: several peripherals have their own independent clock

whatever the system clock. Three PLLs, each having three independent outputs

allowing the highest flexibility, can generate independent clocks for the ADC, USB,

SDMMC, RNG, MDF, ADF, FDCAN1, OCTOSPI and SAI.

•

Startup clock: after reset, the microcontroller restarts by default with MSI. The prescaler

ratio and clock source can be changed by the application program as soon as the code

execution starts.

•

CSS (clock security system): this feature can be enabled by software. If a HSE clock

failure occurs, the master clock automatically switches to HSI16 and a software

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