AN4467 datasheet(17/33 Pages) STMICROELECTRONICS

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AN4467

Clocks

Four different clock sources can be used to drive the system clock (SYSCLK):

•

HSI16 (high-speed internal) oscillator clock

•

HSE (high-speed external) oscillator clock

•

PLL clock

•

MSI (multispeed internal) oscillator clock

The MSI is used as a system clock source after startup from reset, wake-up from Standby

low-power modes.The MSI, HSI16 or HSI16 divided by four, are used as a system clock

source after wake-up from Stop low-power mode.

The devices have the following two secondary clock sources:

•

37 kHz low speed internal RC (LSI RC) which drives the independent watchdog and

optionally the RTC used for auto-wakeup from Stop/Standby mode.

•

32.768 kHz low speed external crystal (LSE crystal) which optionally drives the

real-time clock (RTCCLK)

The STM32L0x2 and STM32L0x3 have HSI48 (high-speed internal) oscillator clock

available for USB and Random generator. This permits a USB communication without the

need for external clock source.

Each clock source can be switched on or off independently when not used, to optimize

power consumption.

Refer to the STM32L0xx reference manuals (RM0367, RM0376, RM0377) for a description

of the clock tree.

2.1

MSI clock

The MSI clock signal is generated from an internal RC oscillator. Its frequency range can be

adjusted by software through the RCC_ICSCR register. Seven frequency ranges are

available: 65.5 kHz, 131 kHz, 262 kHz, 524 kHz, 1.05 MHz, 2.1 MHz (default value) and

4.2 MHz, all are multiple values of 32.768 kHz.

The MSI clock is used as a system clock after a restart from reset.

The MSI RC oscillator has the advantage of providing a low-cost (no external components)

low-power clock source. It is used as a wakeup clock in low-power modes to reduce power

consumption and wakeup time.

The MSIRDY flag in the RCC_CR register indicates wether the MSI RC is stable or not. At

startup, the MSI RC output clock is not released until this bit is set by hardware.

The MSI RC can be switched on and off through the RCC_CR register (default is on).

Calibration

If the application is subject to voltage or temperature variations, this may affect the RC

oscillator speed. You can trim the MSI frequency in the application through the RCC_ICSCR

on clock measurement with TIM21). For more information refer to AN3300 “How to calibrate

an STM32Lxx internal RC oscillator”.

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