AN3216 datasheet(16/33 Pages) STMICROELECTRONICS

16 / 33 page

Clocks

AN3216

16/33

DocID17496 Rev 7

Clocks

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

•

HSI ((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 Stop or

Standby low power modes.

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)

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

power consumption.

Refer to the STM32L1xxx reference manual (RM0038) for a description of the clock tree.

3.1

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. Those frequencies 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).

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

measurement with TIM9/TIM10/TIM11). For more information refer to AN3300 “How to

calibrate an STM32L1xx internal RC oscillator”.

AN3216 Datasheet(PDF) 16 Page - STMicroelectronics

AN3216 Datasheet(HTML) 16 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Datasheet Download

Link URL