AN4631 datasheet(5/30 Pages) STMICROELECTRONICS

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STM32L0xx system clock

The STM32L0xx microcontroller family has various clock sources that can be used to drive

the system clock:

•

16 MHz High-Speed Internal (HSI16) RC oscillator clock

•

1 to 32 MHz High-Speed External (HSE) oscillator clock

•

65.5 kHz to 4.2 MHz Multi-Speed Internal (MSI) RC oscillator clock

•

2 to 32 MHz Phase-Locked Loop (PLL) that is clocked by HSI16 or HSE oscillators

The High-Speed Internal (HSI16) RC oscillator typically has a frequency of 16 MHz and

consumes 100 µA.

The Multi-Speed Internal (MSI) RC oscillator provides seven frequency ranges: 65.5 kHz,

131 kHz, 262 kHz, 524 kHz, 1.05 MHz, 2.1 MHz (default value) and 4.2 MHz. It is designed

to operate with a current proportional to the frequency (refer to the product datasheet for

more details about MSI power consumption versus selected range), so as to minimize the

internal oscillator consumption when the CPU runs at low frequencies. The MSI clock is

used as a system clock after restart from Reset and wake-up from standby. After wake-up

from stop mode, the MSI clock can be selected as system clock instead of HSI16 (or

HSI16/4).

The HSI48 clock signal is generated from an internal 48 MHz RC oscillator and can be used

directly for USB and for random number generator.

The internal RC oscillators (HSI16, MSI and HSI48) have the advantage of providing a low-

cost clock source (no external components required). It also has a faster startup time and

lower power consumption than the external oscillator. HSI16, MSI and HSI48 oscillators can

be calibrated to improve their accuracy. But even with calibration, the internal RC oscillator

frequency is less accurate than the frequency of an external crystal oscillator or ceramic

resonator (tens of ppm).

Note:

The internal RC oscillator MSI can also be used as a backup clock source (auxiliary clock) if

the external oscillator fails.

Figure 1. Simplified clock tree

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