A versatile clock management

AN4445

12/17

DocID025944 Rev 1

5

A versatile clock management

A reset and clock controller (RCC) peripheral manages the five possible clock sources of

the STM32L0.

Two external oscillators can be used for applications requiring high precision:

•

The HSE clock (4-24 MHz high speed external clock), typically used to feed the PLL

and to generate a CPU clock frequency of up to 32 MHz and a 48-MHz frequency for

the USB controller

•

The LSE (typically 32.768 kHz low speed external clock) normally used to provide a

low power clock source to the real time clock but which can also be used as LCD clock.

Four internal oscillators can be selected for various tasks:

•

The LSI clock (37 kHz low speed internal clock) is a low accuracy ultra-low power

source that can feed the real time clock (with a limited accuracy), the LCD controller

and the independent watchdog

•

The HSI16 clock (16 MHz high speed internal clock) is a high speed voltage-

compensated oscillator

•

The HSI48 clock (48 MHz high speed internal clock) is a high speed voltage controlled

oscillator that can be automatically trimmed thanks to LSE or USB's start of frame

signal. It is used to achieve the USB crystal less feature

•

The MSI clock (64 kHz to 4.2 MHz multi speed internal clock) is a medium accuracy

oscillator with adjustable frequency and low current consumption. It is designed to

operate with a current proportional to the frequency, so as to minimize the internal

oscillator consumption overhead for the low CPU frequencies.

The following table summarizes the characteristics and uses of the various oscillators.

Clock

source

Use

Frequency

Consumption

(typical)

Accuracy

Factory

trimming

User

trimmable

HSE

Master clock

(+ RTC & LCD)

1-24 MHz

0.5 to 0.7 mA

Crystal dependent,

down to tens of ppm

Not applicable

LSE

RTC and LCD

32.768 kHz

(typical)

0.45 µA (1.8 V)

0.6 µA (3 V)

Crystal dependent,

down to a few ppm

HSI16

Master clock

16 MHz

100 µA

Yes

Yes

HSI48

USB, Random

number generator

48 MHz

300 µA

Fits USB needs

MSI

Master clock

65.5 kHz

131 kHz

262 kHz

524 kHz

1.05 MHz

2.1 MHz

4.2 MHz

0.75 µA

1.0 µA

1.5 µA

2.5 µA

4.5 µA

8.0 µA

15 µA

0.5% typical(2)

Yes

Yes

LSI

RTC, LCD &

ind. WDG

38 kHz

0.4 µA (3 V)

No

be measured

1. Based on preliminary characterization or design simulations. See product datasheet for detailed electrical characteristics