Cmu - Clock Management Unit; Introduction; Features - Silicon Laboratories EFR32xG21 Wireless Gecko Reference Manual

8. CMU - Clock Management Unit

Oscillators CMU

8.1 Introduction

The Clock Management Unit (CMU) is responsible for switching among various oscillator sources and provide clocks to the peripheral
modules. Oscillators are automatically turned on and off based on demand from the peripherals to minimize power consumption.

8.2 Features

• Multiple clock sources available:
• 38 MHz - 40 MHz High Frequency Crystal Oscillator (HFXO)
• 1 MHz - 80 MHz High Frequency RC Oscillator (HFRCODPLL)
• 1 MHz - 38 MHz Deep Sleep High Frequency RC Oscillator (HFRCOEM23)
• 20 MHz Fast Startup RC Oscillator (FSRCO)
• 1 MHz - 38 MHz External Clock from Input Pins (CLKIN0)
• 32768 Hz Low Frequency Crystal Oscillator (LFXO)
• 32768 Hz Low Frequency RC Oscillator (LFRCO)
• 1000 Hz Ultra Low Frequency RC Oscillator (ULFRCO)
• On-demand oscillator request.
• Low power oscillators.
• Fast start-up times.
• Cascaded prescalers for AHB Clocks (HCLK) and APB Clocks (PCLK).
• Clock gating on an individual basis to all peripherals based on module enable.
• Reset on an individiaul basis for Timer and IADC based on module enable.
• Selectable clocks can be output on external pins and/or PRS.
• Deep Sleep High Frequency RC oscillator (HFRCOEM23), which is asynchronous to the system clock, can be selected for IADC
operation or debug trace functionality.
• Hardware support for calibration of RC oscillators.
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WDOG clock
LETIMER clock
RTCC clock
Peripheral A clock
Peripheral B clock
Peripheral C clock
Peripheral D clock
CPU clock
CMU - Clock Management Unit
Quick Facts
What?
The CMU controls clock switching and distribution.
EFR32xG21 supports 7 different oscillators with
minimized power consumption and short start-up
time. The CMU has HW support for calibration of RC
oscillators.
Why?
Oscillators and clocks contribute significantly to the
power consumption of the MCU. With the low power
oscillators combined with the flexible clock control
scheme, it is possible to minimize the energy con-
sumption in any given application.
How?
The CMU switches different clock sources for vari-
ous peripherals and sets the prescaler for the bus
clocks. The short oscillator start-up times makes du-
ty-cycling between active mode and the different low
energy modes (EM2 DeepSleep, EM3 Stop, and
EM4) very efficient. The calibration feature ensures
high accuracy RC oscillators. Interrupt are available
to avoid CPU polling of flags.
Reference Manual
Rev. 0.4 | 135