AN663 Datasheet, PDF (1/12 Pages) Silicon Laboratories – PRECISION32™ MCU FAMILY CLOCKING OPTIONS

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PRECISION32™ MCU FAMILY CLOCKING OPTIONS
1. Introduction
The 32-bit Precision32™ MCU family features a highly configurable clock structure. The advanced high-
performance bus (AHB) and advanced peripheral bus (APB) have many options available for clocking. In addition,
firmware can disable most peripheral clocks to save power.
The clock to the module must be enabled before firmware can modify the registers. The first initialization
step in all peripheral initialization routines should enable the clock to the module.
The corresponding Precision32 family reference manual contains detailed descriptive and usage information on
each of the clock sources mentioned in this document.
2. Clock Sources
The AHB drives the core and memory on ARM^® Cortex™-M3 devices. The APB drives the peripherals and
peripheral registers.
The AHB on the Precision32 devices has several options for clock sources: Low Power Oscillator, Low Frequency
Oscillator, RTC Oscillator, External Oscillator, PLL Oscillator, and USB Oscillator. The APB clock is either equal to
AHB or a divide by 2 version of it, so it is always synchronized with the AHB clock.
Table 1 describes the oscillators that can serve as clock sources for the AHB bus. Figure 1 shows an example
clock control block diagram.
Table 1. AHB Clock Source Summary
Oscillator Name Short Name
Low Power
Oscillator
LPOSCn
Low Frequency
Oscillator
LFOSCn
RTC Oscillator
RTCnOSC
External Oscillator EXTOSCn
PLL Oscillator
PLLnOSC
USB Oscillator USBnOSC
Module
LPOSCn
RTCn
RTCn
EXTOSCn
PLLn
USBn
Frequency
Use Case
20 MHz or 2.5 MHz
High frequency and lower
power consumption required
16.4 kHz
Low power, slow frequency
required; no room for exter-
nal RTC watch crystal
32.768 kHz
Low power, slow, accurate
frequency required; room for
external RTC watch crystal
Crystal: 10 kHz to 25 MHz
CMOS: 10 kHz to 50 MHz
RC or C: up to 1.6 MHz
Very accurate frequency
required; room for external
components
23 MHz to maximum device Flexible, fast, and accurate
frequency
frequency required
48 MHz
Very accurate fast fre-
quency required; no flexibil-
ity needed
Rev. 0.1 2/12
Copyright © 2012 by Silicon Laboratories
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