AN663

AN663 P RECISION 32™ M CU F A M I L Y C LOCKING O PTIONS 1. Introduction The 32-bit Precision32™ MCU family features a highly configurable clock structure. The advanced highperformance 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 www.DataSheet.net/ Oscillator Name Low Power Oscillator Low Frequency Oscillator RTC Oscillator Short Name LPOSCn LFOSCn Module LPOSCn RTCn Frequency 20 MHz or 2.5 MHz 16.4 ...