System Clock Description
PXN20 Microcontroller Reference Manual, Rev. 1
5-2 Freescale Semiconductor
— Clock source from external oscillator
— Lock detect circuitry continuously monitors lock status
— Loss of clock (LOC) detection for reference and feedback clocks
— On-chip loop filter (for improved electromagnetic interference performance and reduces
number of external components required)
— Programmable frequency modulation
• On-chip crystal oscillator (4 – 40 MHz XTAL) supporting 4 MHz to 40 MHz crystals
• Dedicated 16 MHz internal RC oscillator
— 16 MHz internal RC oscillator supports low speed code execution and clocking of peripherals
through selection as the system clock
— Used as default clock source out of reset
— Provides a clock for rapid start-up from low power modes
— Provides a back-up clock in the event of PLL or external oscillator clock failure
— Provides watchdog timer
— 5% accuracy over the operating temperature range (after factory trim)
— Trimming registers to support frequency adjustment with in-application calibration
• Dedicated internal 128 kHz_IRC oscillator for low power mode operation and self wake-up
— 5% accuracy (after factory trim)
— Trimming registers to support improve accuracy with in-application calibration
• Dedicated 32 kHz_XTAL external oscillator for accurate timed wake-up
5.1.2 Clock Sources
The PXN20 clock sources are:
• High Frequency Crystal oscillator (4 – 40 MHz XTAL)
• Fast on-chip RC oscillator (16 MHz_IRC)
• Slow on-chip RC oscillator (128 kHz_IRC)
• 32 kHz crystal oscillator (32 kHz_XTAL)
• Phase-Locked Loop (FMPLL)
The clock sources available on the PXN20 are shown in Figure 5-1 and discussed in more detail in
subsequent sections.
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