LPC540xx
NXP Semiconductors
32-bit ARM Cortex-M4 microcontroller
7.10.1.3 Crystal oscillator
The LPC540xx include four independent oscillators. These are the main oscillator, the
FRO, the watchdog oscillator, and the RTC oscillator.
Following reset, the LPC540xx will operate from the Internal FRO until switched by
software. This allows systems to operate without any external crystal and the boot loader
code to operate at a known frequency. See Figure 11 and Figure 12 for an overview of the
LPC540xx clock generation.
7.10.2 System PLL (PLL0)
The system PLL accepts an input clock frequency in the range of 32.768 kHz to 25 MHz.
The input frequency is multiplied up to a high frequency with a Current Controlled
Oscillator (CCO).
The PLL can be enabled or disabled by software.
7.10.3 USB PLL (PLL1)
The USB PLL accepts an input clock frequency in the range of 1 MHz to 25 MHz. The
input frequency is multiplied up to a high frequency with a Current Controlled Oscillator
(CCO).
The PLL can be enabled or disabled by software.
7.10.4 Audio PLL (PLL2)
The audio PLL accepts an input clock frequency in the range of 1 MHz to 25 MHz. The
input frequency is multiplied up to a high frequency with a Current Controlled Oscillator
(CCO).
The PLL can be enabled or disabled by software.
LPC540xx
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© NXP Semiconductors N.V. 2018. All rights reserved.
Product data sheet
Rev. 1.8 — 22 June 2018
62 of 168
NXP [ NXP ]
