Lpc17xx using an internal crystal (Internal RC oscillator) as the clock source

In general applications (not using USB, the accuracy requirements are not high) the LPC17XX series in addition to using external crystals, the internal 12MHZ crystals can also be used as the oscillator source. In fact, before running the initialization program, before entering the user program (main function), LPC17XX is using an internal crystal, which is designed to allow the bootstrap code (commonly known as bootloader) to run on a known frequency.

A small experiment using an internal crystal was recently made on the Development Board, and the original program using an external 12MHZ was modified to use an internal crystal. This modification is very simple, as long as the clock Source selection register (Clksrcsel) and the phase-locked Loop 0 Configuration register (pll0cfg) are modified. Under Keil, SYSTEM_LPC177X_8X.C can even visualize changes. Figure as follows:

In order to determine the effect of the experiment, the external crystal can be turned off (Main oscillator enable), PLL1 generally only for USB, so this can also be turned off PLL1 enable. The lpc17xx can run up to 120MHZ, so the M-value of PLL0 can be set to a maximum of ten (0xA)