In general applications (not using USB, not requiring high precision), the LPC17xx series can use an internal 12MHZ crystal as an oscillation source in addition to an external crystal. In fact, before running the initialization program and entering the user program (main function), the LPC17xx uses the internal crystal. This design is to allow the boot code (commonly known as the bootloader) to run at a known frequency.
Recently, I did a small experiment on the development board using the internal crystal. I changed the program that used the external 12MHZ to use the internal crystal. This modification is actually very simple. You only need to modify the clock source selection register (CLKSRCSEL) and the phase-locked loop 0 configuration register (PLL0CFG). You can even visualize the modification in Keil system_LPC177x_8x.c. The details are as follows:
In order to determine the experimental effect, the external crystal enable is turned off (Main Oscillator enable). PLL1 is generally only used for USB, so PLL1 can also be turned off here.
LPC17xx can run at up to 120MHZ, so the maximum M value of PLL0 can be set to 10 (0xA)
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