Hello Daniel Brown,
In the user manual of the LPC824 (https://www.nxp.com/docs/en/user-guide/UM10800.pdf) under section 5.3.2 you can find the following information.
So, you can definitely use the internal oscillator as the main clock. The development board for the LPC824 (LPCXpresso824-MAX Board for LPC82x family MCUs|NXP ) actually uses the IRC in all the examples that we provide (Code Bundle, LPCOpen and SDK) as the input for the PLL and the output of the PLL is the source of the main clock.
Hope it helps!
Victor.
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