Default clock setting on the LPC55S05-EVK

Nikolai_Baschinski — Wed, 24 Jun 2026 15:28:35 GMT

Hi folks,

I'm new to the LPC55 series and currently using the LPC55S06-EVK. I have a very simple bare-metal project that just toggles the LED D4.

What confuses me is that I don't have any clock configuration code, yet the MCU starts up and runs properly. I expected to find a default clock configuration in either SystemInit() or ResetISR(), but there is nothing there.

When I halt the MCU at the very first instruction of ResetISR(), the clock registers are already configured:

MAINCLKSELA = 3
MAINCLKSELB = 0
AHBCLKDIV = 1

However, according to the user manual, the reset values of MAINCLKSELA and AHBCLKDIV are both 0.

How is that possible? Where are these registers being configured before my code starts executing?

Best regards,
Nikolai

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Default clock setting on the LPC55S05-EVK