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

Re: Default clock setting on the LPC55S05-EVK

Harry_Zhang — Fri, 26 Jun 2026 02:38:16 GMT

Hi @Nikolai_Baschinski

The user manual reset value is the hardware reset value.
But your application ResetISR() runs after the LPC55 Boot ROM has already executed. The Boot ROM prepares the device for boot and may configure the main clock path, so when you halt at ResetISR(), those clock registers are no longer at their raw reset values.

Harry

topic Re: Default clock setting on the LPC55S05-EVK in LPC Microcontrollers

Default clock setting on the LPC55S05-EVK

Re: Default clock setting on the LPC55S05-EVK