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Issue with External 1.8V Power on FRDM-MCXN947 Board - MCU Unresponsive

Objective: I am attempting to power my FRDM-MCXN947 development board from an external 1.8V supply. bypassing the default USB 3.3V LDO (U2). My attempts have been unsuccessful, and the microcontroller remains unresponsive until I revert to the default USB power configuration.

Hardware Modifications Made:

Referencing to FRDM-MCXN947 schematic I have disabled the DCDC core and enabled the LDO core by completing the following changes

Populated (Enabled):
- R17.
- R194.
- R20.
Removed / DNP (Disabled):
- R14, R18, R19: These resistors are DNP (Do Not Populate) to disable the DCDC_CORE.

2. Power Supply Setup:

External Supply: 1.8V, connected to J25 (P3V3_MCU), pin 1.
Jumper Configuration: The J24 (VDD_USB) jumper has been removed to isolate the board from the USB 5V input and the U2 LDO.

3. Observed Behavior:

With External 1.8V: The board is completely unresponsive. A pre-flashed "blinky" program does not run. An external debugger detects target voltage but cannot establish communication with the MCU.
With Default USB Power (J24 jumper on): Everything functions perfectly. Probing J25 shows 3.3V (from the U2 LDO), and the program runs correctly.

4. Troubleshooting Steps Already Taken:

Verified the external power supply is stable and providing a clean 1.8V.
Confirmed the power is present on the J25 header.
Tried supplying 3.3V to J25 (with J24 off) as a test, but the MCU was still unresponsive.

5. Core Question / Suspected Issue:

My understanding from the schematics is that the MCU's primary power rail (VDD_CORE) is fed only through the J24/J25 network. Therefore, disconnecting J24 and applying voltage to J25 should power the MCU directly. The fact that it does not, and that even 3.3V fails to work, suggests I may be missing a critical step or a secondary power requirement.

What am I missing? Could there be another voltage rail that must also be powered for the MCU to boot from an external supply on J25? Any insight would be greatly appreciated.

Power

Re: Issue with External 1.8V Power on FRDM-MCXN947 Board - MCU Unresponsive

Hi Habib,

Thanks for the reply but we switch To MCX-N9XX Eval board Since it has the jumper settings we want and with that we were able to run the board successfully with 1.8v

Thanks again

Re: Issue with External 1.8V Power on FRDM-MCXN947 Board - MCU Unresponsive

Hello @ashwinanil,

To better assist you, could you please share a picture showing how you are supplying 3.3V to J25, along with any reworks involving resistors that have been made?

BR
Habib