The STM32MP135 is powered by PMIC1A. After initial power-up, the voltage at BUCK4 is normal at 3.3V, but during U-Boot flashing, the output voltage at VOUT4 drops to only 2.4V, causing the flashing to fail. The VDD and VDDQ pins of the eMMC are powered by the same power rail, VOUT4.
Hi
have you excluded some SW going to change buck4 voltage thru I2C ?
Is eMMC IO supply on STM32MP135 at 3.3V too (check VDDSD1/VDDSD2 if relevant) ?
Could you share the board model you are using. Is it a custom board where HW is not yet trustable ?
Buck component choice and layout could be tricky.
Is the 2.4V clean and stable or oscillating ?
Please look into
AN5474Getting started with STM32MP13x lines hardware development
DS12792 Highly integrated power management IC for micro processor units
AN5431 The STPMIC1 PCB layout guidlines
and maybe
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