NUCLEO-H755ZI-Q does not boot at 480 MHz after power-on; OK after reset

[junwon0425]

Hello,

I am using NUCLEO-H755ZI-Q (STM32H755) and I am observing a reproducible boot issue related to high system clock frequency.

1. Hardware / Environment

• Board: NUCLEO-H755ZI-Q

• MCU: STM32H755

• Power supply: USB (on-board)

• Debugger: ST-LINK (Connect under Reset)

• Supply configuration: LDO

• Toolchain: STM32CubeIDE 1.19.0

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2. Issue Description

• When the system clock is configured to 480 MHz, the board fails to boot after power-on or reset.

  • Flash programming and verification succeed

  • Immediately after reset release, the target becomes unresponsive

  • ST-LINK connection is lost 

• When the same firmware is configured to 400 MHz, the board boots and runs normally.

• Interesting behavior:

  1. Power-on the board

  2. Flash firmware configured for 400 MHz → boots normally

  3. Without removing power, rebuild and flash firmware configured for 480 MHz

  4. The board runs normally at 480 MHz

  5. If power is removed and re-applied, 480 MHz boot fails again

This behavior is fully reproducible.

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3. Observations

• The issue only occurs after a cold power-on reset

• A warm reset / reprogram without power cycle allows 480 MHz operation

• The same firmware behaves differently depending on the previous power state

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4. Question

• Is cold boot at 480 MHz in LDO mode guaranteed on STM32H755 / NUCLEO-H755ZI-Q?

• Are there known limitations or recommended boot sequences (e.g. staged clock increase)?

• Is this behavior expected due to power-up conditions?

Any clarification or recommendation would be appreciated.

Thank you.

[junwon0425]

I tested this on three different NUCLEO-H755ZI-Q boards.
Two out of the three boards show the same behavior: cold boot fails at 480 MHz but works at 400 MHz, and 480 MHz runs normally after a warm reflash without power cycling.

[mƎALLEm]

Hello @junwon0425 and welcome to the community,

NUCLEO-H755ZI-Q board is configured in SMPS mode (by default) not in LDO mode. Unless you modified the board to support the LDO mode.

I'm wondering how you could run your example even at 400Mhz. That doesn't make sense.

Moreover, you can't select a system frequency > 400MHz if you are in SMPS configuration.

From the datasheet:

Boosted mode (> 400Mhz at VOS0) is available only with LDO configuration (which is not the case with the default configuration of the NUCLEO-H755ZI-Q board):

[Pavel A.]

How this is possible at all? STM32H7 starts with the internal clock (HSI, 16 MHz). 400 or 480 MHz is possible only when the software runs and sets up the PLL.

Where the clock & power state can persist after cold boot, if not in the option bytes or in the on-board ST-Link MCU?