STM32F407 STOP MODE FAILURE

[maxtisc]

Hi everyone,

I am facing a bizarre issue with an STM32F407 running a custom PCB. I am trying to put the MCU into STOP mode (Low Power Regulator ON), but it seems to enter deep sleep for a fraction of a second (current drops to almost 0mA) and then immediately wakes up/resets, stabilizing at a constant 9.8mA.

I am programming via DFU and using a custom bootloader that jumps to the main application at 0x08010000.

Here is what I have already done in my "Deep Sleep Test" routine to isolate the issue:

• Hardware Isolation: All external peripherals (W25Q Flash, FRAM, LDOs) are completely disconnected or powered down. The 3.3V line is isolated.

• GPIOs: All GPIOs are set to GPIO_MODE_ANALOG, except for specific pull-up/pull-down pins strictly required by the hardware.

• USB PHY: I am explicitly disabling the internal USB transceiver using USB_OTG_FS->GCCFG |= USB_OTG_GCCFG_PWRDWN; and then disabling the clock.

• Interrupts & SysTick: I disable SysTick->CTRL = 0, clear SCB->ICSR, mask all EXTI lines (EXTI->IMR = 0), and explicitly disable/clear all NVIC lines using NVIC->ICER[i] = 0xFFFFFFFF and NVIC->ICPR[i] = 0xFFFFFFFF.

• RTC/Watchdog: I freeze the IWDG in STOP mode using __HAL_DBGMCU_FREEZE_IWDG(); and I force a backup domain reset to clear any ghost RTC wakeups.

Despite all this, calling HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI); inside a while(1) loop results in a momentary drop to 0mA, followed by an immediate return to 9.8mA.

My suspicion is that the custom bootloader is leaving some peripheral active or a pending flag that completely bypasses my NVIC disabling, or I am triggering a Brown-Out Reset during the current drop.

Has anyone experienced this "instant bounce" back to 10mA on the F4 series? What hidden registers should I check before calling WFI?

Thanks in advance for any pointers!

[Gyessine]

Hello ​@maxtisc 
Sorry for the late reply.
Usually, this kind of issue results from a hardware problem or, as suggested, a code issue.
To troubleshoot this issue, first try the firmware example. It is a simple project that puts the MCU in low-power mode.
If the example does not work correctly, the hardware is likely faulty. Common causes include an unconnected ground, a floating pin, or a similar hardware issue.
If the example works correctly, focus on the software. Immediate wake-up from low-power mode is a common issue.
It is usually caused by a wake-up or reset source is not fully cleared or a bootloader handoff problem. 
I suggest that you read the following registers immediately after reset or wake-up:
RCC->CSR reset flags
PWR->CSR or wake-up flags
EXTI->PR pending bits
RTC wake-up or alarm flags Whether the vector table is correctly set to 0x08010000

BR
Gyessine