NO STM32 TARGET FOUND: Why is there a strong correlation between the 5V source (U5V or E5V) on a Nucleo STM32F401RE board and whether the MCU powers up sucessfully?

CPope.2 · ‎2022-11-03

My Nucleo board is programmed into the Flash from STM32Cube IDE with a Blinking LED program and the USB connection to the Nucleo is always connected.

I am consistently seeing this NO STM32 Target Found" status now on multiple board.

These boards all used to work.

If I set the Nucleo 5V to U5V, the MCU does not start up. Even after a RESET PB is pressed. Then, when connecting to the STM32CubeProgrammer (v2.11.0) still with U5V power, I can't connect. It just reports that there is NO STM32 TARGET FOUND. This status has been discussed many times on the WEB. The typical solution is to APPLY RESET, PRESS CONNECT, RELEASE RESET. but this doesn't work reliably at all.

But then, if I set the JP5 jumper on the Nucleo to E5V and then do a start a power-up with an external 5V supply to the Nucleo board then the MCU powers up without any issue and the program runs successfully. Further, The STM32CubeProgrammer via the ST-Link on the USB port of the Nucleo, can also connect with the Nucleo without any problem.

I am not touching the BOOT pin or applying a RESET to the RESET PB.

Then I switch jumper JP5 over to the E5V selection (with an external 5V bench supply that is NOT powered up. The E5V power will be coming in through CN7 pin 6( for 5V) and pin 8 (for GND).

Then I start the power up the external 5v supply and the MCU starts running perfectly and I can connect with the STM32CubeProgrammer.

This is very repeatable. The No STM32 Target Found Error is strongly linked to how the board power starts up. This doesn't even affect the use of the BOOT pin to the MCU.

I'm wondering if, for the traditional Nucleo setup connections (with USB ST-Link and USB Power and JP5 is set for U5V), has an impact for the "No STM32 Target Found" status because there is something to do with the enabling of the Power FET (T2 on the NucleoF401RE board by U2) that passes the USB 5V power to the USV power in some manner.

My students are using the Nucleo F401RE board in the Lab and there is about a 25% failure rate in the boards with this "No STM32 Target Found" happening. They work on the bench and then .... they don't work.

Sorry for this long-winded file but it helps to provide context I hope.

Any thoughts please?

CP