STM32F411 firmware download issue in DFU mode

[Chayan Roy]

Hi Everybody,

     I am facing a wired issue in programming the  mcu . I have designed  a custom borad with STM32F411CEU6 (QFN-48 package). DFU mode is is properly detected.  But whenever I am trying to connect the mcu  with STM cube programmer in dfu mode mcu gets resets. 

  Some of the things I have observed:

      1. VCAP(1.3067V on vcap pin) Voltage drops  by 0.3V while trying to connect with cube programmer.

      2. MCU connects easily if I apply a little bit of heat . Disconnects again as soon as the mcu cools down. This kind of behaviour  I have never seen with ST mcu. 

      3. DFU mode successfully deteted by the pc.

 

     Actions taken :

         1. MCU supply voltage check:  The supply voltage is steady . No fluctuation observed.

         2. HSE crystal  and their values checked with different capacitors values.  

         3. Able to donload the firmware using STLINKV2. 

        4. But MCU remains under reset.

         3. I have 3 samples of the mcu . Result is same with  all of them.

  I have attached the schematic and pcb layout along with this post. The hardware is tested already with previous pcb design. Due to  some size issue I had to change the layout and found this issue.This is the new layout with fresh components.This is my second iteration. I will be very greatfull if you could guide me a little bit.

    

 

[MM..1]

VBAT require be connected to 3V3 too.

[Chayan Roy]

Hello MM..1,  Thanks for your repsonse.

  We have already tested the hardware in our previous pcb design. Now only the layout hase been changed. Not the circuit. The issue is comming in new layout. But i will try out your suggestion.

[TDK]

Sounds like a hardware assembly issue. VCAP dropping suggests insufficient or invalid power. Perhaps a VSS connection is flaky. NRST pulsing suggests the chip is resetting internally likely due to a power issue.

Does SWD mode work?

Does 3V3 also drop when trying to connect? (Use a scope to see if drops correspond to NRST.)

[Pavel A.]

>  MCU connects easily if I apply a little bit of heat . Disconnects again as soon as the mcu cools down. This kind of behaviour  I have never seen with ST mcu. 

Very unusual indeed. Hints to a shorting somewhere. When you apply heat, things expand and the shorting disappears. Something like that

 

[Chayan Roy]

May be.But where the short is, can't figure out. Can't find any trace of short with measurement. Everything looks ok. The board connects with PC . Windows shows "ST BOOTLOADER " in deice manager list . Where the issue is comming from dont' know. Anyway I am still trying to figure out . Thanks for your reply.

[MM..1]

This isnt short,but cold junction = not connected in cold state. Try resolder MCU with some soldering liquid , after solder use clean liquid (alcohol based) 

[Chayan Roy]

The issue has been solved . VCAP capacitor gorund was missing on pcb. I have soldered a piece of wire to connect vacp negative to ground. Thank you every one for your valuable suggestions .