Voltage Drop from 3.3V to 1V on NRST Pin of STM32F070CBT6

[parvej alam]

I am using STM32F070CBT6. I am experiencing voltage drop on NRST Pin from 3.3V to 2V or 1V randomly for few milliseconds then it goes high to 3.3V. This drop comes randomly in every 2 to 3 minutes. My VDD is stable I checked the same on Oscilloscope.

Below is schematic and oscilloscope screenshot.

[Vangelis Fortounas]

Hi.

From schematic, not all four VDDs , three VSSs pins connected to power rails.(GND and +3V3A)

Also NRST pin is pulled up "strongly" while is already pulled up with 40Kohm Additionally is not connected in the same +3V3A rail but to the other side of MCU jumper.

In case a microAmmeter is connected to jumper, is possible to have this behavior.

"comes randomly in every 2 to 3 minutes" Is it possible to have an interference from GSM module?

[parvej alam]

Hi,

1. From schematic, not all four VDDs , three VSSs pins connected to power rails.(GND and +3V3A) - I don't understand, as per datasheet their are 5 VDDs and 4 GND in STM32F070CBT6.
2. Also NRST pin is pulled up "strongly" while is already pulled up with 40Kohm - I tried removing Pullup resistor but it does not make any changes.
3. Additionally is not connected in the same +3V3A rail but to the other side of MCU jumper - By shorting jumper with solder on PCB it connect +3V3A with VDD. Basically this jumper is placed to repairing purpose by splitting VDD of MCU from other components.
4. In case a microAmmeter is connected to jumper, is possible to have this behavior - Can you please explain this.
5. Is it possible to have an interference from GSM module? - I guess no. It makes no effect whether i switch off GSM or turn it OFF.

[Vangelis Fortounas]

Hi

1. I observed from schematic the Vbat pin and PF6 and PF7 are not exist for this MCU. But the pin numbers are correct connected. So all VDD,VSS are ok.

NRST pin is also an output pin. it can be driven Low from an internal reset source. So check reset flags of clock controller's CSR register just after this described reset to find the reason of unexpected reset.

[parvej alam]

Hi,

Thanks for the suggestion. I have checked CSR register but their is no flag tick, infact MCU keeps running in debugging mode bechase NRST pin not drop below 1V. It just drop between 3.3V and 1.5V. Now my concern is Is it ok to use as it is in production?

Since its not making any reset but fluctuates some times.

[parvej alam]

Hi,

Is their any more suggestion??. My problem still not solved.

[parvej alam]

One More thing i observed today that my BOOT0 pin also goes high from 0 to 1V (zig-zag) at the same time when NRST pin start droping voltage.

I have added external Pull-Up of 10K to BOOT0 pin.

[MikeDB]

Boot0 needs a pull-down, not a pull-up

[Tesla DeLorean]

Exactly, the schematic looks ok. Something else going on here. Perhaps the pins are jumbled up, or an issue with the pcb or fab.

I'm not doing the due diligence on this.

[parvej alam]

Sorry for the mistake. It was Pull down with 10K.