STM32F446RET6 – VCAP1 only 0.4–0.6V and MCU not detected by ST-LINK

[engahmedmaher]

Dear STMicroelectronics Technical Support,

I am working on a custom PCB using the STM32F446RET6 in LQFP64 package, and I am facing a hardware issue where the MCU is not detected by ST-LINK and the VCAP1 voltage is much lower than expected.

I would appreciate your help in identifying the root cause.

MCU and power configuration

• MCU: STM32F446RET6 – LQFP64
• VDD = 3.3V
• VDDA = 3.3V
• VBAT = 3.3V
• VCAP1 = Pin 30
• VSS = Pin 31 → GND
• VCAP1 capacitor = 4.7µF ceramic MLCC, 6.3V, 0603
• VCAP1 capacitor is connected between VCAP1 and GND
• NRST = 3.3V
• BOOT0 has a pull-down resistor
• SWD interface is connected to an ST-LINK debugger

Current symptoms

The 3.3V supply is stable on the MCU.

However:

• VCAP1 is only around 0.4–0.6V
• It does not remain around the expected internal core voltage
• The MCU is not detected by ST-LINK
• The same behavior occurs after replacing the MCU

Troubleshooting already performed

I have already performed the following tests:

1. Checked the 3.3V supply and confirmed that VDD and VDDA are stable at 3.3V.
2. Checked NRST, which is at approximately 3.3V.
3. Verified the VCAP1 connection:
   • Pin 30 → VCAP1
   • 4.7µF capacitor → GND
   • Pin 31 → GND
4. Replaced the STM32F446RET6 MCU with another MCU, but the behavior remained exactly the same.
5. Tried different ST-LINK/debugger connections, but the MCU is still not detected.
6. Checked the PCB schematic and layout to verify the VCAP1 and power connections.
7. I also compared the VCAP circuit with an ST Nucleo board.
8. To eliminate the possibility of a defective VCAP capacitor, I removed the VCAP capacitor from a working Nucleo board and used that same capacitor for testing. The VCAP voltage on my custom board still remained around 0.4–0.6V.
9. I also measured the resistance between VCAP1 and GND with the board powered off. On the Nucleo board I measured approximately 4 kΩ, while on my custom board the resistance is in the MΩ range.

Additional observation

The most important observation is that the 3.3V supply remains stable, while VCAP1 remains significantly lower.

Because replacing the MCU and using a known-good capacitor did not change the behavior, I am currently unsure whether the issue is related to:

• PCB routing or footprint/pin mapping
• An unintended connection on the VCAP1 net
• Power/ground configuration
• VCAP1 capacitor requirements
• Internal VCAP regulator behavior
• Another STM32F446RE power requirement that I may have missed

Request for your assistance

Could you please advise:

1. What voltage should I expect on VCAP1 during power-up and normal operation for STM32F446RET6?
2. Is there a recommended procedure to determine whether the internal VCAP regulator is operating correctly?
3. Is the measured VCAP1-to-GND resistance useful for diagnosing this issue?
4. Could the MΩ-range resistance on my custom board, compared with approximately 4 kΩ on the Nucleo, indicate a PCB connection problem?
5. Are there any other power pins, decoupling requirements, or power-sequencing conditions that could cause VCAP1 to remain around 0.5V and prevent SWD/ST-LINK detection?
6. Could you please review the attached schematic and PCB layout and let me know if you see anything incorrect?

I have attached the schematic, PCB layout, and relevant measurements for your review.

Thank you very much for your support. I would appreciate any recommended measurements or troubleshooting steps that can help identify the root cause before I make further changes to the PCB.

 

[Peter BENSCH]

Welcome ​@engahmedmaher, to the community!

Your first screenshot shows the long-obsolete ST-Link Utility, which you can safely ignore as it was replaced a very long time ago by the STM32CubeProgrammer, which can be seen in the second screenshot.

Please ensure that you are using a genuine ST-Link/V2 and not one of those colourful, illegal counterfeits in the form of brightly coloured tin cans. See the relevant article in the Knowledge Base for more information:

Regards
/Peter

[engahmedmaher]

Thank you for the detailed information.

I understand your point regarding the obsolete ST-Link Utility and the importance of using a genuine ST-Link. For my tests, I am using the embedded ST-Link/V2-1 from a genuine STM32 Nucleo-64 board, connected to my custom STM32F446RET6 board for external programming/debugging, as described in UM1724.

I have also tested the board with STM32CubeProgrammer, and the result is the same.

However, I believe the main issue is not the ST-Link itself. The problem I am trying to investigate is the VCAP1 voltage:

• VDD = 3.3 V stable

• VDDA = 3.3 V stable

• VCAP1 = approximately 0.4–0.6 V, then it drops

• STM32CubeProgrammer cannot connect to the MCU

• I replaced the STM32F446RET6 MCU and observed the same VCAP1 behavior

• I also tested using the genuine Nucleo embedded ST-Link, with the same result

My understanding is that VCAP1 is related to the MCU internal regulator/core supply. If the internal core supply is not correctly generated, the MCU may not start correctly and therefore may not respond to the SWD interface.

Could you please help me determine what could cause VCAP1 to remain around 0.4–0.6 V and then collapse while VDD and VDDA remain at 3.3 V?

This is currently the main hardware issue I am trying to resolve, and I would appreciate any guidance regarding the VCAP circuitry, required capacitor, PCB layout, or possible causes of the internal regulator not generating the expected voltage.

Regards,
Ahmed

[Richard Li]

Where are these documents: “I have attached the schematic, PCB layout, and relevant measurements for your review.”

[engahmedmaher]

 

[AScha.3]

Hi,

Just guessing: one or two of the VDD pins isn't connected to 3v3. Check all VDD pins for connecting to VDD 3v3.

And you should also check current consumption of the chip, compared to values in DS. As current consumption is the most important factor to indicate chip is working/doing nothing/shorted etc.

 

If all VDD and gnd connected, vcap will be there ok.

 

But if still no correct vcap, maybe you have some bad chips?

If no big problem for you, try replacing the CPU with the CPU from the nucleo board to see what happens then. Also could try one of your new CPU on the nucleo board to see, will it run normal there.

[engahmedmaher]

Hi,

Thanks for your suggestions.

We have already checked the VDD and GND connections on the custom PCB. The measurements taken directly at the MCU are:

• VDD = 3.3 V stable
• VDDA = 3.3 V stable
• VBAT = 3.3 V
• NRST = 3.3 V
• VCAP1 (Pin 30) = approximately 0.4–0.6 V
• VSS (Pin 31) = GND

We also verified the VCAP1 connection. Pin 30 is connected to a 4.7 µF ceramic capacitor to GND.

We replaced the STM32F446RET6 with another new MCU from a different source, but the exact same VCAP1 behavior occurs.

We also tested the VCAP capacitor itself by removing a capacitor from a working Nucleo board and using it on our custom PCB. The VCAP1 voltage still remained around 0.4–0.6 V, so the capacitor itself is unlikely to be the cause.

We also compared the resistance from VCAP1 to GND with the boards powered off:

• Nucleo: approximately 4 kΩ
• Custom PCB: several MΩ

We have also tested different ST-LINK/debugger connections, including the embedded ST-LINK from a genuine Nucleo board, but the MCU is still not detected by STM32CubeProgrammer.

Regarding current consumption, we measured approximately 45 mA from the 3.3 V supply. This does not appear to indicate a hard short, but we are not sure whether this is the expected consumption for the STM32F446RET6 under these conditions.

At this point, since the supply voltages have been verified, the VCAP capacitor has been exchanged, two different STM32F446RET6 devices show the same VCAP1 behavior, and the MCU draws approximately 45 mA, we are suspecting a PCB/connection issue or a problem affecting the MCU's internal core voltage generation.

If you recommend any specific measurements to further isolate whether the issue is related to the VCAP/internal regulator, PCB connectivity, or MCU current consumption, we would really appreciate your advice.

Regards,
Ahmed

[AScha.3]

So 45 mA at start of new/never programmed cpu is too much :

from ds it should me more around 3 mA :

+

>The measurements taken directly at the MCU are:

You have to measure every (!) pin : 4x vdd , 4x gnd + vssa+vdda ; all have to be 3v and all gnd , vssa on 0v.

[STOne-32]

Dear ​@engahmedmaher ,

 

Few checks :

 

1. have a photo of the chip soldered . We have seen some designs where the pin 1 position is flipped by 90 degree.
2. Reset pin circuit to add a capacitor to ground and check to disconnect the external circuit 
3. looking to the footprint of C49 I have some  doubt if the value is 4.7uF if possible to measure . A a double check is always good .

Clearly there is a hardware issue here it could be at PCB .

 

Regards,

STOne-32

[engahmedmaher]

Thank you for the suggestions.

Regarding the C49/VCAP capacitor, I actually removed the VCAP capacitor from a working Nucleo board using the same STM32F446 MCU and installed it on our custom PCB. The VCAP1 voltage on the custom board still remained around 0.4–0.6 V, so I believe the capacitor itself is unlikely to be the cause.

I also noticed an additional behavior: sometimes VCAP1 initially rises above 1 V, but then it gradually decreases until it reaches nearly 0 V across the capacitor. When VCAP1 is above approximately 1 V, I still cannot establish a connection to the MCU using STM32CubeProgrammer.

Regarding the MCU orientation and footprint, I have checked this carefully. The MCU is soldered with Pin 1 correctly positioned on the left side, and the pin numbering proceeds counterclockwise, matching the STM32F446RET6 datasheet/package orientation.

So far, the VCAP capacitor and MCU orientation have both been double-checked.

The remaining concern is therefore the PCB/connection or another issue affecting the VCAP/internal regulator.

If you have an email address where I can send the PCB design files, schematic, and PCB layout, I can provide them. Having the actual design files may make it much easier to identify any PCB or connectivity issue causing this behavior.

Regards,
Ahmed

[STOne-32]

Could you provide a photo of the package  and top marking soldered on PCB ? 
 

Thank  you. 

[Richard Li]

If you can replace chip, can you try replace Nucleo board with your chip?

If you measured VCAP_1 on your board  “resistance is in the MΩ range.” most like inside regulator is off, but your chip without bypass_reg pin, so regulator shouldn’t turn off.

Are you waiting longer enough discharge all power before measure this resistance? (If any energy in chip, the inside V12 may keep, turn off regulator.

You can measure the chip pin itself without pcb.

[engahmedmaher]

Hi,

I followed your suggestion and removed the STM32F446RET6 from the custom PCB and measured the resistance directly on the MCU itself, between VCAP1 (Pin 30) and GND/VSS.

The resistance is still in the MΩ range, even when measured directly on the MCU outside the PCB.

This makes the PCB or the VCAP capacitor less likely to be responsible for this particular high-resistance reading.

I will also perform the suggested swap test by placing this MCU on a known-working Nucleo board and testing it there.

What do you think about the MΩ resistance measured directly on the MCU between VCAP1 and GND? Does this behavior suggest that the internal voltage regulator may be damaged or not operating correctly?

Regards,
Ahmed

[Richard Li]

This time I am not work for ST (I was work for ST 26 years ago), so no inside information. waiting for your swap chip result.

[Richard Li]

“measured directly on the MCU outside the PCB.”, is it removed chip or brand new chip? If removed chip, it may already damaged, you have to measure the chip never use on your customer board. 

If your got different result between new and used chip, meaning your board damage inside regulator. 

[engahmedmaher]

It is a completely new STM32F446RET6. It has never been soldered, installed, or powered on the custom PCB.

I measured the resistance directly on this unused MCU, outside the PCB, between VCAP1 (Pin 30) and VSS/GND, and it is also in the MΩ range.

So this is a brand-new, unused chip, not a chip removed from the custom board.

What do you think about this result?

Regards,
 

[Richard Li]

This debug time, only looking for different way try fig-out what’s wrong, I didn’t measure good STM32F446 chip, so maybe my point was wrong, wait for replace Nucleo board with your chip result.  

[Richard Li]

Check gerber files, the pin 12 to GND only one VIA, please measure the PIN 12 to GND connection. Other looks OK.

[TDK]

Serial number of “3” is not a valid serial number. Probably the programmer is fake.

Voltage of “0 V” is not within bounds. Most (but not all) genuine programmers require V_target connected.

Orientation may be off. Show a picture that lets someone else verify correctness. Maybe it’s right, maybe not.

Schematic is missing programming connection. Maybe it’s right, maybe not.