Support Required: STM32F746NGH6 USB OTG FS VBUS Issue

[ipawan]

I am using the STM32F746NGH6 controller in my circuit to read external ADC (16bits) and transmits the same through USB. I have configured USB in OTG FS (device) MODE including AUTO detection.

 

Test Set-Up.

The USB is always connected and controller is powered on for testing and powered off after testing is completed

 

Earlier observation

This was practiced for about two months and was working ok without any issues

 

Current observation

The same is not able to recognize USB (i.e. auto detection is not happening)

 

Further Analysis

 1) On probing the circuit, it was observed that 3V is present at USB V-Bus line always after the board is power on, even without the USB cable connected

  Please clarify does it cause the issues. If yes, why and how

 

Queries

 We would like you to clarify the following points about the user operation sequence

1)    Be the controller power on, when engaging and disengaging USB cable

2)    There is no need for any sequencing in user operation

 

NOTE:

We are in need of fix to avoid these issues in further production lot and please reply as soon as possible

[Gyessine]

Hello ​@ipawan
just to make sure
>The USB is always connected and controller is powered on for testing and powered off after testing is completed

does the USB stay connected even when the chip is not connected to power?
because in that case, connecting USB without board power can back-power the MCU through the USB data pins, causing latch-up or long-term I/O degradation
Can you please provide more details about this part
BR
Gyessine

[ipawan]

Hi Gyessine

For our application, the controller does not need to remain in ON condition continuously. Additionally, the USB cable is connected permanently to the system; there is no provision for removing or reconnect the USB cable.

Considering these constraints, could you please advise on the recommended approach and provide a suitable solution.

Regards,

Pawan 

[Gyessine]

Hello ​@ipawan 

I am still having some difficulty understanding your overall system. Could you please provide a schematic or block diagram showing how the system is structured, especially how the USB port is connected?

If I understand correctly, the USB cable remains connected regardless of whether the system is powered on or off. In that case, the hardware design should ensure that the MCU is not back-powered through the USB lines when the board is unpowered.

In USB device mode, the STM32 should not drive VBUS; it should only detect it. If you are measuring around 3 V on the VBUS line even when no cable is connected, this may indicate a hardware design issue, leakage path, or an incorrect VBUS sensing circuit.

Please also verify that no unintended back-powering when the board is off, correct VBUS sensing and proper protection or isolation on D+ / D- if required

Best regards,
Gyessine

[ipawan]

Hi Gyessine,

Thanks for your response.

Please find the attachment for your reference, include the USB port and microcontroller sections only.

Our application requirement:

The USB remains continuously connected, while the controller is powered on during testing and powered off once testing is completed. This operating scenario has been followed for approximately two months and was working without any issues.

However, with the same setup and operating conditions, the system is now unable to recognize the USB device.

Our expectation:

We intend to design a robust and reliable circuit which is trouble free and works for its specified lifetime under specified operating condition.

Provide your technical recommendations and proposed corrective actions.

 

Regards,

Pawan

[Gyessine]

Hello ​@ipawan 
I reviewed your schematics.
To be honest, I am not familiar with the ESD you used, you need to check if that ESD do isolates Vbus lines from data lines
if no, you need to change your ESD you can check this AN4879 table 11.
if yes, then we need to dig deeper in you issue.
BR
Gyessine
 

[ipawan]

Hi Gyessine,

I checked the document you suggested, specifically Table 11 in AN4879, and reviewed the recommended ESD protection IC, USBLC6-4SC6, for the USB FS OTG application.

I also compared the datasheets of ESDA6V1BC6 with the ESD protection IC currently used in our schematic. Based on the comparison, I could not identify any significant difference in the electrical specifications, except for the specified minimum and maximum parameter ranges.

However, I noticed that the USBLC6-4SC6 datasheet specifically mentions its suitability for USB 2.0 applications in the application section, whereas the ESDA6V1BC6 datasheet does not explicitly mention USB 2.0 compatibility, although it does specify application/devices use USB interface.

Kindly provide suitable solution.

Regards,

Pawan