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July 23, 2018

Management of VBUS sensing for USB device design

  • July 23, 2018
  • 7 replies
  • 81930 views

From a USB specification, a USB device should use VBUS sensing detection: 

When a host connection is detected by the device, the device connects the pull-up resistor either to D+ or to the D- data signal. This allows the host to detect device presence on the bus.

There are two use cases:

  • If the USB device is bus-powered, VBUS sensing is NOT mandatory: USB is connected all the time when the device is powered
  • If the USB device is self-powered, VBUS sensing is mandatory

On STM32F2/F4/F7/L4, peripheral USB OTG_FS (OTG_HS without external PHY) is natively dedicated for VBUS sensing with pin PA9 (PB13 for HS), which is a +5 V tolerant pin.


In the datasheet DS10314 section 6.2, table 11, the "Absolute maximum rating" (AMR) table states that a +5 V tolerant pin voltage can be maximally VDD+4 V. So, the situation where the MCU is not powered and 5 V VBUS is connected to PA9 must be avoided because it is violating AMR.

Laurids_PETERSEN_0-1717499972212.png

1. Solution one: with a dedicated VBUS pin (PA9 or PB13)

To reduce the voltage on PA9 below 4 V, the usage of a voltage divider is recommended, so that AMR cannot be violated. But the internal VBUS detection block inside the OTG peripheral has a certain current consumption. 

You can find this mentioned, for example, in STM32F411 datasheet (section 6.3.19 p. 113) or STM32F446 datasheet (section 6.3.20 p. 136):

“When VBUS sensing feature is enabled, PA9 should be left at their default state (floating input), not as alternate function. A typical 200 µA current consumption of the embedded sensing block (current to voltage conversion to determine the different sessions) can be observed on PA9 when the feature is enabled.”


The recommended resistor bridge is "vbus-- R1=4.7k -- R2=10k -- gnd" and then VBUS detection (PA9) can be taken between R1 and R2.

2. Solution two: without usage of a dedicated VBUS pin

Any 5V tolerant pin with external interrupt functionality can be used with an external resistor divider, to fulfill AMR conditions. In this case, the recommended values are 2x100 kOhms.

However, the disadvantage of this solution is that the software handling of VBUS presence event needs to be implemented by the user. This is not managed within our STM32 USB library for the time being.

Related links

For further reading on VBUS sensing and USB HW implementation, please refer to the AN4879.

    7 replies

    W
    wed
    Visitor II
    May 29, 2024

    This information about VBUS sensing requiring a voltage divider seems to be very recent, both as an addition to AN4879, and this post, only updated a week ago.

     

    In my application, Vdd=3.3v, and the resistor divider in AN4879 (33k / 82k) does not seem to work, the voltages on this divider swing from 3.38v to 3.73v as the USB voltage is swept from 4.75 to 5.25v. However, when the microcontroller is powered up, the voltage at the divider to Vbus pin collapses to 1.59 to 1.75V over the same USB voltage range, which is below the logic high threshold of the device (STM32F676 in this case). 

    Using the values in this post (4k7 / 10k) has a better range of 3.24 to 3.58V as USB voltage is varied from 4.75 to 5.25v, and this time, when the microcontroller circuitry is on, the voltages on Vbus pin range between 2.93v and 3.26v over the same USB voltage range, which seems to satisfy both <4V at all times, and > logic high threshold (2.31v if 0.7xVdd).

    Perhaps it would be worth updating AN4879 since the resistor values don't seem to be suitable?

     

    Another enquiry about this issue would be is this a new emerging problem with Vbus voltages? Since all the literature about voltage dividers is very recently published, were older STM32 chips more immune to issues with the Vbus pin being >4V? Were they manufactured using a more voltage tolerant process? If we used some STM32s we bought a few years ago, are they more resilient? We never experienced any failures connecting Vbus to 5v in the past (plugging 5V USB into a microcontroller when powered off) until very recently, despite older designs having no resistor dividers for several years. 

    N
    Nick Alexeev
    Associate III
    June 12, 2024

    However, the disadvantage of this solution [without dedicated hardware VBUS detection pin] is that the software handling of VBUS presence event needs to be implemented by the user. This is not managed within our STM32 USB library for the time being.

    What should the firmware do when it detects that VBUS is present?  What should the firmware do when VBUS has disappears?  Are there code examples for VBUS detection through GPIO?

    [without dedicated hardware VBUS detection pin] Any 5V tolerant pin with external interrupt functionality [...] 

    Does the VBUS detection have to be done in an interrupt? Or can it be done in the main loop? If it can be done in the main loop, then are there timing constraints?

     

    update:  I received several answers from from the ST’s support, and arrived at the solution. Details here, including my code snippets.

    F
    fanghaiyu
    Visitor II
    April 11, 2026

    fanghaiyu_0-1775905942866.png

    fanghaiyu_1-1775905964469.png
    It is clearly stated here that the VBUS threshold is 4.25V. If you use a resistive voltage divider, it will not work properly. One requires a voltage greater than 4.25V, while the other requires a voltage less than 4V. This is a design bug.

    F
    fanghaiyu
    Visitor II
    April 11, 2026

    It is clearly stated here that the VBUS threshold is 4.25V. If you use a resistive voltage divider, it will not work properly. One requires a voltage greater than 4.25V, while the other requires a voltage less than 4V. This is a design bug.

    E
    eva_402
    Associate II
    April 11, 2026

    You’re asking about proper VBUS sensing and protecting the STM32 pin from overvoltage, yeah this is a common confusion. Basically you’re right, directly putting 5V on PA9 when MCU is unpowered can damage it or violate AMR. Best approach is using a resistor divider (like 4.7k/10k) so voltage stays safe, especially if you’re using the dedicated VBUS pin. If you skip that and use another GPIO, then you’ll have to handle detection in software which is bit extra work. So overall hardware protection first, then decide if you want native OTG sensing or custom logic, both works but depends on your setup.

    F
    fanghaiyu
    Visitor II
    April 11, 2026

     

    4.25.png

    fanghaiyu_1-1775909928830.png

     

    4.25~4.75V？The official response should be that it cannot be achieved, as the voltage division is not met.

    D
    dcm684CM
    Visitor II
    May 20, 2026

    tldr Use the vbus-- R1=4.7k -- R2=10k -- gnd arrangement as recommended above. Higher resistances can result in Vbus sensing not working.

    Initially I tried using 68k and 100k resistors for the voltage divider on my USB OTG CDC peripheral. My circuit did not work. When my STM32L476VGT was powered and initialized the voltage at the output of the divider was at 1.3V not the anticipated 3.07V. When I held the STM32L476 in reset, the divider's output was at the expected 3.07V. The STM32L476 device would not initialize on my Windows PC if I enabled Vbus sensing, but it would if Vbus sensing was disabled.

    The voltage divider arrangement was. v_bus_in -- R1=68k -- v_bus_out -- R2=100k -- gnd

    Noting that the pin, PA9, was 5V tolerant, I removed the 100k pull down resistor. This basically put a current limiting resistor of 68k between the USB connector and PA9. If the USB cable was connected at device start up, the USB connection would be initialized and Windows would recognize the device. I could see output in my terminal. However, if I unplugged the USB cable then plugged it back in, Windows would not see it when plugged back in. I would have to reset my STM32L476 for it to be reenumerated. After the USB cable was removed the divider's output would drop to 1.3V while the input was still 5V.

    I replaced the 68k resistor with the 4.7k as recommended above and the device would reenumerate and be recognized each time it was plugged it. I put the 10k in R2 and it still worked. My final arrangement was what was recommended above. 

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