USB FS unstable on STM32 when using HSI16 + HSI48 (USB3.0 host crash)

[amorftor]

Hello,

I am working on a low-power STM32 system. In my original configuration I was running the core at 32 MHz (PLL from HSI16) with Voltage Scaling 1, and in this setup USB CDC worked without any issues.

To reduce power consumption, I reconfigured the system to run the core at 16 MHz HSI (no PLL) while keeping Voltage Scaling 1. The USB clock source is HSI48.

With this configuration, when connecting to a USB 3.0 host port, the device often crashes during enumeration. Sometimes it enumerates fine, but most of the time it fails. Interestingly, if I connect the device through a USB 2.0 hub, the problem does not appear.

From my research, I found that:

  - USB 2.0 controllers are more tolerant and usually accept HSI48’s +/-1–2% drift.

  - USB 3.0 controllers are stricter, and if HSI48 drift exceeds +/-0.25%, CRC errors occur. This causes descriptors to get corrupted and Windows shows a crash or device not recognized.

My question:
What are the recommended ways to avoid this issue while still keeping the system in low-power mode (16 MHz core)? Should I:
  - Always use HSE + PLL for USB to guarantee clock accuracy?

  - Use CRS synchronization with LSE or USB SOF to stabilize HSI48?

  - Keep the system in Voltage Scale 1 only while USB is active?

Any advice from experience with STM32 USB on USB3.0 hosts would be appreciated.

Thanks!

[TDK]

Two options:

• Using HSE + PLL will work.
• Using HSI48 + CRS will also work. This is easier.

Introduction to USB hardware and PCB guidelines using STM32 MCUs - Application note

 

HSI48 alone will not satisfy the clock requirements for USB (2.0 or 3.0).

[FBL]

Hi @amorftor 

For your USB Full-Speed low-power application, I recommend using CRS to synchronize HSI48 with LSE or USB SOF to improve clock stability although this requires more complex software handling.. For the highest accuracy, especially in high-speed USB, using HSE with PLL for the USB clock is preferred.