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!
