Summary

With MicroPython 1.26.0 and 1.26.1 on ESP32-S3 with PSRAM,machine.USBDevice is broken. Eg making a custom keyboard, keypresses sent to the host arrive as a packet of all zeros.

The root cause is commitespressif/esp-usb@1b18558 in theespressif__esp_tinyusb managed component, which switched USB to use DMA by default. But DMA cannot read from PSRAM. So HID (and other USB) packets that live in PSRAM end up as all zeros when sent to the host.

To fix that, force USB to use IRQ mode when building with PSRAM enabled.

Fixes issuemicropython/micropython-lib#1044.

Testing

Tested with ESP32_GENERIC_S3 firmware, on a UM TinyS3, and the following code:

importtimeimportusb.devicefromusb.device.keyboardimportKeyboardInterface,KeyCodekb=KeyboardInterface()usb.device.get().init(kb,builtin_driver=True)whilenotkb.is_open():time.sleep(0.1)for_inrange(10):print("send key")kb.send_keys([KeyCode.M])time.sleep(0.15)kb.send_keys([])time.sleep(2)

Without the fix in this PR, that code doesn't send any keys. With the fix, it does.

Trade-offs and Alternatives

I'm not sure if this is technically a bug/regression in the IDF, because they did provide an easy option to disable DMA. So this seems the best way to workaround the issue for us.

Alternatively we could allocate a temporary buffer in internal SRAM, and copy the PSRAM buffer into that before passing it to the DMA engine. But it's hard to know when the DMA has finished with the buffer so it can be reused for the next transfer.