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The ESP-S2 comes with a ROM bootloader that already allows you to flash over USB using the serial CDC ACM profile. But this method does not let you restart into the main firmware. The ROM bootloader USB stack even offers DFU capability, and you can flash using dfu-util. But this one also does not let you restart into the main firmware.
To flash using USB serial (can't restart the device):
idf.py -p /dev/ttyACM0 flash ... WARNING: ESP32-S2 chip was placed into download mode using GPIO0. esptool.py can not exit the download mode over USB. To run the app, reset the chip manually. To suppress this note, set --after option to 'no_reset'.
And to flash using DFU:
# generate the DFU binary (not the same as the usual flash binary) idf.py dfu dfu-util --device 303a:0002 --download build/dfu.bin # or idf.py dfu-flash
Note: after detaching, the bootloader claims for be in runtime mode, but this is still the bootloader, and not the flashed firmware.
To switch from the ESP32-S2 ROM bootloader back to the firmware flashed over USB, you need to press the reset button. This is cumbersome when developing firmware, and sometimes impossible if the board is encased or installed remotely.
This USB DFU implementation allows to switch back to runtime mode without pressing a button. The runtime firmware can also reboot into the DFU mode without pressing a button.
cd examples/device/dfu_freertos/ make BOARD=wemos_s2_mini all
This is equivalent to:
idf.py -B_build/wemos_s2_mini -DFAMILY=esp32s2 -DBOARD=wemos_s2_mini -DIDF_TARGET=esp32s2 build
To flash the compiled firmware:
idf.py -B_build/wemos_s2_mini -DFAMILY=esp32s2 -DBOARD=wemos_s2_mini -DIDF_TARGET=esp32s2 -p /dev/ttyACM0 flash
or to use esptool directly over the ROM USB DFU bootloader, without ESP-IDF:
esptool.py -p /dev/ttyACM0 --before no_reset --after no_reset --chip esp32s2 write_flash --flash_mode dio --flash_size detect --flash_freq 80m 0x1000 _build/wemos_s2_mini/bootloader/bootloader.bin 0x8000 _build/wemos_s2_mini/partition_table/partition-table.bin 0xd000 _build/wemos_s2_mini/ota_data_initial.bin 0x10000 _build/wemos_s2_mini/dfu_freertos.bin
To further configure the build:
idf.py -B_build/wemos_s2_mini -DFAMILY=esp32s2 -DBOARD=wemos_s2_mini -DIDF_TARGET=esp32s2 menuconfig
This firmware is just for the DFU mode. The main application needs to implement the runtime mode separately.
To switch from runtime to DFU mode during detach, set the boot partition in
factory, and restart the ESP.
The USB DFU firmware does not act as bootloader.
Meaning, the ESP bootloader does not start the USB DFU firmware, which in turn starts the main firmware.
The main firmware in the OTA0 partition is directly loaded by the ESP bootloader.
Thus is it up to the main firmware to start the DFU mode, as described in
In case the main firmware is defective, and does not allow to switch back to DFU mode, you can still force booting the USB DFU firmware. For that, boot the ESP ROM bootloader (i.e. download mode), and invalidate the OTA data partition:
otatool.py --esptool-args after=no_reset_stub --port /dev/ttyACM0 erase_otadata
When restating the ESP by pressing the reset button, the ESP bootloader will start the firmware which is in the factory partition, which should be the DFU firmware previously flashed.
The ESP-IDF uses tinyUSB, but only offers few profiles (e.g. CDC, MSC). They are defined in 'esp-idf/components/tinyusb/additions', and re-use the tinyUSB device implementations. But there is no DFU implementation (runtime or DFU mode).
TinyUSB is a generic USB stack for micro-controllers. It supports ESP32-S2. DFU is implemented and there is an example. This example can't be used for ESP32-S2 though. I've added freeRTOS to it so it can be used on the ESP32-S2.
This USB DFU implementation is flashed as a factory application image. It will then flash the main firmware image into the OTA0 partition. Thus it requires a custom partition table, with a "small" factory partition, just one OTA application partition, and one OTA data partition. The factory partition should be 200 KB. This is super large for just a "bootloader" to flash firmware images, but since ESP32 often use large external flash memory, this is not too much of an issue. Feel free to reuse the 'partitions.csv' file as example.
TinyUF2 is a UF2 bootloader by Adafruit. It is based on TinyUSB, and supports ESP32-S2. It uses the DFU name (Device Firmware Upgrade), but it's not the DFU specified by USB. Instead it provides a MSC interface where you can copy the firmware binary file to.