ESP8266 SDK reboot loops with cause:2, boot mode:(3,6)

Recently I’ve been building and flashing some projects to the ESP8266 directly using the Espressif SDK for development (not Arduino).

After changing the flash mode, I was still in a reboot loop, but was seeing “rf_cal[0] !=0x05,is 0xFF” just before reboot. This is a known issue that is resolved by reflashing the “init data” (esp_init_data_default.bin) as described here: https://nodemcu.readthedocs.io/en/dev/flash/ (scroll to “SDK Init Data”).

The first thing to try is erase the flash before flashing your code:

esptool.py --port <serial-port-of-ESP8266> erase_flash

If that doesn’t work, flashing the “init” data will likely fix the problem:

esptool.py --port <serial-port-of-ESP8266> write_flash -fm <flash-mode> <flash_addr> esp_init_data_default.bin

Where <flash_mode> would be something like “dout”, “dio” or “qio” (use “dout” for ESP8285, “dio” or “qio” for ESP-12)

and <flash_addr> is determined based on flash size and this table:

• 0x7c000 for 512 kB, modules like most ESP-01, -03, -07 etc.
• 0xfc000 for 1 MB, modules like ESP8285, PSF-A85, some ESP-01, -03 etc.
• 0x1fc000 for 2 MB
• 0x3fc000 for 4 MB, modules like ESP-12E, NodeMCU devkit 1.0, WeMos D1 mini
• 0x7fc000 for 8 MB
• 0xffc000 for 16 MB, modules like WeMos D1 mini pro