yaota8266 is yet another bootloader/over-the-air (OTA) update solution for ESP8266 WiFi SoC. Unlike many other solutions, yaota8266 does not require reserving FlashROM space of 2x the size of the firmware. Instead, it updates the firmware in-place. Of course, this means that if an OTA update fails, there's no previous firmware to fallback to. On the other hand, if OTA update fails, you likely will repeat it again, until it succeeds. So, for many usecases the process of OTA update will be the same - a user will just repeat it until it succeeds, regardless whether there's a fallback firmware or not.

yaota8266 is written with big firmwares and small flash sizes in mind. For example, it allows to have an OTA for full-fledged MicroPython (firmware sizes of 512+KB) on 1MB flash devices, and still have a small, but a filesystem.

yaota8266 consists of two parts:

• 2nd-stage bootloader boot8266
• ota-server application

boot8266 works in the following way:

1. 1st-stage bootloader in ESP8266 BootROM loads boot8266 (from sector 0). It is small and fits within a single FlashROM sector (4K).
2. boot8266 checks whether an OTA button on device is pressed. If it is, it goes in OTA mode.
3. If the button is not pressed, it verifies a checksum of a user application. If it fails (for example, because of unsuccessful, partial previous firmware update), it goes into OTA mode.
4. If OTA mode is requested, boot8266 loads an application starting at the sector 1. This is intended to be the ota-server, but from boot8266's point of view, it's just a standard ESP8266 application, which it loads recursively in the same (or very similar) way as BootROM does it.
5. If OTA mode was not requested, boot8266 loads a user application which lies beyond the ota-server application end (offset is configurable). The same note as above applies - boot8266 just loads one or another application in the same way, and doesn't care what they do (but boot8266 has partially hardcoded knowledge about sizes of these applications, and verifies checksum only of the second one).

ota-server works in the following way:

1. Starts a UDP server on port 8266.
2. Expects consecutive UDP datagram containing chunks of new firmware.
3. Each datagram is signed with RSA private key. Only someone with a valid private key may produce valid datagrams, information from which ota-server will flash as a user application. (The public key is configured when building ota-server.)
4. ota-client host-side application is provided to drive OTA upgrade process for a device in OTA mode.

yaota8266 is a work in progress and is not yet fully working per the spec above.