I just want to let the community to know that I'm trying an ESP8266 port. At the moment
- the core's thread interface is implemented and tested, - the core's irq interface is implemented and tested, - GPIO handling module periph_gpio is implemented and tested, - small applications like ipc_pingpong with interrupts and some I/O are already working.
My next steps are to implement perihph_timer module (I'm currently working on), periph_uart module, periph_spi module and some networking.
I started with two different approaches that are both working until now
- a bare metal implementation, and - an implementation on top of the community SDK esp-open_sdk.
I really like the bare-metal implementation and would prefer this approach since it gives more control over the hardware, requires less resources and fits better RIOT-OS's core implementation approach. Furthermore, it is generic enough that it might run on other Xtensa processors.
However, once networking using the proprietary WIFI hardware is needed, it becomes unfortunately necessary to use proprietary SDK components. So it seems, that I will continue the SDK approach first.
Implementing RIOT-OS on top of the SDK is a bit tricky because the SDK already uses tasks to control the hardware timers, the WIFI hardware, the network connectivity, and so on. Therefore, it is necessary to give away the task control to the SDK from time to time to keep the SDK tasks alive. This is done by reimplementing the ets_run function, which is executed each time before the idle task of the RIOT-OS and after the timer interrupt handling.
One of the two hardware timers is used in the SDK, the other hardware timer is needed for PWM outputs. Therefore, RIOT OS timers are implemented as SDK software timers, which results in an pure accuracy of around 500 us.
That's the status for now and I hope that it might become interesting for someone. I will publish my changes at https://github.com/gschorcht/RIOT-Xtensa-ESP8266 and submit a pull request once I made some more progress.