Comparison of development activity of various RTOS projects. It appears that Zephyr has one to two orders of magnitude more development activity than other projects. Part of the difference is likely that Zephyr is more than just a kernel and includes many drivers for i2c/SPI chips, MCUs peripherals, etc. Traditionally, MCU vendors would likely do some of this driver work internally and then package it with a RTOS kernel. The Zephyr model is completely different.
Note, these metrics are not perfect and likely don’t reflect everything that is going on, but give us some idea.
Zephyr
community-owned, managed by the Linux Foundation
License: Apache-2.0
(1,622 contributors)
FreeRTOS:
owned by Amazon
License: MIT
kernel (123 contributors):
TCP Library (56 contributors):
ThreadX
License: Microsoft
owned by Azure
(11 contributors)
Micrium/uC-OS
owned by SiLabs
License: Apache-2.0
uC-OS3 (7 contributors)
uC-OS2 (3 contributors)
uC-USBD (6 contributors)
uC-TCP-IP (4 contributors)
There are several other repos – the development level is likely similar …