I would suggest that you search around on this forum for some of the discussions regarding the use of an operating system on a microcontroller. You will find out very quickly some very convincing reasons why this is not a good idea, particularly on an 8051 architecture type part. Your problem with RTXTiny seems to directly indicate one reason. You end up with a mass of software that you know only a small about what goes on inside and it does not directly fit to the tsak at hand.

You would be far far better off to make your own simple scheduler in your own code that fits your system requirements like a glove. I really have to repeat something here - in nearly 30 years of making embedded systems I have never had a convincing reason to use a ready-made operating system in one of my projects.

Michael Karas