RTOS does not honour SystemCoreClock and related CMSIS clocking setup

[hugovincent]

The mbed-rtos library does not use or honour SystemCoreClock and is not responsive to changes in SystemCoreClock at run time. To change the core frequency, you must first change the registers and call SystemCoreClockUpdate() mechanism, but then you must update #define OS_CLOCK in mbed-rtos/rtx/RTX_Conf_CM.c. Obviously the later can't be done by code at run time, and this makes run-time clock changes impossible. This furthermore makes the RTOS almost impossible to use in a energy-constrained application. I realise fixing this properly is non-trivial, but I would urge you to find a good solution rather than a quick one.

[emilmont]

Reporting here Hugo's investigation:

After a bit of digging I concluded it would be a fairly major architectural change.
I’m pretty sure that supporting clock-speed changing at run time would break the
“RT” part of “RTOS” (at least hard realtime), since you can be blocked for a
nondeterministic, unbounded amount of time waiting for the PLL to re-lock.
Most RTOS’s are like this, and basically fundamentally incompatible with
low-power/low-energy regimes.

We will not implement this change to the existing mbed-rtos implementation.

Perhaps, in the future, we could consider providing a different implementation optimized for low-power.

Emilio