Where can I find 'sdk_system_get_cpu_frequency' ?

[Lieven Merckx]

As I am trying to have a microseconds counter I use the ccount data with inline assembler : __asm__ __volatile__("esync; rsr %0,ccount" : "=a"(ccount));

However the cycle count per microsecond is dependent on the Cpu frequency, so this needs to be adjusted.

That's why I would need : sdk_system_get_cpu_frequency API call.

It's not clear how to invoke this. Which include file and library should be used ? 

Any suggestions ? Thanks !