This is a dedicated performance counter designed for micro-controllers. For Cortex-M processors, the `perf_counter` uses the SysTick by default. It shares the SysTick with users' original SysTick functions without interfering with them. This library brings new functionalities, such as performance counter, `perfc_delay_us()`, `perfc_delay_ms()` and implements `clock()` defined in `time.h`. It provides a template for porting the `perf_counter` to different architectures or using a different Timer instead of SysTick in Cortex-M processors.