Multiple thread example based on mbed-os

Fork of rtos_basic by mbed official

This is a very basic of RTOS thread example using mbed-os. In this example, LED1 and LED2 are toggling with the interval of 500ms and 1000ms respectively. LED1 blinking is running in the while in main, meantime LED2 blinking is running in a thread. This shows how task(s) can operate independently.