Simply put, without crystal oscillator, there will be no clock cycle. Without clock cycle, the program code can not be executed, and the microcontroller can not work.

When a single chip computer works, it takes instructions from RoM one by one, and then executes them step by step. The time when a single chip computer accesses a memory is called a machine cycle, which is a time benchmark. Mdash; a machine cycle consists of 12 clock cycles. If a single chip computer chooses 12MHz crystal oscillator, its clock cycle is 1/12us, and its machine cycle is 12× (1/12) us, that is 1us.