MCU clock cycle machine cycle

Clock cycle
The clock cycle is also called the oscillation cycle. It is defined as the inverse of the clock pulse and is the most basic and smallest unit of time in a computer.

Machine cycle
In a computer, for ease of management, the execution of an instruction is usually divided into several stages, and each stage completes a task. For example, fetching instructions, reading memory, writing memory, etc. Each of these tasks is called a basic operation.
The time required to complete a basic operation is a machine cycle, and a machine cycle consists of several S cycles (state cycles).

Instruction cycle
The time required to execute an instruction is generally composed of several machine cycles. Different instructions require different machine cycles.
For some simple single-byte instructions, in the instruction fetch cycle, after the instruction is fetched into the instruction register, it is immediately decoded and executed, and no other machine cycles are required.
For some more complex instructions, such as transfer instructions and multiplication instructions, two or more machine cycles are required.
Usually, instructions containing one machine cycle are called single-cycle instructions, and instructions containing two machine cycles are called two-cycle instructions.

8051 microcontroller: 1 machine cycle = 12 clock cycles 12MHZ crystal oscillator It takes 1uS to execute a single-byte instruction;

PIC microcontroller: 1 machine cycle = 4 clock cycles 4MHZ crystal oscillator It takes 1uS to execute a single-byte instruction;

AVR microcontroller: 1 machine cycle = 4 clock cycles 4MHZ crystal oscillator It takes 0.25uS to execute a single-byte instruction.