51 Cycle in Single Chip Microcomputer

1. Clock cycle, also known as oscillation period: defined as the reciprocal of the clock pulse, which is equal to the reciprocal of the crystal oscillator in a single-chip microcomputer.

   In the 51 single-chip microcomputer, one clock cycle is defined as one beat (indicated by P), and two beats are defined as a state cycle (indicated by S).

   The clock cycle is the smallest unit of time in a microcontroller.

   Eg: For a microcontroller with a 12M crystal oscillator, the clock period = oscillation period = 1/12 us.

2. Machine cycle: The time required to complete a basic operation is called a machine cycle.

   In a computer, for ease of management, the execution process of an instruction is divided into several stages, with each stage performing a basic operation.

   Such as: instruction fetch, memory read, memory write, etc.

   In the 51 MCU, one machine cycle consists of 6 state cycles, that is, 12 clock cycles = 12 x 1/12 us = 1 us

   The machine cycle is defined because the clock cycle time is too short to do anything.

3. Instruction cycle: It is defined as the time required to execute one instruction.

   Generally, instructions that contain one machine cycle are called single-cycle instructions, such as MOV instructions, CLR instructions, etc.

         Instructions that take two machine cycles are called two-cycle instructions.

         There are also four-cycle instructions.

   The most reliable way to determine whether an instruction is a single-cycle instruction or a two-cycle instruction is to look up the instruction table. I found a summary of the rules on the Internet.

   This rule should be judged in order. The first principle is higher than the last one (mainly 2 to 6). Whichever one is met in order is the one that belongs to it. 

   one type:

   1. MUL and DIV: the only 4-cycle instructions
   . 2. Instructions including DPTR and PC are all 2-cycle instructions.
   3. All absolute jump and conditional transfer instructions are 2-cycle instructions.
   4. All instructions including the A register are single-cycle instructions.
   5. Among the bit operation instructions, ANL and ORL are 2-cycle instructions.
   6. All instructions containing immediate addresses, except INC direct and DEC direct, are 2-cycle instructions.
   7. The rest are single-cycle instructions.

    Now, the commonly used microcontroller program writing software is Keil. After writing the program, we can use software debugging to determine the time required for program execution, and we can also see each instruction: