How to calculate the instruction cycle of a microcontroller

　　Instruction cycle: The time consumed by the instruction cycle to execute a certain instruction, which is equal to an integer multiple of the machine cycle. Most of the instruction cycles of the traditional 80C51 microcontroller are single-cycle instructions, that is, the instruction cycle = machine cycle, and a small number of them are double-cycle instructions. Now (as of 2012) new microcontrollers can achieve no frequency division, and try to use a single instruction cycle, that is, instruction cycle = machine cycle = clock cycle.

　　Looking at this 8051 microcontroller external data, the changing frequency of ALE and $PSEN$ here is less than one machine cycle. If you use C language to simulate this signal, there is no way to achieve a one-to-one correspondence, so you can only try to match the timing above. Same, the cycle is extended.

　　The instruction cycle is uncertain because it is related to the machine cycle included in the instruction. One instruction cycle = 1 (or 2 or 3 or 4) machine cycle. Multiplication or division contains 4 machine cycles, and a single instruction only contains 1 machine cycle.

　　For most 51 microcontrollers, 1 machine cycle = 12 clock cycles (or oscillation cycles)

　　There are also some microcontrollers whose clock cycle and oscillation cycle are not equal. For example, 1 clock cycle = 2 oscillation cycles.

　　This definition refers to the time required to execute an instruction. Usually an instruction cycle consists of several machine cycles. Different instructions require different number of 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 needed. For some more complex instructions, such as transfer instructions and multiplication instructions, two or more machine cycles are required. Generally, instructions that contain one machine cycle are called single-cycle instructions, and instructions that contain two machine cycles are called two-cycle instructions.

　　PIC microcontroller instruction cycle calculation

　　Every four clock cycles of the PIC microcontroller is an internal instruction cycle

　　For example: for an 8MHz crystal oscillator, the internal command cycle is 1/(8/4) = 0.5 uS

　　Example 1: 35us, 8MHz crystal oscillator, 8-bit timer, frequency division ratio 1/2, initial value E4

　　Example 2: 156.25us, 32768Hz crystal oscillator, 8-bit timer, frequency division ratio 1/32, initial value FC

　　Calculation method one: 35 = = (256-initial value)*frequency division*4/crystal oscillator+14/frequency division=(256-initial value)+14/2

　　Calculation method two: 0.015625 = (256-initial value)*frequency division*4/crystal oscillator = (256-initial value)*32*4/32768

　　Special note: the frequency division ratio 1-8 needs to be added with a 14/division ratio

　　What is prescaler ratio and postscaler ratio?

　　Frequency dividers are generally used in conjunction with counting units.

　　The prescaler ratio takes effect before the value of the counting unit changes (if prescaler is not used, the counter increases by 1 at each rising edge, and now a prescaler with a division ratio of 1:2 is used, Then you must wait until two rising edges arrive before the counter will add 1)

　　The postscaler has the same function as the prescaler, except that it takes effect after the counter value changes. Like the TMR2 postscaler, if it is not used, once the counter overflows, the flag will be set to TMR2IF immediately. However, if there is a 1:2 postscaler, it must overflow twice before it is set.

　　Nowadays, the commonly used microcontroller programming software is keil. After writing the program, we can determine the time required for program execution through software debugging, and we can also see each instruction: