Timing Analysis of MCU

We introduced the delay program earlier, but it is not perfect, because we only know that the sentence DJNZ R6, D2 will be executed 62500 times, but how long will it take to execute so many times? Does it meet our requirements? We don't know yet, so we will solve this problem below.

Let me first ask a question: What is the most important thing in our school? (Bell) The principal can go on business trips, and teachers can take a break, but if there is no bell in the school for a day, there will be chaos. The whole school works in unison and in a coordinated manner under the unified command of the bell. This bell rings according to a certain schedule, which we can call "time sequence - the order of time". A unit composed of people must have a certain time sequence, and computers must have a stricter time sequence. In fact, a computer is more like a big clock. There are strict rules for when the minute hand moves, when the second hand moves, and when the hour hand moves, and there can be no chaos at all. The things that computers have to accomplish are more complicated, so their time sequence is also more complicated.

We know that when a computer is working, it fetches instructions from the ROM one by one and then executes them step by step. We define the time it takes for a computer to access the memory as a machine cycle. This is a time reference, just like we humans use "seconds" as our time reference. Why not just use "seconds"? It's so good and very familiar. As we continue to learn, we will find that using "seconds" is not a habit.

A machine cycle consists of 12 clock cycles. Let's calculate how long a machine cycle is. Suppose a single-chip microcomputer works with a 12M crystal oscillator, and its clock cycle is 1/12 (microseconds). Its machine cycle is 12*(1/12), which is 1 microsecond. (Please calculate the machine cycle of a single-chip microcomputer working with a 6M crystal oscillator).

Among all the instructions of the MCS-51 microcontroller, some are completed relatively quickly and only require one machine cycle, some are completed relatively slowly and require two machine cycles, and two instructions require four machine cycles. This is not difficult to solve, is it? The execution time of the instruction I asked you to sweep the floor is always longer than the instruction of wiping the blackboard. In order to keep the length of the instruction execution time constant, a new concept is introduced: instruction cycle. The so-called instruction cycle refers to the time to execute an instruction. INTEL gives the instruction cycle number for each instruction. Most of these data do not need to be remembered by us, but some instructions need to be remembered, such as the DJNZ instruction, which is a two-cycle instruction.

Let's calculate the delay. First, we must know the frequency of the crystal oscillator. If we assume that the crystal oscillator is 12M, then one machine cycle is 1 microsecond. The DJNZ instruction is a two-cycle instruction, so it takes 2 microseconds to execute once. A total of 62,500 executions is exactly 125,000 microseconds, or 125 milliseconds.