A preliminary study of PIC's delay function and loop optimization

In fact, this is not the case. A deep understanding of the delay function will help you design an ideal frame price.

Generally, we use

for(x=100;--x;){;}This sentence is equivalent to x=100;while(--x){;};

or for(x=0;x<100;x++){;}

Let's write a delay function.

It is important to note here that X=100 does not mean that the loop will exit after only 100 instructions have been executed.

You can look at the compiled assembly:

x=100;while(--x){;}

After compilation:

    movlw 100
    bcf 3,5
    bcf 3,6
    movwf _delay
    l2 decfsz _delay
    goto l2

    return

From the code, we can see that the total number of instructions is 303, and the formula is 8+3*(X-1). Note that the number of cycles is X-1, which is 99.

The summary here is the loop body when x is of type char. When x is int, it is more affected by the value of X.

It is recommended to design a char type loop body, and then use another loop body to call it, so as to achieve accurate and long-term delay.

Here is a function that can accurately control the delay. The assembly code of this function is the most concise and can most accurately control the instruction time:

void delay(char x,char y){
                  char z;
             do{
                           z=y;
                          do{;}while(--z);
                  }while(--x);
}

The instruction time is: 7+(3*(Y-1)+7)*(X-1)

If we add the call instruction for function call, page setting, and 7 instructions for passing parameters.

Then it is: 14+(3*(Y-1)+7)*(X-1).

If the delay requirement is not particularly strict, you can use this function:

void delay(){
 unsigned int d=1000;
 while(--d){;}
}

This function produces a delay of 10003us under a 4M crystal, which is 10MS.

If D is changed to 2000, it will be 20003us, and so on.

Some friends don't understand why we don't use while(x--) followed by decrement to control how many cycles the set X value is?

Now let's look at the assembly code that compiles it:

    bcf 3,5
    bcf 3,6 
    movlw 10
    movwf _delay

  l2
    decf _delay
    incfsz _delay,w

   goto l2

   return

It can be seen that there is one more instruction in the loop body, which is not concise. Therefore, it is best to use the front decrement to control the loop body in PICC.

Let’s talk about this sentence:

for(x=100;--x;){;} and for(x=0;x<100;x++){;}

The two have the same meaning literally, but you can view the code through assembly. The latter code is long, while the former compiles the concise code well.

Therefore, it is best to write the loop body in the former form in PICC. A good C compiler will automatically convert the increment loop into a decrement loop, because this is determined by the hardware characteristics of the processor.

PIC is not a very smart C compiler, so the human brain is still the first priority. Mastering some experience is helpful in writing efficient and concise code.