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Implementing delay()

Implementing delay() without Using Library

• To implement the delay() function yourself without using the library, follow the following steps:

  1. Write a simple for loop and see how many assembly instructions each loop takes. In the following example, it is observed that the 7 instructions in the red box are repeated each loop.

     xxxxxxxxxx
     5
     1
     void delay(void)
     2
     {
     3
         for(uint32_t i = 0; i < 300000; i++);   // at this point the upper limit can be
     4
                                                 // chosen any value
     5
     }
     

      

     

      

  2. Check the frequency of the processor.

     e.g., STM32F407 Discovery board running with 16Mhz of internal RC oscillator

  3. Assume 1 instruction takes 1 clock cycle.

     1 instruction $\approx$$\approx$ 0.0625 micro seconds.

     Then, 7 instructions $\approx$$\approx$ 0.5 micro seconds.

     0.5 micro seconds $\to$$\to$ 1 iteration of the for loop.

     1000 micro seconds (1 ms) $\to$$\to$ 2000 iterations.

     150 ms $\to$$\to$ 150 x 2000 = 300000 iterations.

  $\therefore$$\therefore$ 300000 iterations of the simple for loop introduces about 150 ms of delay.

• This can be used to introduce the debouncing time when implementing the button press detection logic.

References

Nayak, K. (2022). Embedded Systems Programming on ARM Cortex-M3/M4 Processor [Video file]. Retrieved from https://www.udemy.com/course/embedded-system-programming-on-arm-cortex-m3m4/