STM32 Learning 007_Use of Systick (Part 2)

In "Embedded Learning 006_Use of Systick (I)", the use of registers in Systick is introduced in detail, and many functions are used. In fact, in the 3.5 version of the standard firmware library, the relevant driver functions are removed, and users must call the functions defined in CMSIS.h. CMSIS only provides a Systick setting function, which replaces all the original driver functions of STM32. The purpose of doing this may be to simplify the Systick setting, but it reduces the user's controllability of SysTick.

The function provided in CMSIS is SysTick_Config(uint32_t ticks); This function sets the value of the automatic reload counter (LOAD), the priority of the SysTick IRQ, resets the value of the counter (VAL), starts counting and turns on the SysTick IRQ interrupt. The SysTick clock uses the system clock by default.

This function is defined in Core_cm3.h, and the source code is as follows:

   static __INLINE uint32_t SysTick_Config(uint32_t ticks)
{ 
  if (ticks > SysTick_LOAD_RELOAD_Msk) return (1);                                                                                               
  SysTick->LOAD = (ticks & SysTick_LOAD_RELOAD_Msk) - 1;              
  NVIC_SetPriority (SysTick_IRQn, (1<<__NVIC_PRIO_BITS) - 1 );        
  SysTick-> VAL = 0;                                                                                        
  SysTick->CTRL = SysTick_CTRL_CLKSOURCE_Msk | 
                   SysTick_CTRL_TICKINT_Msk | 
                   SysTick_CTRL_ENABLE_Msk;                                                
  return (0); }

As can be seen from the above function, this function completes the initial value of Systick, interrupt priority, enabling interrupts, and starting the timer, which greatly simplifies the program.

The ticks represent the initial value. For example, if the system clock is 72Mhz, then to generate a 1ms time base, we can write it like this.

SysTick_Config(SystemCoreClock/1000); Of course, it can also be written as: SysTick_Config(72000);

Knowing this, we can use it to make a simple delay function delay_ms(u16 time);

        code show as below:

            void delay_ms(u16 time)

             {

                             nTime=time; /nTime is a global variable and can be set as extern u16 nTime;/

                          while(nTime);

             }

       Directly add nTime-- in the interrupt function;

      Add SysTick_Config(72000) during the main function initialization process;