在keil软件中编写一个systick中断程序 使其每次定时时间为0.5秒，即led亮0.5s，熄0.5s

Sure! Here's an example of how you can write a SysTick interrupt program in Keil software to achieve a timer of 0.5 seconds, where an LED is turned on for 0.5 seconds and then turned off for 0.5 seconds:

#include <stm32f4xx.h>

void SysTick_Handler(void)
{
    static uint32_t counter = 0;
  
    counter++;
    if(counter == 500) // Wait for 500ms
    {
        GPIOA->ODR ^= (1 << 5); // Toggle LED on PA5
        counter = 0; // Reset counter
    }
}

int main(void)
{
    // Enable GPIOA peripheral clock
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;
  
    // Configure PA5 as output
    GPIOA->MODER |= GPIO_MODER_MODER5_0;
  
    // Set SysTick interrupt to trigger every 1ms
    SysTick_Config(SystemCoreClock / 2000);
  
    while(1)
    {
        // Main program loop
    }
}

In this example, we use the STM32F4 microcontroller with the LED connected to Pin PA5. The SysTick interrupt is configured to trigger every 1ms (SystemCoreClock / 2000) and a counter is used to keep track of the time.

Inside the SysTick_Handler function, the counter is incremented, and when it reaches 500 (0.5 seconds), the LED is toggled by XORing the corresponding bit in the GPIOA ODR register. The counter is then reset to 0.

In the main function, we enable the GPIOA peripheral clock, configure PA5 as an output, and set up the SysTick interrupt with the desired frequency. The program then enters an infinite loop.

Please note that this example assumes you are using an STM32F4 microcontroller. You may need to modify the code according to your specific microcontroller and LED pin configuration.