GD32F3添加delay_1us函数

原来的函数只有delay_1ms没有delay_1us

#ifndef SYS_TICK_H
#define SYS_TICK_H

#include <stdint.h>

/* configure systick */
void systick_config(void);
/* delay a time in milliseconds */
void delay_1ms(uint32_t count);
/* delay decrement */
void delay_decrement(void);

#endif /* SYS_TICK_H */

#include "gd32f403.h"
#include "systick.h"

volatile static uint32_t delay;

/*!
    \brief      configure systick
    \param[in]  none
    \param[out] none
    \retval     none
*/
void systick_config(void)
{
    /* setup systick timer for 1000Hz interrupts */
    if (SysTick_Config(SystemCoreClock / 1000U)){
        /* capture error */
        while (1){
        }
    }
    /* configure the systick handler priority */
    NVIC_SetPriority(SysTick_IRQn, 0x00U);
}

/*!
    \brief      delay a time in milliseconds
    \param[in]  count: count in milliseconds
    \param[out] none
    \ret