应用层：
#include
1、unsigned int sleep(unsigned int seconds); 秒级
2、int usleep(useconds_t usec); 微秒级：1/10^6
#define _POSIX_C_SOURCE 199309
#include struct timespec {
time_t tv_sec;
long tv_nsec;
};
// The value of the nanoseconds field must be in the range 0 to 999999999.
内核层：
include
1、void ndelay(unsigned long nsecs); 纳秒级：1/10^-10
2、void udelay(unsigned long usecs); 微秒级: 1/10^-6
3、void mdelay(unsigned long msecs); 毫秒级：1/10^-3
