第一步：
在stm32f4xx.h中定义宏__FPU_PRESENT ; __FPU_USED

#define __CM4_REV                 0x0001  /*!< Core revision r0p1                            */
#define __MPU_PRESENT             1       /*!< STM32F4XX provides an MPU                     */
#define __NVIC_PRIO_BITS          4       /*!< STM32F4XX uses 4 Bits for the Priority Levels */
#define __Vendor_SysTickConfig    0       /*!< Set to 1 if different SysTick Config is used  */
#define __FPU_PRESENT             1       /*!< FPU present                                   */
#define __FPU_USED                1

第二步：
在stm32f4xx.h中增加头文件arm_math.h

#include "core_cm4.h"             /* Cortex-M4 processor and core peripherals */
#include "system_stm32f4xx.h"
#include 
  
   
#include "arm_math.h"
  

第三步：
在system_stm32f4xx.c的SystemInit()中添加如下代码：

void SystemInit(void)
{
  /* FPU settings ------------------------------------------------------------*/
  #if (__FPU_PRESENT == 1) && (__FPU_USED == 1)
    SCB->CPACR |= ((3UL << 10*2)|(3UL << 11*2));  /* set CP10 and CP11 Full Access */
  #endif

第四步：
在配置的c++界面添加宏定义ARM_MATH_CM4, __CC_ARM

第五步 ：
编译写个测试程序测试下代码

static void TaskProcess(void)
{
    static uint8_t flag = 0;
    static uint8_t eeprom_cmd = 1;
    static uint16_t i = 0;
    static float num_f = 0;
    static float a = 3.1415926;
    static float b = 87.6;
    static float c = 76.987;

    sd_card_test();
    TIM_SetCounter(RUNTIMER_TIM, 0);
    num_f = a * b / c;

        printf("t %dus \r\n",TIM_GetCounter(RUNTIMER_TIM)); 

  while(1); 
    while (1)
    {   
    printf("Hello STM32");
    }
}

定时器时钟为84M，不分频。
开启FPU运算时长为22×1/84 us;
不开启FPU运算时长为192×1/84 us；
可见开启FPU浮点运算乘除法快了8.7倍!!!