Interfaz de sensor ultrasónico HC-SR04 con STM32L

Me gustaría compartir el código actualizado, que realmente funciona (no se necesita biblioteca):

#include <stdio.h>
#include "stm32l1xx.h"                  // Keil::Device:Startup

        //Initialize the timers variables.
    volatile int timespan = 0;                              // Total pulse width
    volatile int lastcounter = 0;                           // Timer counter value of the last event
    volatile int newcounter = 0;                            // Timer counter value of the current event
    volatile int overflow = 0;                              // Count the number of overflows


    void SysTick_Handler(void);
    void SetHSI(void);
    void LED_GPIO(void);    
    void TIM4_C1_Init(void);
    void TIM2_C3_Init(void);
    void TIM4_IRQHandler(void);
    void LED (void);

        void setSysTick(void){
    // ---------- SysTick timer (1ms) -------- //
    if (SysTick_Config(SystemCoreClock / 1000)) {
    while (1);  // Capture error
    }
}
    volatile uint32_t msTicks=0; //counts 1ms timeTicks 
    void SysTick_Handler(void) {
    msTicks++;
}

static void Delay(__IO uint32_t dlyTicks){ 
  uint32_t curTicks; 
    curTicks = msTicks;
    while ((msTicks - curTicks) < dlyTicks);
}

        int main(void){
          SysTick_Handler();
            setSysTick();
            SetHSI();                             
            LED_GPIO();

            TIM2_C3_Init();
            TIM4_C1_Init();
while(1){

    LED();

Delay(100);
    }
}

/*----------------------------------------------------------------------------
  set HSI as SystemCoreClock (HSE is not populated on STM32L-Discovery board)
 *----------------------------------------------------------------------------*/
void SetHSI(void) {

// Turn on HSI (16MHz)
RCC->CR |= RCC_CR_HSION;
// Wait until HSI is ready
while( (RCC->CR & RCC_CR_HSIRDY) == 0);
// Select HSI as system clock
RCC->CFGR &= ~RCC_CFGR_SW_HSI;
RCC->CFGR |= RCC_CFGR_SW_HSI;
while( (RCC->CFGR & RCC_CFGR_SWS)!=RCC_CFGR_SWS_HSI ); // Wait till HSI
}

// Configure GPIO Port B
void LED_GPIO(void){


    RCC->AHBENR |= RCC_AHBENR_GPIOBEN;        // Enable GPIOB clock 

//PB7 LED ON/OFF
    GPIOB->MODER   |=   GPIO_MODER_MODER7_0;     // General purpose output mode
  GPIOB->OSPEEDR |=   GPIO_OSPEEDER_OSPEEDR7;  // Max High speed 50MHz


}

// CONFIGURE TIM2 FOR SENDING OUTPUT SIGNAL
void TIM2_C3_Init(void){

    RCC->AHBENR |= RCC_AHBENR_GPIOBEN;        // Enable GPIOB clock 

//PB10 Pluse Generating Pin
    GPIOB->MODER   &=   ~(0x03 << (2*10));     // Clear bit 12 & 13 Alternate function mode 
    GPIOB->MODER   |=   0x02 << (2*10);                 // set as Alternate function mode 
    GPIOB->OSPEEDR &=   ~(0x03<< (2*10));           // 40 MHz  speed 
    GPIOB->OSPEEDR |=   0x03<< (2*10);              // 40 MHz  speed 
    GPIOB->PUPDR &=         ~(1<<10);                           // NO PULL-UP PULL-DOWN 
    GPIOB->OTYPER &=        ~(1<<10);                           // PUSH-PULL 
    GPIOB->AFR[1] |=        0x1 << (4*2);                   // set PB pin 10 as AF1 (TIM2_CH3)

    RCC->APB1ENR |= RCC_APB1ENR_TIM2EN;                 // ENABLE TIM2 CLOCK
    TIM2->PSC = 159;                                                        // SET APPROPRAIT PRESCALER TO SLOW DOWN THE CLOCK
    TIM2->ARR = 0XFFFF;                                                 // SET MAX PULSE WIDTH OF 65536us FOR 16-BIT TIMER
    TIM2->CR1 |= TIM_CR1_DIR;                                       // Set downcounting counter direction
    TIM2->CCMR2 &= ~(TIM_CCMR2_OC3M);                       // Clear OC3M (Channel 3)
  TIM2->CCMR2 |= TIM_CCMR2_OC3M_1 | TIM_CCMR2_OC3M_2;
    TIM2->CCMR2 |= TIM_CCMR2_OC3PE;                         // CH3 Output Preload Enable
    TIM2->CR1 |= TIM_CR1_ARPE;                                  // Auto-reload Prelaod Enable
    TIM2->CCER |= TIM_CCER_CC3E;                                // Enable Output for CH3
    TIM2->EGR |= TIM_EGR_UG;                                        // Force Update
    TIM2->SR &= ~TIM_SR_UIF;                                        // Clear the Update Flag
    TIM2->DIER |= TIM_DIER_UIE;                                 // Enable Interrupt on Update
    TIM2->CCR3 &= ~(TIM_CCR3_CCR3);                     // Clear CCR3 (Channel 3) 
    TIM2->CCR3 |= 0x1;                                            // Load the register 
    TIM2->CR1 |= TIM_CR1_CEN;                           // Enable the counter
}


// CONFIGURE TIM4 FOR RECEIVING INPUT SIGNAL
void TIM4_C1_Init(void){
    RCC->AHBENR |= RCC_AHBENR_GPIOBEN;        // Enable GPIOB clock 
    GPIOB->MODER   &=   ~(0x03 << 12);     // Clear bit 12 & 13 Alternate function mode 
    GPIOB->MODER   |=   (0x02 << 12);               // set as Alternate function mode 
    GPIOB->OSPEEDR &=   ~(0x03<< 12);           // 40 MHz  speed 
    GPIOB->OSPEEDR |=   (0x03<< 12);                // 40 MHz  speed 
    GPIOB->PUPDR &=         ~(0X3<<12);                         // NO PULL-UP PULL-DOWN 
    GPIOB->OTYPER &=        ~(1<<6);                            // PUSH-PULL 
    GPIOB->AFR[0] &= ~GPIO_AFRL_AFRL6;  // Clear pin 6 for alternate function
    GPIOB->AFR[0] |=        0x2 << (4*6);                   // set PB pin 6 as AF2 (TIM4_CH1) 

    RCC->APB1ENR |= RCC_APB1ENR_TIM4EN;                 // ENABLE TIM4 CLOCK
    TIM4->PSC = 15;                                     // SET APPROPRAIT PRESCALER TO SLOW DOWN THE CLOCK                                              
    TIM4->CCMR1 &= ~TIM_CCMR1_CC1S;                         // CLEAR CAPTURE/COMPARE REGISTER
    TIM4->CCMR1 |= 0X1;                                                 // SELECT CH1 INPUTE CAPTURE 
    TIM4->CCMR1 &= ~TIM_CCMR1_IC1F;                         // DISABLE DIGITAL FILTERING
    TIM4->CCER |= (1<<1 | 1<<3);                                // SELECT BOTH RISING AND FALLING EDGE DETECTION CC1P & CC1NP
    TIM4->CCMR1 &= ~(TIM_CCMR1_IC1PSC);                 // INPUT PRESCALER 0 TO CAPTURE EACH VALID EDGE
    TIM4->CCER |= TIM_CCER_CC1E;                                // ENABLE COUNTER CAPTURE
    TIM4->DIER |= TIM_DIER_CC1IE;                               // ENABLE CH1 CAPTURE/COMPARE INTERRUPT
    TIM4->DIER |= TIM_DIER_CC1DE;   
    TIM4->DIER |= TIM_DIER_UIE;                                 // UPDATE INTERRUPT ENABLE
    TIM4->CR1 &= ~TIM_CR1_DIR;                                      // Set downcounting counter direction
    TIM4->CR1 |= TIM_CR1_CEN;                                       // Enable the counter
    NVIC_SetPriority(TIM4_IRQn, 1);                         // SET PRIORITY TO 1
    NVIC_EnableIRQ(TIM4_IRQn);                                  //ENABLE TIM4 INTERRUPT IN NVIC


}

void TIM4_IRQHandler(void){

    if ((TIM4->SR & TIM_SR_UIF) != 0){                  // Check the update event flag
        overflow++;                                 // if UIF = 1, increment overflow counter
        TIM4->SR &= ~TIM_SR_UIF;                                    // clear UIF
    }
    if ((TIM4->SR & TIM_SR_CC1IF) != 0){                // Check capture event flag 
    newcounter = TIM4->CCR1;                                        // read capture value, store as newcounter
    timespan = (newcounter - lastcounter)+(65536 * overflow);   // calculate the total pulse width
    lastcounter = newcounter;                               // save the value of newcounter as lastcounter to be used for the next cycle
    overflow = 0;                                                       // clear overflow counter
    }

}

void LED (void){

    float Distance;                                             // actual distance in cm
    Distance = (timespan / 58.0);

    if (Distance > 0.0 && Distance <= 100.0){

        GPIOB->BSRRL = (1<<7);
    }
        else {
        GPIOB->BSRRH = (1<<7);

        }   
    }