TM4C1294NCPDT: timer capture mode to get frequency

Hello 

i am trying to calculate frequency of an input  signal with timer capture mode.

however it running good but sometime the result get 0 value .

how could this happen ?

how  eliminate  this error?

#include <stdbool.h>
#include <stdint.h>
#include "inc/hw_ints.h"
#include "inc/hw_types.h"
#include "inc/hw_memmap.h"
#include "driverlib/gpio.h"
#include "driverlib/fpu.h"
#include "driverlib/interrupt.h"
#include "driverlib/pin_map.h"
#include "driverlib/sysctl.h"
#include "driverlib/systick.h"
#include "driverlib/timer.h"
#include "driverlib/uart.h"
#include "utils/uartstdio.h"
void calc_freq(void)
;
volatile uint32_t  g_ui32Counter = 0;
uint32_t ui32SysClock=0,    time=0, start=0,end=0;
bool flag=false;
void
SysTickIntHandler(void)
{
    //
    // Update the Systick interrupt counter.
    //
    if(g_ui32Counter>10000000){
        g_ui32Counter=0;}
    g_ui32Counter++;


}

void
InitConsole(void)
{

    SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOA);
    GPIOPinConfigure(GPIO_PA0_U0RX);
    GPIOPinConfigure(GPIO_PA1_U0TX);
    SysCtlPeripheralEnable(SYSCTL_PERIPH_UART0);
    UARTClockSourceSet(UART0_BASE, UART_CLOCK_PIOSC);
    GPIOPinTypeUART(GPIO_PORTA_BASE, GPIO_PIN_0 | GPIO_PIN_1);
    UARTStdioConfig(0, 115200, 16000000);
}
void edge_capture(void)
{
    TimerIntClear(TIMER1_BASE, TIMER_CAPB_EVENT);
  if (flag==false){
    start = TimerValueGet(TIMER1_BASE, TIMER_B);
    flag=true;
  }
    else if (flag==true){
        end=TimerValueGet(TIMER1_BASE, TIMER_B);
        time=end-start;
       flag=false;
       TimerLoadSet(TIMER1_BASE, TIMER_B,(10000-1));

   }
    }
void Configure_timer(void)
{
    SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOD);
    SysCtlPeripheralEnable(SYSCTL_PERIPH_TIMER1);
    GPIOPinTypeTimer(GPIO_PORTD_BASE, GPIO_PIN_3);
    GPIOPinConfigure(GPIO_PD3_T1CCP1);
    TimerConfigure(TIMER1_BASE,(TIMER_CFG_SPLIT_PAIR | TIMER_CFG_B_CAP_TIME_UP));
    TimerControlEvent(TIMER1_BASE,TIMER_B,TIMER_EVENT_POS_EDGE);
    TimerPrescaleSet(TIMER1_BASE, TIMER_B, (120-1));
    TimerLoadSet(TIMER1_BASE, TIMER_B,(10000-1));
    IntRegister(INT_TIMER1B, edge_capture);
    TimerIntClear(TIMER1_BASE, TIMER_CAPB_EVENT);
    TimerIntEnable(TIMER1_BASE, TIMER_CAPB_EVENT);
    IntEnable(INT_TIMER1B);
    TimerEnable(TIMER1_BASE, TIMER_B);
    }

int main(void)
{
    FPULazyStackingEnable();
    ui32SysClock = SysCtlClockFreqSet((SYSCTL_XTAL_25MHZ |
                                       SYSCTL_OSC_MAIN |
                                       SYSCTL_USE_PLL|SYSCTL_CFG_VCO_480), 120000000);
    InitConsole();
    Configure_timer();
    SysTickPeriodSet(ui32SysClock/1000000);
    IntMasterEnable();
    SysTickIntEnable();
    SysTickEnable();
    while(1)
    {
if(time!=0)
{
calc_freq();
}
    }
}
void calc_freq(void)
{
float y;
y=time/1200 ;
y=1/y;
time=100000*y;
UARTprintf("start :%d\n",time);
time=0;
end=0;
start=0;
}