problem with capacitance measuring with stm32f103RE

hi, I'm trying to measure a capacitor's value and send it to the uart terminal. I'm using stm32f103re and I'm using registers. i attached the circuit I'm using and also my code.  when I connect the circuit to stm32 or when i connect the A1 pin to 3.3 volts the result is the same and I attached the response of the terminal. the problem is that the timer interrupt routine only executes once and i can't figure out why.  

#include "stm32f10x.h"
#include "stdio.h"

void SysTick_Handler (void);
void Delay_ms (uint32_t dlyTicks);
volatile uint32_t msTicks;
unsigned int value = 0;
int Data=0;

int counter=0;
char string[50];
char string2[50];
int time=0;
double charge_time=0;
double c=0;
double period=1.0/72000000;
int flag=0;
int val_Tx = 0;              /* Globals used for display */
int AD_last=0;
int flag2;

#include "stm32f10x.h"
#include "usart.h"
	char str[200];
	char data;

#include "STM32F10x.h"
#include "ADC.h"


void ADC_Init (void) {

  RCC->APB2ENR |=(1<<2); /* enable periperal clock for GPIOA */
  GPIOC->CRL &= ~0xF0;                 /* set ADC1_IN1 (PA1) as analog input  */
//  /* Setup and initialize ADC converter                                       */
  RCC->CFGR    |= ( 3UL << 14);           /* ADC clk = PCLK2 / 8              */

  RCC->APB2ENR |= ( 1UL <<  9);           /* enable periperal clock for ADC1  */

  ADC1->SQR1    =  0;                     /* Regular chn. Sequence length = 1 */
  ADC1->SQR2    =  0;                     /* Clear register                   */
ADC1->SQR3    = ( 1UL <<  0);           /* 1. conversion = channel 1        */
  ADC1->SMPR2   = ( 5UL <<  3);           /* sample time channel 1  55,5 cyc. */
  //ADC1->CR1     = ( 1UL <<  8);           /* Scan mode on                     */
  ADC1->CR2     = ( 7UL << 17)         /* select SWSTART                   */
                  |( 1UL << 20) ;          // enable external Trigger          
  ADC1->CR1    |= ( 1UL <<  5);/* enable for EOC Interrupt         */
//  NVIC_EnableIRQ(ADC1_2_IRQn); /* enable ADC Interrupt             */
//  NVIC_SetPriority(ADC1_2_IRQn,1);
  ADC1->CR2    |= ( 1UL <<  0);           /* ADC enable                       */

  ADC1->CR2    |=  1 <<  3;               /* Initialize calibration registers */
  while (ADC1->CR2 & (1 << 3));           /* Wait for init to finish          */
  ADC1->CR2    |=  1 <<  2;               /* Start calibration                */
  while (ADC1->CR2 & (1 << 2));           /* Wait for calibration to finish   */
}

//// initialize usart
void usart_init(void) 
{
RCC->APB2ENR |=(1<<2); /* Enable GPIOA clock*/
RCC->APB2ENR |=(1<<14);  /* Enable USART1 clock*/
GPIOA->CRH   &= ~(0xFF <<  4);        /* clear PA9, PA10                  */
GPIOA->CRH   |=  (0xB <<  4);        /* USART1 Tx (PA9) alternate function output push-pull */
GPIOA->CRH   |=  (0x4 <<  8);        /* USART1 Rx (PA10) input floating  */
USART1->BRR=0x1D4C;     // baudrate 9600 assuming f=8MHz
USART1->CR1=((1<<2)|(1<<3)|(0<<12)|(1<<13));  // enable transmit and receive word length = 8 bit and usart enable 
//USART1->CR1  |= ( 1UL << 5);     // RXNE interrupt enable
//NVIC_SetPriority(USART1_IRQn,0); /* Default priority group 0, can be 0(highest) - 31(lowest) */
//NVIC_EnableIRQ(USART1_IRQn);     /* Enable UART0 Interrupt */
	USART1->CR2   = 0x0000;                 /* 1 stop bit                       */
  USART1->CR3   = 0x0000;                 /* no flow control                  */
}	
	





void usart_sendchar(char data){
	while(!(USART1->SR&(1<<7)));
	USART1->DR=data;
}



void usart_sendstring(char *s) 
{
  	while (*s) 
	{
   	usart_sendchar(*s);
		s++;
	}
}


void Timer_init(void)
	{

RCC->APB2ENR |= (1<<11);             /* enable clock for TIM1    */

  TIM1->PSC   = ( 1 - 1 );   /* set prescaler   = 72000000Hz  */
  TIM1->ARR   = ( 1000 - 1);   /* set auto-reload = 13/89 us */
  TIM1->RCR   =  0;           /* set repetition counter   */

TIM1->DIER = (1<<0);   /* Update Interrupt enable  */
TIM1->CR1  |= (1<<0);   /* 0x0001 timer enable       */
NVIC_EnableIRQ(TIM1_UP_IRQn);


  
//		NVIC_SetPriority(TIM1_UP_IRQn,1);
}


void TIM1_UP_IRQHandler (void) {

  if (TIM1->SR & (1<<0)) { /* UIF set    */

   
	time++;
	TIM1->SR &= ~0x1;         /* clear UIF flag           */
		
	}
 
}

int main()
{
	SysTick_Config(SystemCoreClock/1000); // setup systick timer for 1ms interrupts
	usart_init();
	ADC_Init();
RCC->APB2ENR |= (1<<3);
GPIOB->CRL &=~0xFF;
GPIOB->CRL |= 0x33;            //////// GPIOB.1  output


	
	
while (1)
{
	char m;
	
if (flag==0){

Delay_ms(500);
GPIOB->BSRR=(1<<17);
Delay_ms(500);	
GPIOB->BSRR=(1<<1);
flag=1;

ADC1->CR2    |=  (1<<22);	      // start A/D conversion
Timer_init();	
}

else if (flag==1)
{
//	ADC1->CR2    &= ~(1<<22); /* stop A/D conversion     */
//	NVIC_DisableIRQ(ADC1_2_IRQn);
	if (ADC1->SR & (1 << 1)){
AD_last = ADC1->DR;
 val_Tx = (AD_last & 0xFFF);        /* use upper 12 bits of ADC  */
	            if (val_Tx>2048)
								{
									ADC1->CR2    &= ~(1<<22); /* stop A/D conversion     */
	                flag=0;
  	              TIM1->CR1  &= 0;
	                counter=TIM1->CNT;
	                charge_time=(time*1000*period)+(counter*period);
	                c=14.42695*charge_time;
	                sprintf(string,"the capacitance value is : %f uF\n", c);
	                sprintf(string2,"charge time is  %f\n time is %i\n counter is %i\n ",charge_time,time,counter);
	                usart_sendstring(string);
	                usart_sendstring(string2);
									
	                time=0;
                  flag=0;	
                  counter=0;
                 charge_time=0;
                 c=0;
  
	              }
}
	
	
}
}	
	
}

void SysTick_Handler(void) {
	msTicks++;
}
void Delay_ms (uint32_t dlyTicks) {
	uint32_t curTicks;
	curTicks = msTicks;
	while ((msTicks - curTicks)< dlyTicks);
}