Why can't I enter USART3_IRQHandler? - stm32

Question

#if EN_USART2
u16 _distance1=0;
u16 distance1=0;
void USART2_Init(u32 bound) {
GPIO_InitTypeDef GPIO_InitStructure;
USART_InitTypeDef USART_InitStructure;
NVIC_InitTypeDef NVIC_InitStructure;
RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA, ENABLE);
RCC_APB1PeriphClockCmd(RCC_APB1Periph_USART2, ENABLE);
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_2;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;
GPIO_Init(GPIOA, &GPIO_InitStructure);
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_3;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;
GPIO_Init(GPIOA, &GPIO_InitStructure);
NVIC_InitStructure.NVIC_IRQChannel = USART2_IRQn;
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 3;
NVIC_InitStructure.NVIC_IRQChannelSubPriority = 3;
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
NVIC_Init(&NVIC_InitStructure);
USART_InitStructure.USART_BaudRate = bound;
USART_InitStructure.USART_WordLength = USART_WordLength_8b;
USART_InitStructure.USART_StopBits = USART_StopBits_1;
USART_InitStructure.USART_Parity = USART_Parity_No;
USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
USART_Init(USART2, &USART_InitStructure);
USART_ITConfig(USART2, USART_IT_RXNE, ENABLE);
USART_Cmd(USART2, ENABLE);
}
void USART2_IRQHandler(void){
u8 a;
if(USART_GetITStatus(USART2, USART_IT_RXNE) != RESET){
a =USART_ReceiveData(USART2);
if(a!=0xa5) _distance1=(_distance1<<8)|a;
else
{
distance1=_distance1;
_distance1=0;
}
}
}
#endif
#if EN_USART3
u16 _distance2=0;
u16 distance2=0;
void USART3_Init(u32 bound) {
GPIO_InitTypeDef GPIO_InitStructure;
USART_InitTypeDef USART_InitStructure;
NVIC_InitTypeDef NVIC_InitStructure;
RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOB, ENABLE);
RCC_APB1PeriphClockCmd(RCC_APB1Periph_USART3, ENABLE);
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_10;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;
GPIO_Init(GPIOB, &GPIO_InitStructure);
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_11;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;
GPIO_Init(GPIOB, &GPIO_InitStructure);
NVIC_InitStructure.NVIC_IRQChannel = USART3_IRQn;
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 3;
NVIC_InitStructure.NVIC_IRQChannelSubPriority = 3;
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
NVIC_Init(&NVIC_InitStructure);
USART_InitStructure.USART_BaudRate = bound;
USART_InitStructure.USART_WordLength = USART_WordLength_8b;
USART_InitStructure.USART_StopBits = USART_StopBits_1;
USART_InitStructure.USART_Parity = USART_Parity_No;
USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
USART_Init(USART3, &USART_InitStructure);
USART_ITConfig(USART3, USART_IT_RXNE, ENABLE);
USART_Cmd(USART3, ENABLE);
}
void USART3_IRQHandler(void){
u8 a;
if(USART_GetITStatus(USART3, USART_IT_RXNE) != RESET){
a =USART_ReceiveData(USART3);
if(a!=0xa5) _distance2=(_distance2<<8)|a;
else
{
distance2=_distance2;
_distance2=0;
}
}
}
#endif
This is part of the code of usart.c. EN_USART2 and EN_USART3 are all 1. main.c is as follows:
#include "stm32f10x.h"
#include "delay.h"
#include "system.h"
#include "usart.h"
// Device header
int main(void)
{
SystemInit();
USART1_Init(115200);
USART2_Init(115200);
USART3_Init(115200);
while(1)
{
printf("difference: %dmm\r\n",distance1-distance2);
delay_ms(100);
}
}
It runs on STM32F103C8T6. PA2 and PA3 connects to an ultrasound module, and PB10 and PB11 connects to another ultrasound module. I found that the interruption service function of USART2 can be implemented, but that of USART3 cannot. Why is that? USART3 and USART2 are all written similarly. (As for how do I know that USART2 doesn't work: I added printf("p") behind a =USART_ReceiveData(USART3);, but I didn't see any "p" on the serial debug window(however, messages printed in main() all displayed well, which means that USART1 is normal).
I searched for the template code, but I can't find anything wrong in my own code.