Because the chip serial port is not enough, I have to use IO port to simulate the serial port. I downloaded a program to simulate the serial port on the Internet, which can be run. However, it is found that the output of the serial port will be garbled after several characters. The main part of the code is as follows:

#define OI_TXD	PAout(12)
#define OI_RXD	PAin(11)

#define BuadRate_9600	100

u8 len = 0;	//Receive count
u8 USART_buf[11];  //Receive buffer

u8 recvStat = COM_STOP_BIT;
u8 recvData = 0;

void IO_TXD(u8 Data)
{
	u8 i = 0;
	OI_TXD = 0;  
	delay_us(BuadRate_9600);
	for(i = 0; i < 8; i++)
	{
		if(Data&0x01)
			OI_TXD = 1;  
		else
			OI_TXD = 0; 	
		
		delay_us(BuadRate_9600);
		Data = Data>>1;
	}
	OI_TXD = 1;
	delay_us(BuadRate_9600);
}
	
void USART_Send(u8 *buf, u8 len)
{
	u8 t;
	for(t = 0; t < len; t++)
	{
		IO_TXD(buf[t]);
	}
}
	
 void IOConfig(void)
 {
	GPIO_InitTypeDef  GPIO_InitStructure;
	NVIC_InitTypeDef NVIC_InitStructure;
 	EXTI_InitTypeDef EXTI_InitStruct;
  RCC_APB2PeriphClockCmd(RCC_APB2Periph_AFIO|RCC_APB2Periph_GPIOA, ENABLE);	 //Enable PB,PC port clock 
	 
	 //SoftWare Serial TXD
	GPIO_InitStructure.GPIO_Pin = GPIO_Pin_12;	    
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP; 		 //Push pull output
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;		 //IO port speed is 50MHz	 
  GPIO_Init(GPIOA, &GPIO_InitStructure);	  				
  GPIO_SetBits(GPIOA,GPIO_Pin_12); 						
	 
	 
	//SoftWare Serial RXD
	GPIO_InitStructure.GPIO_Pin = GPIO_Pin_11;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IPU;
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;		
	GPIO_Init(GPIOA, &GPIO_InitStructure);	 

	GPIO_EXTILineConfig(GPIO_PortSourceGPIOA, GPIO_PinSource11);
	EXTI_InitStruct.EXTI_Line = EXTI_Line11;
	EXTI_InitStruct.EXTI_Mode=EXTI_Mode_Interrupt;
	EXTI_InitStruct.EXTI_Trigger=EXTI_Trigger_Falling; //Falling edge trigger interrupt
	EXTI_InitStruct.EXTI_LineCmd=ENABLE;
	EXTI_Init(&EXTI_InitStruct);


	NVIC_InitStructure.NVIC_IRQChannel= EXTI15_10_IRQn ; 
	NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority=2; 
	NVIC_InitStructure.NVIC_IRQChannelSubPriority =2;  
	NVIC_InitStructure.NVIC_IRQChannelCmd=ENABLE;  
	NVIC_Init(&NVIC_InitStructure);  
	
}

 int main(void)
 {		
	delay_init();
        IOConfig();
        while(1)
	{
		USART_Send("100",3);
		delay_ms(100);
	}
}

In general, there is output scrambling, which may be caused by insufficient delay in the output part. Therefore, check the use of delay in the output part, and find that there is no delay in USART_Send. Add a 1ms delay in USART_Send, and the error is removed. The changed code is as follows:

void USART_Send(u8 *buf, u8 len)
{
	u8 t;
	for(t = 0; t < len; t++)
	{
		IO_TXD(buf[t]);
		delay_ms(1);
	}
}

Although the delay is simple, it can really play a great role sometimes.