INTRODUCTION
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The universal synchronous asynchronous receiver transmitter (USART) offers a flexible means of full-duplex data exchange with external equipment requiring an industry standard NRZ asynchronous serial data format. The USART offers a very wide range of baud rates using a fractional baud rate generator.

It supports synchronous one-way communication and half-duplex single wire
communication. It also supports the LIN (local interconnection network), Smartcard Protocol and IrDA (infrared data association) SIR ENDEC specifications, and modem operations (CTS/RTS). It allows multiprocessor communication.

High speed data communication is possible by using the DMA for multibuffer configuration.

MAIN FEATURES
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● Full duplex, asynchronous communications

● NRZ standard format (Mark/Space)

● Configurable oversampling method by 16 or by 8 to give flexibility between speed  and clock tolerance

● Fractional baud rate generator systems

– Common programmable transmit and receive baud rate (refer to the datasheets
for the value of the baud rate at the maximum APB frequency.

● Programmable data word length (8 or 9 bits)

● Configurable stop bits - support for 1 or 2 stop bits

● LIN Master Synchronous Break send capability and LIN slave break detection
capability

– 13-bit break generation and 10/11 bit break detection when USART is hardware
configured for LIN

● Transmitter clock output for synchronous transmission

● IrDA SIR encoder decoder

– Support for 3/16 bit duration for normal mode

● Smartcard emulation capability

– The Smartcard interface supports the asynchronous protocol Smartcards as
defined in the ISO 7816-3 standards

– 0.5, 1.5 stop bits for Smartcard operation

● Single-wire half-duplex communication

● Configurable multibuffer communication using DMA (direct memory access)

– Buffering of received/transmitted bytes in reserved SRAM using centralized DMA

● Separate enable bits for transmitter and receiver

● Transfer detection flags:

– Receive buffer full

– Transmit buffer empty

– End of transmission flags

● Parity control:

– Transmits parity bit

– Checks parity of received data byte

● Four error detection flags:
– Overrun error
– Noise detection
– Frame error
– Parity error

● Ten interrupt sources with flags:

– CTS changes

– LIN break detection

– Transmit data register empty

– Transmission complete

– Receive data register full

– Idle line received

– Overrun error

– Framing error

– Noise error

– Parity error

● Multiprocessor communication - enter into mute mode if address match does not occur

● Wake up from mute mode (by idle line detection or address mark detection)

● Two receiver wakeup modes: Address bit (MSB, 9th bit), Idle line

USART BLOCK DIAGRAM
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.. image:: /usart_block_diagram.png

USART SYNCHRONOUS MODE
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The synchronous mode is selected by writing the CLKEN bit in the USART_CR2 register to

1. In synchronous mode, the following bits must be kept cleared:

● LINEN bit in the USART_CR2 register,

● SCEN, HDSEL and IREN bits in the USART_CR3 register.

The SCLK pin works in conjunction with the TX pin. Thus, the clock is provided only if the transmitter is enabled (TE=1) and a data is being transmitted (the data register USART_DR has been written). This means that it is not possible to receive a synchronous data without transmitting data.

The LBCL, CPOL and CPHA bits have to be selected when both the transmitter and the
receiver are disabled (TE=RE=0) to ensure that the clock pulses function correctly. These bits should not be changed while the transmitter or the receiver is enabled.

.. image:: /synchronous_transmission.png

CODE SECTION
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main()
{
//初始化GPIO
GPIO_InitTypeDef GPIO_InitStructure;

GPIO_InitStructure.GPIO_Pin = GPIO_Pin_6 | GPIO_Pin_7;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
GPIO_Init(GPIOB, &GPIO_InitStructure);

GPIO_PinAFConfig(GPIOB, GPIO_PinSource6, GPIO_AF_USART1); 
GPIO_PinAFConfig(GPIOB, GPIO_PinSource7, GPIO_AF_USART1);

//初始化UART
SART_InitStructure.USART_BaudRate = 115200;//設定baudrate
USART_InitStructure.USART_WordLength = USART_WordLength_8b;//設定word長度為8位
USART_InitStructure.USART_StopBits = USART_StopBits_1;//1位停止字節
USART_InitStructure.USART_Parity = USART_Parity_No;//無奇偶檢驗
USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;//無流量控制
USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;//開啟Rx接收和Tx發送功能

USART_Init(USART1, &USART_InitStructure);
USART_Cmd(USART1, ENABLE);//啟動usart1


while(1)
{
    while(i < 100){
	send_byte('G');
	i++;
     }
    while(USART_GetFlagStatus(USART1, USART_FLAG_RXNE) != RESET)//等到有data才變為RESET
	b = (USART_ReceiveData(USART1) & 0x7F);



}

}
void send_byte(uint8_t b)
{
	
	USART_SendData(USART1, b);

    while(USART_GetFlagStatus(USART1, USART_FLAG_TXE) == RESET);//直到USART1是空
}

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