USART - Serial Communication

The Universal Synchronous Asynchronous Receiver Transmitter (USART) module is one of the two serial I/O modules. (USART is also known as a Serial Communications Interface or SCI.) The USART can be configured as a full-duplex asynchronous system that can communicate with peripheral devices, such as CRT terminals and personal computers, or it can be configured as a half-duplex synchronous system that can communicate with peripheral devices, such as A/D or D/A integrated circuits, serial EEPROMs, etc.

The USART can be configured in the following modes: - Asynchronous (full-duplex) - Synchronous – Master (half-duplex) - Synchronous – Slave (half-duplex)

Setting Up USART

1. Associated Registers - TXSTA Register:

7	6	5	4	3	2	1	0
CSRC	TX9	TXEN	SYNC	-	BRGH	TRMT	TX9D

• RCSTA Register:

7	6	5	4	3	2	1	0
SPEN	RX9	SREN	CREN	ADDEN	FERR	OERR	RX9D

2. Configure USART module

• Set up TXSTA register

    TXSTAbits.TX9 = 0; // set 8 bit transmission
    TXSTAbits.TXEN = 1; // enable data transmit
    TXSTAbits.SYNC = 0; // select asynchronous mode
    TXSTAbits.BRGH = 1; // set high baud rate

• Set up RCSTA register

    RCSTAbits.SPEN = 1; // enable serial ports
    RCSTAbits.RX9 = 0; // select 8 bit reception
    RCSTAbits.CREN = 1; // enable continuous receive

3. Set the BAUD RATE of communication:

• Baud Rate = FOSC/(16 (SPBRG + 1))

    void __set_baud_rate(int baud_rate) {
      SPBRG = ((_XTAL_FREQ / 16) / baud_rate) - 1;
    }

4. Writing a character

• Wait for the previous transmission to complete. When it is completed the TXIF bit is set.
• Write the character in TXREG

    void __write_char(char c) {
        while(!TXIF) {
            // wait until byte is sent
        } 
        TXREG = c; // transmit a bit
    }

5. Writting a String

• Call the __write_char(c) function everytime looping through the string.

    void __usart_write_ln(char data[]) {
        int i;
        for(i=0; i<(strlen(data)); i++) {
            __write_char(data[i]);
        }
        __write_char('\r');
    }

Circuit Diagram

License

This work is licensed under GNU General Public License v3.0.