1.8 Getting Started with UART

To use the High-Frequency Internal Oscillator (HFINTOSC) at 1 MHz, two settings must be made in the registers. First, select HFINTOSC as the oscillator source in the OSCCON1 register by writing to the NOSC bit field:

/* Set HFINTOSC as new oscillator source. */
OSCCON1bits.NOSC = 0b110;

Then, select the nominal frequency of 1 MHz in the OSCFRQ register:

/* Set HFFRQ to 1 MHz. */
OSCFRQbits.HFFRQ = 0;

EUSART2 will be configured for 9600 baud rate and the standard 8-N-1 (eight data bits, no parity bit and one Stop bit) frame format.

Given the configured frequency and the desired baud rate of 9600, the 16-bit Baud Rate Generator (BRG16) must be used, and High Baud Rate (BRGH) must be enabled. The SPEN bit enables the Serial Port (EUSART) as a whole, while TXEN enables its transmitter.

/* Transmit Enable */
TX2STAbits.TXEN = 1;
/* High Baud Rate Select */
TX2STAbits.BRGH = 1;
/* 16-bit Baud Rate Generator is used */
BAUD2CONbits.BRG16 = 1;
/* Serial Port Enable */
RC2STAbits.SPEN = 1;

The value to be written in SP2BRG is found in the table below, taken from the device data sheet.

/* Baud rate 9600 */
SP2BRGL = 25;
SP2BRGH = 0;

Since only the transmission is necessary for this use case, only the TX pin must be configured. The pin used as TX in this example, is the RD0 pin and it must be configured using the Peripheral Pin Select (PPS). To find the value that needs to be written to the RD0PPS register, inspect the Peripheral PPS Output Selection Codes table below, taken from the device data sheet.

The following line will route the EUSART2 TX to RD0:

/* RD0 is TX2 */
RD0PPS = 0x0B;

The pin direction is set by default as output, but if it were not, the following line sets it.

/* Configure RD0 as output. */
TRISDbits.TRISD0 = 0;

Before sending data, the user needs to check if the previous transmission is complete by checking the PIR3.TXnIF bit field. The following code example waits until the transmit buffer is empty, then writes a character to the TXnREG register:

static void EUSART2_write(uint8_t txData)
{
    while(0 == PIR3bits.TX2IF)
    {
        ;
    }

    TX2REG = txData; 
}  

Use MPLAB X Data Visualizer as described in the appendix, How to Receive Data in MPLAB X Data Visualizer.

To use the HFINTOSC at 1 MHz, two settings must be made in the registers. First, select HFINTOSC as the oscillator source in the OSCCON1 register by writing to the NOSC bit field:

/* Set HFINTOSC as new oscillator source. */
OSCCON1bits.NOSC = 0b110;

Then, select the nominal frequency of 1 MHz in the OSCFRQ register:

/* Set HFFRQ to 1 MHz. */
OSCFRQbits.HFFRQ = 0;

EUSART2 will be configured for 9600 baud rate and the standard 8-N-1 (eight data bits, no parity bit and one Stop bit) frame format.

Given the configured frequency and the desired baud rate of 9600, the 16-bit Baud Rate Generator (BRG16) must be used and High Baud Rate (BRGH) must be enabled. The SPEN bit enables the Serial Port (EUSART) as a whole, while TXEN enables its transmitter.

/* Transmit Enable */
TX2STAbits.TXEN = 1;
/* High Baud Rate Select */
TX2STAbits.BRGH = 1;
/* 16-bit Baud Rate Generator is used */
BAUD2CONbits.BRG16 = 1;
/* Serial Port Enable */
RC2STAbits.SPEN = 1;

The value to be written in SP2BRG is found in the table below, taken from the device data sheet.

/* Baud rate 9600 */
SP2BRGL = 25;
SP2BRGH = 0;

Since only the transmission is necessary for this use case, only the TX pin must be configured. The pin used as TX, in this example, is the RD0 pin and it must be configured using the Peripheral Pin Select (PPS). To find the value that needs to be written to the RD0PPS register, inspect the Peripheral PPS Output Selection Codes table below, taken from the device data sheet.

The following line will direct the EUSART2 TX to RD0:

/* RD0 is TX2 */
RD0PPS = 0x0B; 

The pin direction is set by default output, but otherwise, the following line sets it.

/* Configure RD0 as output. */
TRISDbits.TRISD0 = 0;

Before sending data, the user needs to check if the previous transmission is complete by checking the PIR3.TXnIF bit field. The following code example waits until the transmit buffer is empty, then writes a character to the TXnREG register:

static void EUSART2_write(uint8_t txData)
{
    while(0 == PIR3bits.TX2IF)
    {
        ;
    }

    TX2REG = txData; 
}  

The function that handles STDIO is void puthc(char c). Implementing this function will transmit its char argument over EUSART and will cause the printf to be redirected to EUSART.

void putch(char txData)
{
    EUSART2_write(txData);
}

The following line will send messages over EUSART:

printf("%c%c%c%c", START_DATA_STREAM_PROTOCOL, cnt, cnt * 2, STOP_DATA_STREAM_PROTOCOL);

The Data Stream Protocol frame has start and stop tokens, as inverse/one’s complement each other. In between those, the payload may contain any number of values. The protocol must be sent in binary format.

In this example, the %c specifier is used, because the row binary value will be transmitted.

Use MPLAB X Data Visualizer as described in the appendix, How to Configure MPLAB X Data Visualizer to Decode Data Stream Protocol.

To use the HFINTOSC at 1 MHz, two settings must be made in the register. First, select HFINTOSC as the oscillator source in the OSCCON1 register by writing to the NOSC bit field:

/* Set HFINTOSC as new oscillator source. */
OSCCON1bits.NOSC = 0b110;

Then, select the nominal frequency of 1 MHz in the OSCFRQ register:

/* Set HFFRQ to 1 MHz. */
OSCFRQbits.HFFRQ = 0;

EUSART2 will be configured for 9600 baud rate and the standard 8-N-1 (eight data bits, no parity bit and one Stop bit) frame format.

Given the configured frequency and the desired baud rate of 9600, the 16-bit Baud Rate Generator (BRG16) must be used and High Baud Rate (BRGH) must be enabled. The SPEN bit enables the Serial Port (EUSART) as a whole, while TXEN enables its transmitter.

In Asynchronous mode, the EUSART supports two types of receive: Single Receive and Continuous Receive. Since a command can be received at any time, Continuous Receive mode will be used.

/* 16-bit Baud Rate Generator is used */
BAUD2CONbits.BRG16 = 1;
/* Transmit Enable */
TX2STAbits.TXEN = 1;
/* High Baud Rate Select */
TX2STAbits.BRGH = 1;
/* Continuous Receive Enable */
RC2STAbits.CREN = 1;
/* Serial Port Enable

The value to be written in SP2BRG is found in the table below, taken from the device data sheet.

/* Baud rate 9600 */
SP2BRGL = 25;
SP2BRGH = 0;

In this example, the pin used as TX is RD0 and it must be configured using the Peripheral Pin Select (PPS). To find the value that needs to be written to the RD0PPS register, inspect the Peripheral PPS Output Selection Codes table below, taken from the device data sheet.

The following line will direct the EUSART2 TX to RD0:

/* RD0 is TX2 */
RD0PPS = 0x0B; 

The pin direction is set by default output, but if it were not, the following line sets it.

/* Configure RD0 as output. */
TRISDbits.TRISD0 = 0;

The pin used as RX is RD1, so using PPS, EUSART2 RX must be routed to this pin. For this, the RX2PPS register is used. The value to be written is determined based on the xxxPPS register definition:

Therefore, the following line routes RX2 to pin RD1.

RX2PPS = 0b00011001;

Since the pin is not a digital input by default, this needs to be configured (enable digital input buffers using the ANSELD register and set as input using the TRISD register).

/* Configure RD1 as input. */
TRISDbits.TRISD1 = 1;
/* Enable RD1 digital input buffers.*/
ANSELDbits.ANSELD1 = 0;

/* Configure RE0 as output. */
TRISEbits.TRISE0 = 0;

static void EUSART2_write(uint8_t txData)
{
    while(0 == PIR3bits.TX2IF)
    {
        ;
    }
    TX2REG = txData;
}

Before reading the data, the user must wait for it to be available by polling the PIR3.RCnIF flag.

uint8_t EUSART2_read(void)
{
    while(0 == PIR3bits.RC2IF)
    {
        ;
    }

    return RC2REG;
}

The function that handles STDIO is void puthc(char c). Implementing this function will transmit its char argument over EUSART and will cause printf to be redirected to EUSART.

void putch(char txData)
{
    EUSART2_write(txData);
}

Similarly, to implementing putch, char getch(void) must be implemented so that the EUSART incoming data are mapped to STDIO.

char getch(void)
{
    return EUSART2_read();
}

The following code snippet reads one line of data and stores it in an array. A valid line is shorter than the array length in this example.

The array index is reset to zero when reaching the array end, to avoid a bugger overflow error in case of longer lines received. The characters, ‘\n’ (line feed) and ‘\r’ (carriage return), are ignored, because they are part of the line terminator. When ‘\n’ is found, the string end (NULL) is added to the command, and the function ‘executeCommand’ will interpret it and change the state of the LED .

char command[MAX_COMMAND_LEN];
uint8_t index = 0;
char c;
while(1)
{
    c = getch();
    if(c != '\n' && c != '\r')
    {
        command[index++] = c;
        if(index > MAX_COMMAND_LEN)
        {
            index = 0;
        }
    }

    if(c == '\n')
    {
        command[index] = '\0';
        index = 0;
        executeCommand(command);
    }
}

Using the MPLAB X Data Visualizer, ‘LED ON’ and ‘LED OFF’ commands can be sent to the board.

Use the MPLAB X Data Visualizer as described in the appendix, Send Commands from MPLAB X Data Visualizer.