Hardware UART

An example showing how to use the hardware UART on the MSP430G2553 to transmit and receive characters between a terminal on the computer and the MSP430 launchpad over the USB connection. In this example sending an ‘R’ or ‘G’ will turn the red and green LEDs on and sending an ‘r’ or ‘g’ will turn them off, respectively. To send and receive characters over the serial port you can use the serial monitor that is built into Energia, click on the magnifying glass button on the top right of the window. Make sure the baud rate is set to the same rate as that specified in the code. You can also use a serial terminal program such as Br@y Terminal or if you are on a Mac Screen. Another very good terminal with multi-platform support is CoolTerm. This also allows for the display of the received characters in HEX. Spark fun has a nice getting started with CoolTerm post. Don’t forget to connect to the right serial/com port. Also it pays to disconnect the terminal program before trying to upload more code using CCS or Energia. In Screen you need to press ctrl+A+K and then enter y to disconnect the serial terminal. You may also need to physically disconnect and reconnect the USB cable sometimes to free up the USB connection, especially if your MSP430 code continually sends data.

Energia Serial Monitor window

Note: Because this uses the hardware UART on the MSP430G2553 it is necessary to rotate the RXD and TXD jumpers on J3 by 90 degrees on the lauchpad. When in the correct position the jumpers should be as shown below. The jumpers are shown in the correct orientation in the top left of the picture.

RXD and TXD jumper orientation for using the hardware UART

/* Example code demonstrating the use of the hardware UART on the MSP430G2553 to receive
 * and transmit data back to a host computer over the USB connection on the MSP430
 * launchpad.
 * Note: After programming it is necessary to stop debugging and reset the uC before
 * connecting the terminal program to transmit and receive characters.
 * This demo will turn on the Red LED if an R is sent and turn it off if a r is sent.
 * Similarly G and g will turn on and off the green LED
 * It also transmits the received character back to the terminal.
 */

#include "msp430g2553.h";
void UARTSendArray(unsigned char *TxArray, unsigned char ArrayLength);

static volatile char data;

void main(void)

{
 WDTCTL = WDTPW + WDTHOLD; // Stop WDT

P1DIR |= BIT0 + BIT6; // Set the LEDs on P1.0, P1.6 as outputs
 P1OUT = BIT0; // Set P1.0

BCSCTL1 = CALBC1_1MHZ; // Set DCO to 1MHz
 DCOCTL = CALDCO_1MHZ; // Set DCO to 1MHz

/* Configure hardware UART */
 P1SEL = BIT1 + BIT2 ; // P1.1 = RXD, P1.2=TXD
 P1SEL2 = BIT1 + BIT2 ; // P1.1 = RXD, P1.2=TXD
 UCA0CTL1 |= UCSSEL_2; // Use SMCLK
 UCA0BR0 = 104; // Set baud rate to 9600 with 1MHz clock (Data Sheet 15.3.13)
 UCA0BR1 = 0; // Set baud rate to 9600 with 1MHz clock
 UCA0MCTL = UCBRS0; // Modulation UCBRSx = 1
 UCA0CTL1 &= ~UCSWRST; // Initialize USCI state machine
 IE2 |= UCA0RXIE; // Enable USCI_A0 RX interrupt

__bis_SR_register(LPM0_bits + GIE); // Enter LPM0, interrupts enabled
}

// Echo back RXed character, confirm TX buffer is ready first
#pragma vector=USCIAB0RX_VECTOR
__interrupt void USCI0RX_ISR(void)
{
data = UCA0RXBUF;
UARTSendArray("Received command: ", 18);
UARTSendArray(&data, 1);
UARTSendArray("\n\r", 2);

switch(data){
 case 'R':
 {
 P1OUT |= BIT0;
 }
 break;
 case 'r':
 {
 P1OUT &= ~BIT0;
 }
 break;
 case 'G':
 {
 P1OUT |= BIT6;
 }
 break;
 case 'g':
 {
 P1OUT &= ~BIT6;
 }
 break;
 default:
 {
 UARTSendArray("Unknown Command: ", 17);
 UARTSendArray(&data, 1);
 UARTSendArray("\n\r", 2);
 }
 break;
 }
}

void UARTSendArray(unsigned char *TxArray, unsigned char ArrayLength){
 // Send number of bytes Specified in ArrayLength in the array at using the hardware UART 0
 // Example usage: UARTSendArray("Hello", 5);
 // int data[2]={1023, 235};
 // UARTSendArray(data, 4); // Note because the UART transmits bytes it is necessary to send two bytes for each integer hence the data length is twice the array length

while(ArrayLength--){ // Loop until StringLength == 0 and post decrement
 while(!(IFG2 & UCA0TXIFG)); // Wait for TX buffer to be ready for new data
 UCA0TXBUF = *TxArray; //Write the character at the location specified py the pointer
 TxArray++; //Increment the TxString pointer to point to the next character
 }
}