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MSP430G2553: Problem sending ADC values over UART

James Drewelow
Prodigy 50 points
Part Number: MSP430G2553

I am trying to read sensor data and send it to my computer through Bluetooth to be graphed. For testing purposes I am just trying to send a sine wave from a function generator before connecting my actual sensors, but I can only get a clean sine wave of about 2Hz to come through. My guess is that the issue is caused by differing speeds in the ADC sampling and UART transmission but I am pretty new to this so I can't figure out what my issue is. 

#include <msp430g2553.h>
/**
 * main.c
 */
int main(void)
{
    WDTCTL = WDTPW | WDTHOLD;	// stop watchdog timer


    //--------------------Disable Port 2---------------------//
    P2OUT &= 0x00;
    P2DIR &= 0x00;

    //------------------- Configure the Clocks -------------------//
    DCOCTL  = 0;             // Select lowest DCOx and MODx settings
    BCSCTL1 = CALBC1_1MHZ;   // Set range
    DCOCTL  = CALDCO_1MHZ;   // Set DCO step + modulation
	

    //------------------- Configure pins--------------------------//

    P1SEL  |=  BIT1 + BIT2;  // P1.1 UCA0RXD input
    P1SEL2 |=  BIT1 + BIT2;  // P1.2 UCA0TXD output
    P1DIR  |=  BIT3 + BIT4 + BIT5; //P1.3,1.4,1.5 set as outputs
    P1DIR  &=  ~BIT0 + ~BIT6; //P1.0,1.6 set as input


    //--------------------Configure ADC--------------------------//
    ADC10CTL0 &= ~ENC;
    ADC10CTL0 = ADC10SHT_1 + ADC10ON + ADC10SR + MSC + ADC10IE;  
    ADC10CTL1 = INCH_0 + SHS_0 + ADC10DIV_0 + ADC10SSEL_0 + CONSEQ_0;
    ADC10AE0 |= BIT0;


    //------------ Configuring the UART(USCI_A0) ----------------//

    UCA0CTL1 |=  UCSSEL_2 + UCSWRST;  // USCI Clock = SMCLK,USCI_A0 disabled
    UCA0BR0   =  104;                 // 104 From datasheet table
    UCA0BR1   =  0;                   // -selects baudrate =9600,clk = SMCLK
    UCA0MCTL  =  UCBRS_1;             // Modulation value = 1 from datasheet
    UCA0CTL1 &= ~UCSWRST;             // Clear UCSWRST to enable USCI_A0

    //---------------- Enabling the interrupts ------------------//

    IE2 |= UCA0TXIE + UCA0RXIE;       // Enable the Tx and Rx interrupts
    _BIS_SR(GIE);                     // Enable the global interrupt



    while(1)
    {
        ADC10CTL0 &= ~ENC;
        while(ADC10CTL1 & ADC10BUSY)
        {}
        __low_power_mode_0();
        ADC10CTL0 |= ENC + ADC10SC;




    }

}


//-----------------------------------------------------------------------//
//                Transmit and Receive interrupts                  //
//-----------------------------------------------------------------------//

#pragma vector = USCIAB0TX_VECTOR
__interrupt void TransmitInterrupt(void)
{
  UCA0TXBUF = ADC10MEM >> 2; // UART stored and sent in UCA0TXBUF
  IFG2 &= ~UCA0TXIFG; // Clear TX flag
}

#pragma vector = USCIAB0RX_VECTOR
__interrupt void ReceiveInterrupt(void)
{

}

#pragma vector = ADC10_VECTOR
__interrupt void ADC10_ISR(void)
{
    __low_power_mode_off_on_exit();
}

 

  • 0
    Guru 99915 points
    As near as I can tell, the code presented will just generate a stream of 0x00 bytes (or maybe just stall, depending on what the compiler generates), since it doesn't start the ADC (ADC10SC) before going into low_power_mode_0 ("LPM0"). The loop sequence is out of order, but I'm going to recommend you not use LPM for the ADC at all.

    Each ADC conversion is taking about (8+12+2)/5=~4 usec to run. This is 4 MCLKs, or about the cost of a single (typical) CPU instruction. This is so fast that (a) you're pretty much never in LPM and (b) you're tempting a race where the ADC completes before you go into LPM0 and your program stalls. So:

    1) Don't set ADC10IE (you can get rid of the ISR), and reduce your while(1) loop to:
    ADC10CTL0 |= ENC + ADC10SC; // Start conversion
    while(ADC10CTL1 & ADC10BUSY) /*EMPTY*/; // Wait for completion
    Also:
    2) Don't clear UCA0TXIFG in the TX ISR, since setting UCA0TXBUF clears it.
    3) Don't enable UCA0RXIE until you put some code in the RX ISR. If you don't read UCA0RXBUF in the ISR, the ISR will just loop forever.

    At 9600bps, your UART is "sampling" at 9600/10=960sps -- much slower than your ADC is sampling -- so this is what will limit your signal. Nyquist says you can get 480Hz out of 960sps, but only a much slower signal will have any fidelity.
  • 0 in reply to Bruce McKenney
    Prodigy 50 points

    Thanks for you response. I tried the changes that you recommended but I'm still not getting a wave to come through well. I attached a picture of what the graph that I'm getting looks like from a 50 Hz sine wave. Is there maybe something I should do to make sure that each value sends before the ADC reads another value? The fastest wave that I will need to capture will only be around 10 Hz, so 480 Hz from 9600 baudrate should be more than sufficient. Thanks in advance!

  • 0 in reply to James Drewelow
    Guru 99915 points
    This graph seems to suggest that each cycle has about 4 samples rather than the expected (960/50)=~20 samples, so maybe there's something else going on. (Or it could be an illusion.)

    Can you show your new code? As it was structured, it was sampling much faster (~250:1) than it needed to , but as a consequence the sample sent at the UART rate was always (I believe) fresh.
  • 0 in reply to James Drewelow
    TI__Genius 12315 points
    Hi James,

    I think you should send the ADC data in your ADC ISR or after wake up from LPM0, rather then using a UART TX interrupt. In your code, if ADC doesn't complete the conversion, the UART TX ISR will send a old ADC data.

    Best Regards,
    Winter Yu
  • 0 in reply to Bruce McKenney
    Prodigy 50 points 
    #include <msp430g2553.h>
/////////////////////ZERO IS INPUT/////////////////////
/**
 * main.c
 */

int main(void)
{
    WDTCTL = WDTPW | WDTHOLD;	// stop watchdog timer


    //--------------------Disable Port 2---------------------//
    P2OUT &= 0x00;
    P2DIR &= 0x00;

    //------------------- Configure the Clocks -------------------//
    DCOCTL  = 0;             // Select lowest DCOx and MODx settings
    BCSCTL1 = CALBC1_1MHZ;   // Set range
    DCOCTL  = CALDCO_1MHZ;   // Set DCO step + modulation

    //------------------- Configure pins--------------------------//

    P1SEL  |=  BIT1 + BIT2;  // P1.1 UCA0RXD input
    P1SEL2 |=  BIT1 + BIT2;  // P1.2 UCA0TXD output
    P1DIR  |=  BIT3 + BIT4 + BIT5; //P1.3,1.4,1.5 set as outputs
    P1DIR  &=  ~BIT0 + ~BIT6; //P1.0,1.6 set as input


    //--------------------Configure ADC--------------------------//
    ADC10CTL0 &= ~ENC;
    ADC10CTL0 = ADC10SHT_1 + ADC10ON + ADC10SR + MSC; //+ ADC10IE;  //ADC interrupt enabled
    ADC10CTL1 = INCH_0 + SHS_0 + ADC10DIV_0 + ADC10SSEL_0 + CONSEQ_0;
    ADC10AE0 |= BIT0;


    //------------ Configuring the UART(USCI_A0) ----------------//

    UCA0CTL1 |=  UCSSEL_2 + UCSWRST;  // USCI Clock = SMCLK,USCI_A0 disabled
    UCA0BR0   =  104;                 // 104 From datasheet table
    UCA0BR1   =  0;                   // -selects baudrate =9600,clk = SMCLK
    UCA0MCTL  =  UCBRS_1;             // Modulation value = 1 from datasheet
    UCA0CTL1 &= ~UCSWRST;             // Clear UCSWRST to enable USCI_A0

    //---------------- Enabling the interrupts ------------------//

    IE2 |= UCA0TXIE;// + UCA0RXIE;       // Enable the Tx and Rx interrupts
    _BIS_SR(GIE);                     // Enable the global interrupt



    while(1)
    {

        ADC10CTL0 |= ENC + ADC10SC;
        while(ADC10CTL1 & ADC10BUSY);


    }

}


//-----------------------------------------------------------------------//
//                Transmit and Receive interrupts                        //
//-----------------------------------------------------------------------//

#pragma vector = USCIAB0TX_VECTOR
__interrupt void TransmitInterrupt(void)
{
  UCA0TXBUF = ADC10MEM >> 2; // UART stored and sent in UCA0TXBUF
  //IFG2 &= ~UCA0TXIFG; // Clear TX flag
}

//#pragma vector = USCIAB0RX_VECTOR
//__interrupt void ReceiveInterrupt(void)
//{
//
//}

//#pragma vector = ADC10_VECTOR
//__interrupt void ADC10_ISR(void)
//{
//    __low_power_mode_off_on_exit();
//}

    This is my code after making the changes. The more I've been thinking about it though, the more I think that my problem might actually be with either my Bluetooth module or the Python code I'm using to receive and plot the data. I'm open to any further suggestions as far as the embedded code though.

  • 0 in reply to Winter Yu
    Prodigy 50 points
    I tried changing the location of my line of code for transmitting but then I wasn't getting anything on my computer. Would I have to write the code differently to transmit in a different location rather than just setting UCA0TXBUF = ADC10MEM? I'm still pretty new to writing embedded code besides for Arduino, so I still have quite a bit to learn.
  • 0 in reply to James Drewelow
    Guru 99915 points
    I agree that this isn't necessarily the most efficient way of doing this, but as I read your code (I don't have the equipment to try it) it should be giving much better results.

    Is there a way to check the other elements in the signal chain (Bluetooth, Python)?
  • 0 in reply to Bruce McKenney
    Prodigy 50 points

    That's what I was trying to work on today. I verified that Python code is receiving the hex data from the BT module and converting it to decimal values correctly so now I think my next step is to see if something weird is happening in the BT module itself. The module I'm using is an AMS001 incase anybody has experience with it and might be able to give some advice. I'm also new to BT communication so it's completely possible I'm making a simple mistake there as well. If not, I might just have to see if I can find another forum somewhere where I can get some help troubleshooting the BT.

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