MSP430FR5949: ADC issue when measuring small value

Xiaowei Yu

We are having a issue with using port P1.4 to measure low voltage, below 5mV. The input of P1.4 is buffered. When the input is 2mV, we have to add a 30sec delay after 'ADC12CTL0 |= ADC12ENC | ADC12SC' sentence, otherwise, the measurement result is 0 or 1 count. Does anybody have a clue what's the reason for this? Thanks in advanced.

    //Turn on Power
   Power_Port_DIR |= Acc_On;
   Power_Port_OUT |= Acc_On;
   // Configure ADC
   ADC_PORT_SEL1 |= RMS_ADC;
   ADC_PORT_SEL0 |= RMS_ADC;
   ADC12CTL0 &= ~ADC12ENC;                    //enable modification
   ADC12CTL0 = ADC12SHT0_2 + ADC12ON + ADC12MSC;        // ADC12ON, S&H=64 ADC clks; 5M/(64+15)=65.8KHz
   ADC12CTL1 = ADC12SSEL_0 + ADC12SHS_0 + ADC12SHP + ADC12DIV_5 + ADC12CONSEQ_2 ;        // ADCCLK = MODOSC = 5MHz
                                                                                        //ADC12SC bit,sample-and-hold source select
                                                                        //SAMPCON signal is sourced from the sampling timer //Divide by 6= 833333
                                                                                       //Repeat-single-channel
   ADC12CTL2 = ADC12RES_2;     // 12-bit conversion results
   ADC12MCTL0 = ADC12INCH_4 +ADC12VRSEL_1;
    while(REFCTL0 & REFGENBUSY);            // If ref generator busy, WAIT
    REFCTL0 |= REFVSEL_1+REFON;             // Select internal ref = 2.0V
   ADC12CTL0 |= ADC12ENC | ADC12SC ;       // Start conversion

   configureTimer_B0(150);     //150*0.2 = 30 sec
   while(!timed_outB0) __bis_SR_register(LPM3_bits + GIE);
   TB0CTL = MC_0 ;         //MC==00 HALTED
   timed_outB0 = 0;

   for (i=0; i<tSample; i++)
       {
           // movingAVG = 0;
           // ADCresult = 0;
           // for (j=0; j<256; j++)
           // {
                while (!(ADC12IFGR0 & BIT0));
           //    movingAVG += ADC12MEM0;
           // }
           // ADCresult = movingAVG >> 4;
           // ADCresult = movingAVG/10;
       //        __delay_cycles(1000000);
           *FRAM_write_ptr++ = ADC12MEM0;
       }
   ADC12CTL0 &= ~ADC12SC;

over 9 years ago

0 Cameron LaFollette over 9 years ago

TI__Genius 14875 points

Hi Xiaowei,

I suspect that the REF isn't ready. Try modifying your code to the following:

while(REFCTL0 & REFGENBUSY);            // If ref generator busy, WAIT
REFCTL0 |= REFVSEL_1+REFON;             // Select internal ref = 2.0V
// TODO: Add the line below.
while(!(REFCTL0 & REFGENRDY));          // Wait for reference generator to settle

ADC12CTL0 |= ADC12ENC | ADC12SC ;       // Start conversion

I'd also say to ensure your Sample Hold Time is long enough, but it appears that you've already calculated that.

0 Xiaowei Yu over 9 years ago in reply to Cameron LaFollette

Intellectual 510 points

Hi Cameron,
Thank you for your suggestions. I just tried to increase the SH time to 512 clocks and added that line to wait REF, but still measured 0 or 1 without 30sec long delay.

Xiaowei

0 Agustin Villavedra over 9 years ago in reply to Xiaowei Yu

Intellectual 510 points

The ADC input has a RC circuit, which input resistance are you seeing? It might take a while for the actual ADC input to reach the input voltage.
See 28.2.6.3 on the user guide.

0 Cameron LaFollette over 9 years ago in reply to Agustin Villavedra

TI__Genius 14875 points

Hi Xiaowei,
Did Agustin's advice help? If not please let us know and we can continue to debug.

0 Cameron LaFollette over 9 years ago in reply to Cameron LaFollette

TI__Genius 14875 points

Hi Xiaowei,
I'm going to close this thread, please reply if this did not answer your question and it will re-open. If not, click 'Verify Answer' to confirm it.

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MSP430FR5949: ADC issue when measuring small value