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How to get 200 ksps conversion rate from 12-b ADC of MSP430 using repeat single-channel mode

Esa Korhonen
Prodigy 70 points
Other Parts Discussed in Thread: MSP430F155

Hi,

 

I'm trying to get as fast conversion rate as possible by using msp430f155 12-b ADC. I'm using repeat single-channel mode and I'm quite far from 200 ksps.  What I'm doing wrong or is it impossible to get such conversion rates using repeat single-channel mode?

 

Thanks in advance

 

--------------------------

 

void main(void){

WDTCTL = WDTPW + WDTHOLD;

ADC_Init();

ADConversions();

}

 

 

void ADConversions(void){

index = 0;

ADC12IE = 0x01;                           // Enable ADC12IFG.0

ADC12CTL0 |= ENC;                         // Enable conversions

ADC12CTL0 |= ADC12SC;                     // Start conversion

_BIS_SR(LPM3_bits + GIE);                 // Enter LPM3,Enable interrupts

}

 

 

#pragma vector=ADC12_VECTOR

__interrupt void ADC12ISR (void){

Badc = ADC12MEM0;

index++;

}

 

 

void ADC_Init(void){

 

 

 P6SEL |= BIT1; 

  ADC12CTL0 = ADC12ON+SHT0_0+MSC+REFON+REF2_5V; 

  ADC12CTL1 = SHP+CONSEQ_2;

  ADC12MCTL0 = SREF_1+INCH_1;

}

 

  • 0
    Guru 17650 points

    Hi Esa,

    at what clock speed do you run the controller? In addition, you should think about using the DMA for ADC data handling.

    Rgds
    aBUGSworstnightmare

  • 0 in reply to Joerg Quinten
    Prodigy 70 points

    If I use default DCO speed, the conversion rate is about 30 ksps. If I use DCOCTL = DCO0 + DCO1 + DCO2, the conversion rate is about 50 ksps. Maybe I should use DMA so that no CPU interventions are required. I understood that it is not possible to use ADC memory registers to store 16 samples when repeat single-channel mode is used?

  • 0 in reply to Esa Korhonen
    Guru 12160 points

    Hi Esa,

    Your ADC configuration looks good as far as I can tell.  For the kind of speeds you want, don't let the DCO stop.  200ksps yields one conversion every 5 microseconds.  The MSP430 1xx can barely start the DCO in that time.  Even the DMA is powerless to keep up if you let the DCO stop.

    So as an experiment, don't use LPM.  Just do a while(1) loop.  Your ISR will probably keep up just for this experiment as long as you run MCLK fast.  Then work on switching to DMA.

    Jeff

  • 0 in reply to Jeff Tenney
    Guru 227245 points

    On 1X series, the typical maximum DCO frequency is about 5MHz. That's in the range of the ADC12OSC.

    So a conversion takes (fastest case) 17 (M/ADC10)CLK cycles (~ 3µs). That's almost less than entering and exiting the ISR takes, even if there is no LPM. For 200kHz, it is still 5µs and then there is the problem of properly clocking the ADC, as you need 13+x clock cycles per conversion, where x is 4, 8,16,32...) Well, 5.8MHz MCLK/ADC12CLK and a S&H value of 16 clock cylces would do the trick. Giving you 29MCLK cycles per conversion.

    Using the 16 memory locations won't help. It would relax the timing a bit, bu tnot much. Sure, you then had 16*29 * 464 MCLK cycles between two interrupts on teh first run, but then you'll have only 29 more to copy the first of the sampled 16 results, then another 29 to copy the next etc. So it's still tight. And then I'm sure you want to do something with these samples too.

    Teh use of DMA would allow you to samle a large number of values into a large memory array., costing only 4 MCLK cycles per transfer (including all the triggering). Giving you 25 MCLK cycles per sample for processign the data. That's a value you can hope to get things done with.

    If you use double-buffering, you might even find some time for going to LPM after you processed a buffer.

    You can, however, go for a newer MSP form the 5x series. It can run on up to 25MHz, which gives you more than four times as much processing power.

  • 0 in reply to Jens-Michael Gross
    Prodigy 70 points

    Thank you very much for good tips. Unfortunately, the solution was much simpler. I just added BCSCTL1 = RSEL0 + RSEL1 + RSEL2;  (and DCOCTL = DCO0 + DCO1 + DCO2;) and now the ADC takes 200000 samples per second. (I don't know how and why ;)

     

    void main(void){

      WDTCTL = WDTPW + WDTHOLD;

      DCOCTL = DCO0 + DCO1 + DCO2;

      BCSCTL1 = RSEL0 + RSEL1 + RSEL2;

      ADC_Init();

      ADConversions();

    }

  • 0 in reply to Esa Korhonen
    Guru 227245 points

    Esa Korhonen said:
    and now the ADC takes 200000 samples per second.

    Oh, yes, I didn't notice. The 155 is of the old 1x series. These devices run on <1MHz CPU clock per default. I always have my standard startup code setting the 1611 (which we still  use) to 8Mhz with external crystal, so I forgot this initial slow setting.
    The ADC was already taking the samples at the desired speed, yet the CPU was running so slow that it was unable to pick them up. With the typical 800kHz, there were only 4 clock cycles per conversion, which is just enough to do a single copy operation, but not for the loop around or for incrementing the counter, let alone anthing beyond that. No wonder you didn't get the 200kSps. For each result you got, the ADC has done and discarded several more.

    Esa Korhonen said:
    I don't know how and why

    Well, you kicked the MSP in the A**, making it run on highest possible frequency available from the DCO. Which is somewhere between 5 and 6MHz. Now the CPU core is fast enough to pick up the samples at the desired speed.
    But beware: on the newer MSPs, teh DCO supports frequencies much above the maximum operating frequency (>100MHz), so just setting everything to the max will only work on teh 1x devices up to 1612.

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