TMS320F28388D: Changing the ADC resolution does not change the processing time.

Hi Guoki,

How were you able to measure 1.8uS exactly?  Please refer to the timing diagrams in TRM section 20.  ADC throughput calculation starts from when SOC trigger is received.  This will connect the input channel to the ADC sampling capacitor for charging for a duration of ACQPS+1 (ADC H+H in the timing diagram) and after this duration has passed the sampling switch is opened and the charge in the sampling capacitor will be quantized by the ADC module for the duration of tLAT (in the timing diagram).  After tLAT, the ADC results will be available in the ADC results register.  There are two timing tables in the same TRM section, one for 12-bit and another for 16-bit mode and you can get tLAT information from the timing tables depending on ADC prescaler used.

Based from the settings you provided (and assuming that SYSCLK is 200MHz, 5nS period) and using the timing diagrams and tables, the time it takes for the results to be available in the results register is given below:

12-bit: t = ((ADC S+H) + tLAT) * 5nS = (19 + 1 + 44)*5nS = 330nS

16-bit: t = ((ADC S+H) + tLAT) * 5nS = (68 + 1 + 120)*5nS = 945nS

If you are not getting above conversion times, check that your measurement methodology is measuring the actual time from when trigger is provided until time when interrupt from end of conversion (tEOC) has occured.  There are probably some overhead that are included in your routine that adds up to the conversion time that you are measuring.

Regards,

Joseph

Hi Joseph.

I use an oscilloscope to check.
It outputs the "Timer Start - Timer End" time described in (Process) in the main text.
Regardless of the resolution, the same time is output here.

I thought the interrupt flag would turn ON in "330ns*2" time when using 12bit, but
It is taking the same time as at 16bit.

As for the trigger, it is based on PWM, but in the form that the PWM interrupt operates at the same time as the trigger.
ADC interrupt flag clearing is done in that.
The PWM cycle is 25us cycle.

(In 12bit, ADCINT1 == 1 before initializing the ADC flag, so the judgment is performed only by ADCINT2.)
================================================================
__interrupt void epwm1_isr()
{
  GpioDataRegs.GPASET.bit.GPIO21 = 1; // Timer Start

  /* ADC Interrupt flag Clear. */
  AdcaRegs.ADCINTFLGCLR.bit.ADCINT1 = 1;
  AdcaRegs.ADCINTFLGCLR.bit.ADCINT2 = 1;

  flag1 = 1;

  // Clear INT flag for this timer
  EPwm1Regs.ETCLR.bit.INT = 1;
  // Acknowledge this interrupt to receive more interrupts from group 3
  PieCtrlRegs.PIEACK.all = PIEACK_GROUP3;
}

main()
{
  (Setting)

  while(1)
  {
    if( flag1 == 1 ){
      while (1) { if ( AdcaRegs.ADCINTFLG.bit.ADCINT2 == 1 ) break; } // check ADCINT2
      GpioDataRegs.GPACLEAR.bit.GPIO21 = 1; // Timer End
      flag = 0;
    }
  }
]
=================================================================

Since this is the first process after the interrupt flag,
and since there is no other interrupt present,
I do not expect it to be disturbed.

Regards.