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Questions about adc_ex6_soc_continuous_dma routine

Katherine Wang
Intellectual 2210 points
Part Number: TMS320F28379D
Other Parts Discussed in Thread: TMS320F28335

Hi Team,

Question 1

As shown in the figure above, the source address of DMA channel 2 in the routine is ADCBRESULT_BASE. This might be incorrect as the ADC module of ADCD is used. Could you please check it for me?

Question 2

As shown in the figure above, ADCA and ADCD are sampled synchronously, so ADCA2 (ADCA unit's interruption 2) and ADCD2 (ADCD unit's interruption 2) occur at the same time, but in principle, using DMA_TRIGGER_ADCD2 as the trigger signal of DMA channel 2 is more reasonable, isn't it?

Question 3

The default sampling window of the project is 75nS. I don't know why in the routine the sampling window is set as such, but for the LaunchPad development board of TMS320F28379D, this sampling time is too short. The sampling results of the input signal of 1.632V are as follows.

In theory, with an input voltage of 1.632V, a value of 2228 should be generated. But the value in the figure is obviously too small.

After increasing the sampling window, accurate results can be obtained, as shown in the figure below. ACQPS is set as 163.

My calculation is based on the relevant chapter 11.15.2 Choosing an Acquisition Window Duration in TRM, as shown in the figure below.

Since each ADC channel of the LaunchPad does not have Rs and Cs, I assumed a set of values, so it may not be accurate. However, I would like to remind everyone to calculate the minimum sampling window when using the ADC unit and be aware that the Cp value of each channel is different.

The CH value of the ADC channel of TMS320F28379D is 14.5pF, while the CH value of the ADC channel of TMS320F28335 is 1.64pF. Although the Ron resistance of TMS320F28379 is 425Ω, which is smaller than the Ron resistance of TMS320F28335, namely 1KΩ. But as the CH value of TMS320F28379D is relatively large, it is recommended to use Relatively smaller Rs resistor to reduce the minimum sampling window to reduce the impact on time-sensitive applications.

Question 4

I don't understand the function of the highlighted statement in the code snippet above well. After commenting out, the code runs well without any abnormality

It seems that the initialization of counter 2 is enabled.

But the routine does not use counter2. I did not find where the SOCPSSEL is configured as 1.

Question 5

Will SOCBCNT2 (SOCACNT2) be cleared when SOCB (SOCA) is generated? If it will not be cleared, is it necessary to manually clear it?

Question 6

The srcStep is -14 because 16 result registers are transmitted in 32bit size. The address pointer of the last transmission points to ADCRESULT14. Thus, when starting a new burst transmission, the address pointer needs to be subtracted by 14 to make the pointer point to ADCRESULT0. Is my understanding correct?

The destStep is 2. My understanding of this is: because it is transmitted in 32bit size, the address pointer points to addr1.14 in the last transmission, and 16 16-bit results are transmitted each time, so it is necessary to make addr1.14+2 = addr2.0 to start a new storage of 16 16-bit results. Is this understanding correct?

Kind regards,

Katherine

  • 0
    TI__Mastermind 30185 points

    Hello Katherine,

    For your first question, the code does conflict with the comments above it. I will request this change be made. Please give me another day to look into your other questions.

    Best regards,

    Omer Amir

  • 0
    TI__Mastermind 30185 points

    Hello Katherine,

    To answer your questions:

    1. I've made the request for this to be fixed
    2. Because ADCA/D are used synchronously and their end of conversion will happen at the same time, it doesn't really matter which one you pick
    3. It seems like the example got designed with an op-amp or driver that is not native to the launchpad, and so that may be why it's set up the way it is. Calculating your sample and hold window is the safest way to go about this.
    4. This is more of an ePWM question, so I can include another expert to help out with this.
    5. Looking at the reference manual it seems SOCBCNT2 gets cleared when SOC B pulse is generated
    6. Your understanding is correct

    Best regards,

    Omer Amir

  • 0 in reply to Omer Amir
    TI__Intellectual 2210 points

    Hello Omer,

    Thank you for your reply.

    Then, the only remaining question is Question 4, and frankly speaking, I am not very clear about the functions of Event Counter INIT.

    Regards,

    Katherine

  • 0 in reply to Katherine Wang
    TI__Mastermind 30185 points

    Hello Katherine,

    After talking with someone who has worked with ePWM a little more than me, it seems that line 315 is only really necessary when you are triggering the ADC multiple times. Because the example only triggers the first SOC once and then disables the ADC interrupt, commenting out line 315 does not affect the results. However, if you comment out line 240 where the ePWM ADC trigger is disabled, then commenting out the ADC Trigger Event Count initialization function should produce incorrect results/behavior. The example's comments do not seem to correctly reflect this, so I will try to put in a fix for it.

    Best regards,

    Omer Amir