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ADS8556: We would like to set the "CR" only once, retain the "CR" setting, and obtain only the AD conversion data "SDO_x" every time.

Shinichi Inoue
Guru 22016 points
Part Number: ADS8556


Tool/software:

Dear Specialists,

My customer is considering ADS8556 and has a question.

I would be grateful if you could advise.

---

We have a question about Figure 2. Serial Operation Timing Diagram (All Three SDOs Active) on page 15 of the ADS8556 data sheet.

In this timing table, it looks like the "Control Register (CR)" bit needs to be set on SDI every time the ADC is referenced. Is that correct?

I would like to set the "CR" on the ADS8556 only once when powered on, and then retain the initial "CR" setting, and obtain only the AD conversion data "SDO_x" without having to set the "CR" every time the ADC is used.

If this is possible, please let us know how to set it up.

We plan to use it only in Software Mode.

ｰｰｰ

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Genius 9766 points

    Hi Shinichi-San,

    Thanks for your question. It looks like CR must be updated with every read/write access when the ADC is used with software mode with serial interface (as the CR is updated with every 32 SCLKs). Parallel interface in combination with software mode may allow more flexibility to not have to update the CR every time. However, once you set the CR once, the best way to use the device without having to set CR again would be to switch to hardware mode. 

    Hope this helps.

    Best regards,

    Samiha

  • 0 in reply to Samiha Sharif
    Guru 22016 points

    Hi Samiha-San,

    Thank you for your reply.

    I understand that in order to retain the data that has been set in the CR, it is necessary to change to hardware mode.

    I'll share this information with the customer.

    When the customer has an additional question, I consult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 22016 points

    Hi Samiha-San,

    I shared your suggestion with the customer.

    Then the customer has additional questions.

    Could you please advise?

    ---Questions

    Question 1
    In serial interface + software mode, if you set the "CR" bit once,
    and then fix SDI to "0" (SDI to GND level),
    is it correct to understand that only the bit values ​​in the "CR" bit C[31:24] range will be updated by SDI due to the "CR" bit C[16]"C23:2_EN"="0"?

    Question 2
    When setting the "CR" bit (HW/SW(62pin)="H"), setting REFDAC to C[9:0]=0x3FF(2.5V) and RANG to C[28:26]="1"(2×Vref) to ±5V as the input range, and then setting HW/SW to "L" to change to the hardware setting, do you also need to set RANGE(27pin) to "H" at the same time?

    Currently, I am not using an external clock in software mode, so I always connect it to "GND".

    Question 3

    Currently, the input range is REFDAC C[9:0]=0x3FF (2.5V), RANG C[28:26]=all "1" (2×Vref), and ±5V is used as the input range, so it is necessary to set "1" during this period. If I set C[9:0]=0x1FF (1.25V), can I fix SDI to "0" (C[28:26]=all "0" (4×Vref)) after setting the "CR" bit and maintain the exact same state as it is now?

    Please let me know if there are any changes to the measurement specifications such as resolution and offset.

    ---

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Genius 9766 points

    Hi Shinichi-San,

    Thanks for the questions! Here is my feedback:

    1. Yes, you are correct.
    2. Yes, if the device is configured in hardware mode during data capture, the RANGE pin should be set high for 2xVref range.
    3. The issue with leaving SDI = 0 here would be that you may want C29:C31 to be 1, as otherwise the channels would be disabled. Also C25 = 1, to enable the internal reference. The best thing would be to set the CR as needed in SW mode, switch to HW mode, and either leave the RANGE pin high or low depending on the settings in SW mode. If you select the reference to be 1.25V in SW mode, you could leave the RANGE pin = 0 in HW mode as 4 x 1.25V would reach your desired +-5V range. Resolution and offset should be comparable.

    Best regards,

    Samiha

  • 0 in reply to Samiha Sharif
    Guru 22016 points

    Hi Samiha-san,

    Thank you for your reply.

    I'll share your answer with the customer.

    When the customer has an additional question, I consult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi