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ADS7951 programming - number of frames needed for Auto-2 mode

Steve April
Prodigy 100 points
Other Parts Discussed in Thread: ADS7951

For the ADS7951, I need to capture all channel with auto increment starting from Ch0.  During this capture, the range settings will not change for each channel.  

As I understand the specification:  Auto-2 mode is entered by programming the first 3 frames with SDI contents as described in Table 5. At the end of 3rd frame, the ADC enters Auto-2 mode.   From 4th frame onwards I do not need to send any data on SDI (continued operation mode) and SDI should be low.  In this mode, the ADC will auto increment channel with each frame.  

What I am seeing on the bench is a little different:   In my case, I only need to program one frame, not three as described in the specification.  I perform the same procedure as outlined above, but instead of three frames, the ADC seems to enter Auto-2 mode only after one frame.  From 2nd frame onward I do not need to send any data on SDI and SDI is low.  In this mode, it the ADC appears to be auto incrementing channel with each frame.  Is there an explanation as to why only one frame seems to get the device into Auto-2 mode VS the three frames described in the specification?  

  • 0
    TI__Expert 4405 points

    Steve,

    Your interpretation of the datasheet is correct. The behavior you are observing is more to do with the implementation of the mode selection logic in the ADS7951.

    For every frame, the D15..D12 bits received on SDI are decoded to determine the intended mode of operation. If this is different from the current mode, the Mode Control Register is updated. If these bits are the same as the current mode, or if they are all zeros (as described in Section 8.4.6 and Table 7), the device remains in the current mode.

    So strictly speaking, after a power cycle the Mode Control Register needs to be updated at least once, unless you intend to keep the device in the default Manual mode. From that point on, the D15..D12 bits can be written for either the current mode or all zeros, with no impact to mode.

    However, since the first data in the newly selected mode is only available from the 3rd frame onwards, the first 3 frames are explicitly described in the datasheet.

    Regards,

    Sandeep