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Original question:

DAC8742H: Hardware changes to use HART and Field bus

DAC8742H: Connecting an external CMOS clock.

Jan Stewart Rambech
Prodigy 10 points
Part Number: DAC8742H

Hi !

We are using the DAC8742H in HART mode and have configured XEN, CLK_CFG0 and CLK_CFG1 to accept an external CMOS clock source (generated with a function generator) running at 3.6864 MHz (line 1 in Table 1 on page 16 of the data sheet). In accordance with the description of this configuration, we have connected the CMOS clock between XTAL1 (X1, pin 28) and GND. The CMOS clock has a peak-to-peak amplitude of 3.0 V and an offset of + 1.5 V, so the logic low level is 0 volts and the logic high level is + 3.0V. Se screen shot #1.

However, when we connect the CMOS clock in this way to pin X1, the clock signal as measured at this pin is level shifted up to IOVDD (+ 3.3V) and appears to be ac-coupled, see screen shot #2. Does this mean that the CMOS clock signal must be ac-coupled to pin X1? I can not see that the data sheet mentions this.

Have a nice day and thank you in advance for an illuminating answer !

Best regards,

Jan

  • +1
    TI__Guru* 93945 points

    Jan,


    I'm not sure what it happening here. Can you send a schematic of the connections to the device? Just to confirm, the /XEN pin is high, while the CLK_CFG1 and CLK_CFG0 pins are grounded? If you are using an external clock, the clock signal should be connected directly to the XTAL1 pin. It should not need to be AC coupled into the device.

    There is one issue that I noticed in the second plot. In the scope shot that shows the connected clock, the clock signal goes from about 2V to 5V. With a 3V clock and IOVDD of 3.3V, the signal should never go that high. Is there voltage on the board that goes as high as 5V? I would verify that the ground connection from the board to the clock source is correct. What are you using as a clock source?


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 10 points

    Hi Joseph, 

    Thank you for your quick and informative response. This is somewhat embarrassing, but one of my good colleagues has just checked the connection between the HART device and the external clock source and found that I had connected it to pin 30, REF_EN, instead of pin 28, . REF_EN is tied to + 3.3V.Rolling eyes

    As I said, embarrassing.

    Thank you again, and sorry for taking up your time.

    Best regards,

    Jan 

  • 0 in reply to Jan Stewart Rambech
    TI__Guru* 93945 points

    Jan,

    Those things do happen! Thanks for letting me know. 

    If you have any other questions please let me know.

    Joseph Wu