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SN74LV123A: Differences in data sheet description

cafain
Mastermind 6755 points
Part Number: SN74LV123A
Other Parts Discussed in Thread: SN74LVC1G123, SN74LV221A

SN74LV123A Differences in data sheet description

Hi, Staff

I was investigating the data sea about the terminal processing method when not using Rext/Cext, and there was a difference in the contents.

The predecessor designer referred to the SCLS393O data sheet when designing.
page5 NOTE5:
Unused Rext / Cext terminals should be left unconnected.
All remaining unused inputs of the device must be held at VCC or GND to ensure
proper device operation. Refer to the TI application report, Implications of Slow or Floating CMOS Inputs, literature number SCBA004.

Designers in recent years have designed with reference to the SCLS393Q data sheet.
page5 (1)
All unused inputs of the device must be held at VCC or GND to ensure proper device operation. Refer to the TI application report,
Implications of Slow or Floating CMOS Inputs, SCBA004.


The SCLS393O datasheet states that Rext/Cext is Open,
The SCLS393Q datasheet does not describe the Rext/Cext terminal processing method, and all unused terminals have Vcc or GND.

Q1:
If you are currently designing, how do you handle unused terminals in Rext/Cext?

Q2:
What is the reason why the datasheet of SCLS393Q does not describe the unused terminal processing of Rext/Cext?


best regards
cafain

  • 0
    Guru 317260 points

    The Cext terminals (pins 6 or 14) are connected to ground; the Rext/Cext terminals (pins 7 or 15) are analog inputs. That footnote applies only to digital inputs.

    To prevent the analog inputs from floating, you can simply short them to the corresponding Cext pin.

    Anyway, the easiest way to handle an SN74LV123A with unused inputs is to replace it with the SN74LVC1G123.

  • 0 in reply to Clemens Ladisch
    Mastermind 6755 points

    Ladisch san

    Thank you for your prompt reply.
    e2e has similar questions and different answers, so please check again.

    The question is SN74LV123A、I think the following is a function block diagram of SN74AHC123.
    In this function block diagram, it is thought that the operation according to the datasheet cannot be performed unless CLR = GND and Rext/Cext are open, or CLR = GND and Rext/Cex = VCC (pull up to VCC with an or resistor).

    Also, on the same functional device SN74LV221A, the same question was answered on e2e as follows.
    This is inconsistent with your answer.

    e2e.ti.com/.../faq-how-do-i-terminate-an-unused-channel-of-a-monostable-multivibrator-one-shot

    Also, looking at the Figure 7 Input and Output Triming Diagram of the SN74LV123A below, Rext/Cext is forcibly set to High when the CLR changes from Hi to Low.
    From this, the operation similar to the above function block diagram can be considered.

    best regards
    cafain

  • 0 in reply to cafain
    TI__Guru* 97046 points

    Hi cafain,

    From the revision table in the current datasheet:

    You can see that there were many formatting changes and additions to the datasheet from rev O to rev Q. It appears that "NOTE 5" from the original datasheet was lost in the conversion to the new format and was replaced by a more generic statement used in most CMOS logic datasheets. I will mark this in our errata to be fixed.

    -

    Clemens is correct that the Rext/Cext pin is an analog input and thus does not require termination, however we do recommend terminating at VCC as explained in the FAQ because it prevents the floating node from causing internal fluctuations / oscillations if the voltage crosses the internal threshold of the comparator. These changes will not cause damage to the device, but they can cause some additional noise and power consumption.

  • 0 in reply to Emrys Maier
    Mastermind 6755 points

    Hello Emrys san

    Thank you for your comment.

    Vcc processing is desirable for the Rext / Cext pin to prevent oscillation due to noise near the threshold value of the comparator.
    I understand as above.

    In the previous answer, the connection to GND was commented, but the connection to Vcc is preferable.
    I understand as above.

    best regards

    cafain

  • 0 in reply to cafain
    TI__Guru* 97046 points

    Hi cafain,

    Yes, I believe your understanding is correct.

    If this resolved your issue, please click the green "This resolved my issue" button on the appropraite post.