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New multi-vibrator in an old design

Other Parts Discussed in Thread: CD74HC4538

I am working with a legacy product that uses a Dual Mono-stable Multi-vibrator. The part designed into the product is an MC74HC4538AD. The component uses an old design architecture that defines pins 1 and 15 as ground pins. The capacitor in the RC timing network has one side connected to pin 2 (C/R) and the other side terminated to ground. Sources here would like to use a TI CD74HC4538M as a drop-in replacement. However the pin-out is a little different. Pins 1 and 15 are to be used to terminate the negative side of the timing capacitor instead of grounding the capacitor externally.

My questions are

Will there be any ill effects to the CD74HC4538M using it in the old style configuration with pins 1 and 15 grounded?

Why was this minor architectural change made to the device?

  • John,

    This would be better addressed in the logic forum. I'll move the post.
  • John,

    The CD74HC4538M is a pin-to-pin replacement for the MC74HC4538AD.  The two pins that you are referencing are internally connected to ground, and connecting them externally to ground will not affect the IC's operation.

    I can only theorize as to the designers' motives - but the change makes sense to me considering that the old version had a requirement to tie pins 1 and 15 directly to ground, and the new version has that done internally, removing that restriction.

    You can manually verify this by measuring the resistance between pins 1, 8, and 15 on a chip.  You can also see the internal connection in the logic diagram (figure 2) in the CD74HC4538 datasheet.

  • Thank you for the reply. This is a production issue and I needed verification before clearing it for production usage.