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Is it safe to connect inputs to VCC directly

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tkamiya:
We've been talking about resisters.  Like what values?  Normal pull up like 470 ohm to 10K ohm range?  And the "test" that has been referred to, they are extreme tests like overlaying KV pulse on power line, etc, or zap the input with huge noise.  Right?  We are not talking about normal usage in office.

David Hess:

--- Quote from: ataradov on February 04, 2020, 11:29:24 pm ---But what does SCR latch-up has to do with connecting inputs to the VCC directly?
--- End quote ---

If you had more than one Vcc supply, which used to be more common, and the input was connected to the other one, then power supply sequencing could cause SCR latch-up.  Now this would be more likely to happen at the interface between different voltage domains.


--- Quote from: tkamiya on February 04, 2020, 11:45:59 pm ---And.....  Why do input of TTL logic gate have to be pulled up or down?  It is completely safe to just leave it unconnected.  At least I've never done it and never seen any one else do it that way.  I thought the practice was for ?mos type devices only.
--- End quote ---

Some parts in the later bipolar TTL families, I think the ones built on complementary bipolar processes so some AS and FAST parts at least, had buffered inputs and present a much higher impedance to the input making them susceptible to capacitive coupling into an unconnected input.  I would not put it past having the same problem with advanced low-power schottky or even low-power schottky parts.

tkamiya:
Thank you.  Learned something new today.  It was a simpler time when chip was made by Texas Instruments and started with 74, it was always TTL, and if the chip was made by Motorola and started with 4, it was always CMOS.  Yes, I'm that old....

aix:

--- Quote from: ataradov on February 05, 2020, 02:37:45 am ---Let's say I use 4-NAND and I only need 3 inputs.

--- End quote ---

I do realise that your question is broader than this.  However, this particular example got me wondering: to get 3-NAND from 4-NAND, what are the pros and cons of tying one input high vs tying two of the four inputs together.

ataradov:

--- Quote from: aix on February 05, 2020, 06:25:54 am ---I do realise that your question is broader than this.  However, this particular example got me wondering: to get 3-NAND from 4-NAND, what are the pros and cons of tying one input high vs tying two of the four inputs together.

--- End quote ---
I have no idea. I don't think there is a significant difference. Intuition tells me that having one input fixed is better, it avoids one more thing changing, but I don't know for sure.

In this case question came up for XOR gates, so no way to tie things together.

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