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| Is it safe to connect inputs to VCC directly |
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| MasterT:
--- Quote from: Siwastaja on February 05, 2020, 01:56:46 pm --- --- Quote from: MasterT on February 05, 2020, 01:51:48 pm ---If I asked, is it safe to apply 5.5V? Everyone would say, yes, DS specifies acceptable voltage level. But reliability numbers would fall from 10^-7 to 10^-6 or like that. So, from reliability point of view, increasing voltage (or using safety resistors) should be considered based on Acceptable failure rates. And this strongly depends on a market, cost of device, and things that difficult to estimate - brand name ratings. Nobody would care if led-blinking toy breaks down, OTOH, if a scope failed because someone save on single resistor - would you buy another one from this maker? --- End quote --- What does this comment has to do with this topic, though? And what the heck is a "safety resistor"? Do note that sprinkling resistors randomly without a reason won't increase reliability, and if they are in series with actual nets (instead of, say, on the side of the board unconnected), each resistor slightly decreases the reliability (not by much, of course). I would expect the scope manufacturer to only design in resistors that are needed or do good for the circuit, and don't put in any excess. --- End quote --- May be won't. Since you don't provide specific numbers to estimate reliability, and me nether, what I'm trying to say is that question has no sense w/o strictly confidential manufacturer's reliability report for specific part number. How much failure rate goes up if not follow to recommended design rules. OP says, that doesn't need a highest military/ aerospace reliability marks. But consumer product is also demands reliability numbers not equal to zero. |
| TimFox:
This question probably arises because of a real feature of original 7400-series TTL, which was actual TTL, using a multiple emitter input transistor instead of DTL diodes to do logic. The breakdown voltage of the input structure was less than the smoke level of the rest of the circuit. Therefore, with a direct pull-up to Vic, there was an unnecessary risk of damage if Vcc overshot on power-on. The situation is different with later bipolar and CMOS families. |
| Wimberleytech:
--- Quote from: thinkfat on February 05, 2020, 03:03:57 pm --- --- Quote from: Wimberleytech on February 05, 2020, 02:44:07 pm --- --- Quote from: ataradov on February 04, 2020, 03:56:01 am ---Ok, I did some searching and I can't find a good definitive answer. Is it safe to connect to inputs of the 74-series CMOS logic directly to VCC instead of using pull-ups? What are realistic pros and cons? I'm not interested in what may happen if device fails catastrophically and the input becomes an output and stuff like that. I'm only interested in normal operation. I'm also not interested in TTL logic. The devices will never be substituted on accident. Seriously, some people still make this argument. I guess the question may be rephrased as: are there realistic use cases where it is not safe to connect inputs directly to VCC? --- End quote --- My answer is: Other than for reasons of testing, there is no realistic use case for using a resistor pullup (versus a direct connection) to VCC. I read this thread before answering. The testing idea was not something I thought of, but it is an interesting idea. For production, you would not want to waste lots of resistors for testing features used only during qualification and/or debug. --- End quote --- Unless you're really space-constrained, is it really an issue to have a couple of additional resistors sprinkled onto the board? Especially if they're not introducing another unique BOM item. --- End quote --- A couple? No big deal. Ten? Dunno. This boils down to designer's choices really. There is no "correct" answer I suppose. |
| wraper:
--- Quote from: thinkfat on February 05, 2020, 03:03:57 pm ---Unless you're really space-constrained, is it really an issue to have a couple of additional resistors sprinkled onto the board? Especially if they're not introducing another unique BOM item. --- End quote --- Adding parts "just because" without any understanding or measurable evidence of any benefits is not a good design practice IMHO. Over-engineer here and there and result is simply worse, as minimum because of larger size and manufacturing price. |
| wraper:
--- Quote from: Wimberleytech on February 05, 2020, 07:03:36 pm ---There is no "correct" answer I suppose. --- End quote --- IMHO the correct answer is what isn't needed shouldn't be there. You'd better add some real ESD protection where it might help instead of sprinkling useless resistors. |
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