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| Is it safe to connect inputs to VCC directly |
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| MarkF:
Very very old... |
| ataradov:
But what does SCR latch-up has to do with connecting inputs to the VCC directly? |
| wraper:
--- Quote from: David Hess on February 04, 2020, 11:12:36 pm ---If multiple Vcc supplies are used then that would be a good reason to use series input resistors to limit the current but in the common case, I do not know of any reason the input cannot be directly connected to either power rail. Early TTL is a special case because of its common base multiple emitter input structure which required series resistors if an input was pulled up to Vcc. --- Quote from: wraper on February 04, 2020, 09:35:29 pm --- --- Quote from: EEEnthusiast on February 04, 2020, 04:08:58 am ---Direct shorting of inputs to Vcc or Ground can lead to latchup in some CMOS families. It is always safe to use some pull-up to limit this current. --- End quote --- How resistor is supposed to limit current through MOSFET gates when latch-up does not even cause current flowing through them? --- End quote --- It does not have anything to do with the gate; it is insulating after all. Latchup occurs when carriers are injected into the substrate and they activate the parasitic SCR structure present in a junction isolated CMOS process. This can happen if the input or output ESD protection diodes are forward biased with enough current. I have personally witnessed it a few times when static jumped between a finger and a CMOS logic board and one CMOS IC became an SCR crowbar across the power supply. --- End quote --- Exactly, you need to bring terminals above Vdd or below GND. Direct input connection to the power rail actually prevents it. So I don't get what current is supposed to be limited by using resistor instead of direct connection. Certainly not current which flows through IC when latch-up already happened. |
| tkamiya:
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. |
| james_s:
It's just good practice to do so, it can prevent strange problems. A lot of modern "TTL" stuff is actually CMOS. |
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