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I was reading this intro article:

https://learn.sparkfun.com/tutorials/logic-levels/all#active-low-and-active-high

  • Is there a particular time when an active low signal is preferred over an active high signal or vice versa?
  • Does it make a different for simple buttons/switches, or maybe ICs?

Just curious if one has benefits over the other.

Thanks!

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    \$\begingroup\$ I think it boils down to the fact that the ground net is (and always has been, to a large extent) ubiquitious in a circuit and you would want active to mean asserted. It's cheap and easy to assert an active-LO using an open-collector using almost any available supply rail other than ground. In the earlier days, active devices (tubes or transistors) were expensive. And the fewer, the better. Now, it's not such a problem. But the practice still makes sense as the ground net is still almost everywhere available. Just my opinion, though. \$\endgroup\$
    – jonk
    Nov 30 '18 at 5:21
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    \$\begingroup\$ electronics.stackexchange.com/questions/60401/… \$\endgroup\$
    – EasyOhm
    Nov 30 '18 at 6:21
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    \$\begingroup\$ @jonk, that's the answer I was looking for, thanks! \$\endgroup\$ Nov 30 '18 at 22:02
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Many things come into play when choosing whether a switch is active high or active low. As others have mentioned, it can depend on your IC and what it needs, safety concerns, other circuit needs, industry standards, or if none of these are relevant, personal preference.

Is there any safety related to your device, and do you need a "dead man" switch? In other words, if part of your circuit fails to work properly, which state does the switch need to default to in order to prevent something dangerous from occurring?

For example, say you have a circuit that controls a crane holding a large weight with a quick-release switch. If, for some reason, the trace were to be broken or a component removed, causing the switch to no longer be "forced" to a certain state, which state would it default to? You would want to set up your circuit so that it defaults the "inactive" state, whether high or low for the given device. Pull-up and pull-down components would also come into play here.

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Active low is much more common. I pretty much always wire switches that way, and most switches I see on other peoples' designs are also active low. I don't actually have a good reason why. I think conceptually active low switches are similar to "open-collector" outputs, and people are accustomed to thinking about open-collectors. That may be the only reason.

I only remember doing active high once, and the reason was that the button had to wake up the processor from deep sleep, and it needed and active high signal to do that.

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Selection of enable polarity largely depends on desired behavior of whatever is controlled during power up of the device.

For example, if you are using some high-side voltage switch that delivers some power to another device (USB port power as example), and if you want to avoid a glitch on power on, you select "active high", because the control signal is likely to stay at zero for some time until your MCU boots and start running. A small capacitor to ground will ensure that the signal stays at logic zero.

If you use "active low", the control signal will stay low until MCU/controllers comes to mind, which will temporarily enable the controlled unit. Practically it all depends on ramping rate of the power supply rails, various built-in "undervoltage protections", etc., so it all depends, and no universal recommendations are possible..

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  • \$\begingroup\$ This is not entirely true. It depends on the circuit which polarity can cause glitches. The active low signal usually has a pull-up resistor, which pulls the signal high when the voltage rail rises. If the pull-up is to the correct rail, it will not cause glitches. \$\endgroup\$
    – TemeV
    Nov 30 '18 at 6:55

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