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I noticed that pullup resistors for reset pins that are pulled to ground with a switch are commonly placed between the power rail and the pin, for example the EN pin on an ESP32.

Could the resistor just as well be placed between ground and the switch, with the reset pin connected directly to the power rail?

I always see it done this way, so I assume there's a reason to not do it the other way?

pullup resistor placement

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Could the resistor just as well be placed between ground and the switch, with the reset pin connected directly to the power rail?

enter image description here

Figure 1. The pull-down resistor idea.

I suspect that if you had drawn the circuit out as shown in Figure 1 that you would see the error of your thinking. (Note that it makes no difference whether R3 is above or below SW1 as it is a simple series circuit.)

When SW1 is press EN is still at V+ and R3 is now connected between V+ and ground so, for a 5 V supply 0.5 mA will flow through it. EN will not be affected.

The idea with a pull-up is that it is strong enough to hold EN high in normal conditions but weak enough that when a direct short to ground is applied only a limited current will flow. You can think of R3 in your original diagram as acting a little like a rubber band pulling EN high and when EN is pulled low the band stretches without harm and pulls EN high again when it is released.

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  • \$\begingroup\$ I interpreted the part of the question you've quoted differently. I think your schematic still needs to swap R3 and SW1, though the result is effectively the same. \$\endgroup\$ – Kevin Kruse May 29 at 17:44
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    \$\begingroup\$ It was a Paint job so I was limited in how much manipulation I was prepared to do! I explained in the first paragraph that order of R3 and SW1 make no difference. Thanks for the feedback though. \$\endgroup\$ – Transistor May 29 at 17:48
  • \$\begingroup\$ Indeed now that I see it like this it's pretty clear that EN wouldn't be affected. Thanks for taking the the time! \$\endgroup\$ – Marc Liyanage May 29 at 18:30
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What you suggest won't work. The resistor is meant to keep reset voltage high when button is not pushed, and when button is pushed the reset pin voltage goes low.

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