Cannot detect interrupt properly on a pulled up line

Question

This is my simple setup

^{simulate this circuit – Schematic created using CircuitLab}

This is my test program, using nodejs and rpi-gpio.

var gpio = require('rpi-gpio');

gpio.on('change', function(channel, value) {
    console.log('Channel ' + channel + ' value is now ' + value);
});
gpio.setup(36, gpio.DIR_IN, gpio.EDGE_BOTH);

I expect when I close the switch the voltage at point A to drop to 0 and trigger a falling edge interrupt, however all I see in my program is rising edge interrupts, i.e.:

Channel 36 value is now true
Channel 36 value is now true
Channel 36 value is now true

I had my doubts, as this is not the best library for GPIO manipulation, that there might be some kind of bug, but I physically separated GPIO16 from A (pulled out the jumper cable on the pin) and then I saw a falling edge interrupt

Channel 36 value is now true
Channel 36 value is now true
Channel 36 value is now true
Channel 36 value is now false

I also measured the voltage at point A which is about 2.7V regardless of whether the switch is open or closed.

Something that may be worth mentioning is that the distance between A and SW1 is about 4 meters. And that's also why I'm pulling up, as I can't move SW1 physically and there is already ground there, this way I'm saving 1 cable of about 4 meters.

I'm missing something which is probably really silly, but oh well..

Oh and I'm aware there are internal pull-up resistors but I'd rather use external ones.

Answer 1

Apparently all pins have enabled pull up/down resistors by default, listed in this document on page 102.

The pins I was using have a default pull-down setting, which explains the 2.7V at A given that the internal resistors are 50k.

I don't know how, but removing the default pull-down resistance solved my problem. Usually more advanced libraries have APIs for this but in my case I had to edit /boot/config.txt and add the following line

gpio=16,19,26=pn