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I have a situation where I tried setting pins on my Atmel Mega328P to INPUT_PULLUP. I was told to expect an inverted input: a +5V would result in LOW reading and a GND would result in HIGH reading. However, what I found is the exact opposite. I tried checking my +5V and GND with some LEDs and they are definitely +5V and GND.

Below is my arduino code; I know it's not very efficient, but that's besides my point. My IDE is Eclipse C++.

#include "Arduino.h"
int count = 0;
void setup() {
    Serial.begin(115200);
    pinMode(PIN2, INPUT);
    pinMode(PIN4, INPUT);
    pinMode(PIN5, INPUT_PULLUP);
    pinMode(PIN6, INPUT_PULLUP);
}

void loop() {
    byte i = 0;
    if(digitalRead(PIN2) == HIGH) i += 1;
    if(digitalRead(PIN4) == HIGH) i += 2;
    if(digitalRead(PIN5) == HIGH) i += 4;
    if(digitalRead(PIN6) == HIGH) i += 8;
    Serial.println(i, BIN);
    delay(1000);
}
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    \$\begingroup\$ Why are you expecting an inverted input? If I remember right, the input_pullup mode just connects an internal pullup resistor, no? \$\endgroup\$ – Hearth May 5 '17 at 1:10
  • \$\begingroup\$ Perhaps you are mis-remembering a comment about what would happen with a pullup and a swtich to ground - switch "off" is a "1" while switch "on" is a "0" \$\endgroup\$ – Chris Stratton May 5 '17 at 1:27
  • \$\begingroup\$ Sorry, I actually mis-remembered. Thanks for pointing out though. \$\endgroup\$ – krismath May 5 '17 at 1:56
  • \$\begingroup\$ Should I remove this question then? \$\endgroup\$ – krismath May 5 '17 at 2:18
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    \$\begingroup\$ Accepting an answer is generally a better way to mark it resolved; the available one isn't outstanding but it isn't wrong either. \$\endgroup\$ – Chris Stratton May 5 '17 at 2:53
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It isn't possible with the Atmega328P to switch how the IO pins translate a voltage to a binary value in a register. (And the internal pull-up has nothing to do with this)

Just check for the opposite condition in the code:

if(digitalRead(PIN5) == LOW) i += 4;
if(digitalRead(PIN6) == LOW) i += 8;
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