Digital pins only outputting 0.013v

Using Arduino Project Guidance
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Hello, I'm doing a project with a arduino mega, where I use 20 of the digital pins as output.
When I set the 20 pins pinMode to output and set one to high, I only get 0.013v.
When I don't set a pinMode, but just set one high, I also only get 0.013v. Only when I only set one of the pins to output, and set that one to high, it outputs about 5v.
I've tried both with a USB connection and with a 12v, 1a power supply.
What could cause this?

2

Until you specify a given pin as an output with pinMode(), it will be an input ( the default on power-up) and will not output a logic level.

Allan

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What do you have attached to the outputs?

4

APersonH:
When I set the 20 pins pinMode to output and set one to high, I only get 0.013v.

Just to make sure, you set (i.e., dont know all the digital pins on the mega)

pinMode(5,OUTPUT);
pinMode(5,OUTPUT);
...
pinMode(25,OUTPUT);

and then

digitalWrite(5,HIGH);

you measure on pin 5 then, right?

It wasn't clear to me which Pin you set HIGH and which pin you perform the measurement on. maybe some code?

5

In fact even if a pin set to INPUT mode is written HIGH to, a multimeter should measure 5V on that pin (thanks to the built-in pull-up resistor).

My preferred sketch to check the working of a pin: blink. Just set the pin to test as LED pin, upload the sketch, connect your multimeter, and you should see the voltage go from 0V to 5V.

6

When I set the 20 pins pinMode to output and set one to high, I only get 0.013v.

An you post the code for that please. Use the select all the copy for forum on the computer and paste the result into the reply.

7

You know how to measure a voltage?

Are you sure that your sketch uploaded properly and is running?

8

wvmarle:
In fact even if a pin set to INPUT mode is written HIGH to, a multimeter should measure 5V on that pin (thanks to the built-in pull-up resistor).

Surely not unless you specified pinMode( , INPUT_PULLUP) ?

Allan

9

allanhurst:
Surely not unless you specified pinMode( , INPUT_PULLUP) ?

Allan

Instead you can write:

pinMode(pin, INPUT);
digitalWrite(pin, HIGH);

Which would be the case if OP forgot to set the pinMode to OUTPUT, but does digitalWrite HIGH to the pin. A multimeter will read 5V. A connected LED would light up very dimly. Switching a MOSFET would probably not work at all.

One of the odd and confusing Arduino things... Look at how the underlying registers work to really understand this :slight_smile:

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The digitalWrite() to an input pin is an AVR hack, works only on specific controllers. For compatibility the mode INPUT_PULLUP should be used.

MOSFETs need no special current for switching, unlike BJT do. If 5V is too low for turning a MOSFET on, it's regardless of input or output mode.

11

OP uses a Mega, so ATmega. It'll work on that processor.

MOSFETs commonly have a 10k resistor connected between gate and source, forming a voltage divider with the internal pull-up resistor. In that case it will not work with INPUT_PULLUP, even if it would switch fine with a proper 5V output.

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