What is the minimum required voltage required for a digital input high?

Other Hardware General Electronics
1

Hi, I'm using an arduino nano that will be used on a board where the Vcc is set to 3.3V. However, since the arduino nano operates on 5V, my colleague was worried that the arduino nano will never register digital input high since you need 5V*0.7=3.5V. Since 3.5V is higher than the Vcc the arduino nano will never receive a digital input high. I have searched online for an answer and some places mention 3V as the minimum needed for digital high, but didn't clarify much.

So my question is what is the minimum input voltage required to register a digital high in arduino nano?

2

Your friend is right to be concerned. It might work if you are lucky. It might not. Why take the risk? Why use a 5V Nano rather than a 3.3V Pro Mini?

3

I have searched online for an answer

Section 30.2 of the [u]ATmega datasheet[/u] says an input-high is 0.6Vcc or more, and an input-low is 0.3Vcc or less (at Vcc = 5V). In-between is undefined and may read either high or low.

will be used on a board where the Vcc is set to 3.3V.

That device/chip also has tolerances so you also need to know the minimum output-high for that device.

(Output-high on an ATmega chip operating at 3V can be as low as 2.1V so that would be unpredictable into a similar chip running at 5V.)

4

Thanks for the replys, I'm thinking about changing the Arduino I use for one that uses 3.3V instead

5

Good idea. Unfortunately I don't think there is any such thing as a 3.3V Nano. Which is strange because there are 3.3V Pro Micro and 3.3V Pro Mini! But a 3.3V Pro Mini is very similar indeed to a Nano, except that the USB-serial chip is not included and has to be purchased as a separate module.

6

A 3.3V signal will reliably read high on a Nano running at 5V. But of course you can add a level shifter of sorts to bring the signal to 5V.

The 3.3V Pro Mini works great; main differences are that it runs at 8 MHz and requires a separate FTDI USB adapter cable for programming, and the SCL/SDA pins are unbelievably awkwardly placed. Now if they did that with A6/A7 I could kinda understand it...