# SORTED Uno digital input high / low voltage levels

Hi all,

I know that digital high and low are nominally 5v and 0v respectively, but what are the actual voltages that the Uno sees as high and low on input? I'm guessing a "high zone" of (say) 3-5 and a "low zone" of (say) 0-2 and a "no-man's-land" in the middle?

Reason I'm asking is that I have a sensor that looks like it outputs about 3v to my DMM. I haven't tried it to a digital pin yet because the Uno is currently deployed in my daughter's robot.... (Note to self: might need to buy another one so we can have one each \$)). I guess if its high output is too low I could use it on an analog input, and set a level in the 0-1023 range as the threshold.

But I'm interested to know what the digital voltage high / low levels really are.

Ciao,

Jim

It is in the data sheet and is related to the supply voltage. Off the top of my head it is about 0.7 Vcc for a one or greater and less than 0.3 Vcc for a zero.

For an Uno, it is about <0.3Vcc for low, and >0.6Vcc for high (about 3V)

If it is too low, then your best bet would be either the analog comparator (with reference of 1.1v), or a simple mosfet level shifter (any n-channel enhancement MOSFET, and a 10k resistor).

EDIT: Form the datasheet:

VIL
Input Low Voltage, except
XTAL1 and RESET pin
Vcc = 1.8V - 2.4V MIN = -0.5 MAX = 0.2Vcc
Vcc = 2.4V - 5.5V MIN = -0.5 MAX = 0.3Vcc

VIH
Input High Voltage, except
XTAL1 and RESET pins
Vcc = 1.8V - 2.4V MIN = 0.7Vcc MAX = Vcc+0.5
Vcc = 2.4V - 5.5V MIN = 0.6Vcc MAX = Vcc+0.5

Thanks chaps..... I'll test it on a digital pin (I have negotiated time on the Uno and may have to sacrifice 2 cable ties to get it loose, more \$)), but at least I now know what the 0-1 levels are likely to be.

If it's too low I'm happy to use it on an analog pin.

Cheers,

Jim

With +5V power supply 2.2V low and 2.7V high. Your 3V sensor should be reliable detected as high.

With +5V power supply 2.2V low

No you do not get a low,it has to be below 1.5V for a logic zero.

AtMega is not regular logic type IC, it has “build-in” Schmitt Trigger on each input.

I agree with Magician. I mesure hystérésis on an UNO board. I don't have the results with me but hysteresis was only a few mV. I think a larger hysteresis would have been better.

Well those last two responses meant very little to me, but thanks anyway 8)

Main thing is, I now know there are three zones of high, low and no-mans’-land. If the sensor’s high is too low I’ll cheat and use an analog input.

Thanks for all the replies,

Jim

I don’t have the results with me but hysteresis was only a few mV.

All digital pins will show some form of hysteresis. It does not detract from the fact that the data sheet give a specific value for a guaranteed logic zero.
Sure you will get a logic zero at other voltage levels like you measured but they are not guaranteed.