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Topic: UNO specs, logic low level (Read 315 times) previous topic - next topic

Railroader

Hi!
I have been searching for the data specs regarding safe high and low logic levels for my 5 volt UNO. Vill 0.77 volt be read as a low logic level? I only have free analog input and I wish to connect 3 such signals. It is the output of opto couplers.



Use Your knowledge. If that's not enough, look for education.
Having knowledge, think outside the box to gain more of it. Only trains run like the train, on the rails. The rest run between the rails.

larryd

#1
Jul 20, 2019, 08:08 pm Last Edit: Jul 20, 2019, 08:11 pm by larryd
From testing done here, some time ago, 0V to 1.3V on a 5V UNO were detected as a LOW.


"I only have free analog input "

A0-A5 can be read as digital inputs too, or am I misinterpreting what you are saying?





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CrossRoads

Analog input is just another function added to a digital input.
D14 to D19 can be used for A0 to A5.

For a 5V Uno, max level for a valid low input is 0.3 x Vcc, so with Vcc pins at 5V that is 1.5V.
Min level for a valid high is 0.6 x Vcc,  so with Vcc pins at 5V that is 3V.

Your Vcc may be less than 5V if powered from USB.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Railroader

Thanks for Yor answer Larryd.

Oh, such a high voltage as 1.3 is recognized as low. Then I am safe. The opto coupler has a Darlington output stage but using "INPUT_PULLUP" the digital input has no problem reading a low.


My project is controlling a CNC. It has an I2C LCD, 3 analog pots for manual stepping, 2 I/O for high temperature alert communication and 3 steppers connected so D9, D10 and D11. So, not much left…


I got this part of the project working.
Use Your knowledge. If that's not enough, look for education.
Having knowledge, think outside the box to gain more of it. Only trains run like the train, on the rails. The rest run between the rails.

larryd

.77 to 1.3V (in my testing) is .53V, lots of head room, you'll be fine.




As Crossroads mentioned, a controller with 5V Vcc 'should' give anything <= 1.5V as a LOW.

I don't like the word 'should' ;) .









No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

Railroader

Thanks CrossRoads.

Using A0 - A5 being named D14 - D19 is new to me. However that will not change the impact of I2C and other stuff connected to the A-inputs I assume.

I once got hold of the technical specs but today they seam to be hiding. Being an old TTL logic level fighter low levels around 0.7 volt looked questionable to me.

Summing up, thanks for Your time. Things look good here for the moment.
Use Your knowledge. If that's not enough, look for education.
Having knowledge, think outside the box to gain more of it. Only trains run like the train, on the rails. The rest run between the rails.

Railroader

I'm now sure I'm safe.
 I know thart "5 volt" is uncertain when using USB, as I do, but today it is 4.96 volt!
Use Your knowledge. If that's not enough, look for education.
Having knowledge, think outside the box to gain more of it. Only trains run like the train, on the rails. The rest run between the rails.

pert

The place to look for this sort of thing is the microcontroller's datasheet. In the case of the Uno, you would find the ATmega328P datasheet:
http://ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061A.pdf
In that document, you find the information in table 30-1 that the maximum input low voltage at Vcc 2.4V-5.5V is 0.3*Vcc.

Railroader

Thanks pert! That was what i found some time ago but now I couldn't. Using "worst case" data has made my constructions work in the past and I never drop the data in the specs.
Use Your knowledge. If that's not enough, look for education.
Having knowledge, think outside the box to gain more of it. Only trains run like the train, on the rails. The rest run between the rails.

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