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Topic: Understanding digital pin output voltage (Read 6638 times) previous topic - next topic

RMurphy195

I wasn't too clear about the digital pin output voltage when using differing power sources so decided to do a test
USB power came from the USB port, battery power via the power connector
Code is the "Blink" example with a couple of extra lines to declare pin 12 and set it to High

Board is Uno R3
Pin 12 set to high

RESULTS
    Power supplied by USB: output voltage 5v
    Power supplied by 6 volt battery producing 6.5v: Pin 12 output voltage 4.9v
    Power supplied by 9 volt battery producing 10v : Pin 12 output voltage 5v

So I've found out what I wanted to know - that I don't need to change resistors on any LED's driven by the Arduino if I change it's power source.

But - is the apparent 1.6 voltage drop as per expectations?
Is this why the recommended minimum input voltage is 7v?
And - is this sort of information documented somewhere that I've missed?

Many thanks - Richard
Have a nice day

graynomad

#1
Jan 14, 2014, 01:03 pm Last Edit: Jan 14, 2014, 01:11 pm by Graynomad Reason: 1
5v, 4.9v, 5v, where is the 1v6 difference?

7v is recommended because regulators have a "head room" they need above the output voltage, typically this is in the 1v5 range (depending on the regulator being used), so speccing 2v is safe.

Quote
6.5v: Pin 12 output voltage 4.9v

Ah you mean this, the two non-USB voltages are regulated down to 5v, the USB input is already 5v and used directly.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

TomGeorge

Hi, the recommended minimum input voltage is 7V because the regulator on the arduino board needs an input higher than its output to work, in this case 5V.
Usually you allow 2V differential for a linear regulator, as is this case.
You noticed that 6.5V input the output dropped 0.1V, this is good, but under load the output would have dropped more and that would not be satisfactory for 5V digital circuitry.

I admire that fact that you have done some work to get the answers yourself, well done.

Tom.... :)
Everything runs on smoke, let the smoke out, it stops running....

Grumpy_Mike

Section 28.2 of the data sheet says:-
The output voltage from a pin will always be at least 0.7 of the supply voltage to the chip with 20mA of current flowing out of it, proving the voltage to the chip is between 2.5 and 5.5V.

MarkT

#4
Jan 14, 2014, 02:17 pm Last Edit: Jan 14, 2014, 02:26 pm by MarkT Reason: 1
The on-board 5V regulator has a fairly low dropout I think, about 1V, but the DC power jack
has a protection diode in series with it that loses about 0.7V, so that when powered from
the jack you need about 6.7V or more, from Vin about 6.0V or more to get a stable
5V on Vcc.  Different boards and different versions of a board may have different regulators
so take these drop-out voltages with a pinch of salt.  7V is a good lower limit that is reliably
going to work.

Unloaded output pins are spot-on the 5.0V rail since CMOS outputs are driven to the rail
(FETs are like resistors, the output FETs are about 40 ohm when on at 5V supply).

Once you draw significant current the voltage will droop - 10mA will perhaps pull the output
down 0.4V (all very rough figures, note).

You can do some more measurements to characterise your board if you want, but don't
try to reach/exceed the abs max output currents from a pin...

[ actually just measured my Uno, which has a MC33269-5G regulator (nominally 1.0V
drop-out voltage).  From Vin pin a minimum of 6.08V needed, from jack minimum of
6.84V needed to get 5V stable ]
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

RMurphy195

Thanks for all the replies, my understanding broadens with each day and each piece of information from the Forum :)

Just one question - can anyone provide a link to the datasheet that Grumpy_Mike refers to, when I google it I get loads of stuff but nothing that seems to have the information referred to. And presumably there's a lot more useful stuff in there!

Cheers - Dick.
Have a nice day

MarkT

[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

CrossRoads

Datasheets are here
http://www.atmel.com/products/microcontrollers/avr/megaAVR.aspx
There is a ~30 page summary, and a hundreds of page complete version.
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.

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