Pull-down Arduino Pin when Power Up

[imranahmed]

Please let me know I make simple project for ON and OFF DC motor with relay using Arduino UNO. When I power up Arduino a HIGH pulse is going to relay for few milliseconds it turn on the relay for some moment I want to avoid it. When I goes to power on Arduino all pins should be in LOW state. Is it possible?

 

[FvM]

Arduino GPIO lines are high "Z" at startup. You are probably using an unsuitable application circuit, e.g. missing pull-down resistor.

 

[danadakk]

Normally you would sequence power thru some gate mechanism that stays off until processor
can run and take control. Most processor datasheets, the ones well characterized, show you
a "safe area" of operation versus its Vdd. So you use a switch that has high threshold to hold
off any transients of GPIO, like a MOSFET with Vgson that is higher than the processor Vdd when it
takes control.


Regards, Dana.

 

[KlausST]

Hi,

The text in the picture of post#3 contains some doubtful statements.
I recommend to keep on microcontroller datasheets and application notes.

Klaus

 

[danadakk]

I agree with Klaus, confirm with Atmel as I could not find in datasheet
adequate description of power up effects on GPIO.

Typically a field engineer (FAE) would be a good contact point, or a post
on Atmel web site.

Power sequencing is not a trivial design consideration, one can get some
pretty strange behavior in circuits not designed for this, as well as outright
device failure. True for both power up and power down.

I ran across another datasheet recently that stated GPIO was in tri-state on
power up. What was not stated was that for all chip Vdd voltages ? In fact
it was not, eg. GPIO was indeterminate at voltages below chip operating specs.
If your chip drives into a bipolar transistor for power switching, like relays, etc..,
say an NPN, it turns on at 1 Vbe, in fact its on partially << Vbe. So a 3.3V chip with
transients at levels of a Vbe as its power ramps up can cause premature Relay
turn on. Good datasheets cover this. Some do not.

A common approach is to use, in the above case, a pulldown R. The trade off
here is too low a pulldown reduces operating noise margin, too high not enough
reduction in transient amplitude. And leakage absorption issues where leakage
thru the R can turn on an external device. choose carefully.


Regards, Dana.