Go Down

Topic: reset pin pull up ? if yes , only 3.3V? (Read 1 time) previous topic - next topic

zmatek

hello all,

I have question regarding arduino due reset circuit, i have checked schematic, but still not clear for me.
Question is if Arduino reset pin can be pulled up by external resistor? and if yes, I guess it should be only 3V3?

Because I have one puss button used for "software reset" by MCP23017 which is running at 5V, I would like to make jumper option for direct connection between pushbutton and arduino reset.
But that push button has pull up resistor to 5V because of MCP23017.

Any advice helpful, thanks in advance. Miro

Grumpy_Mike

As a general rule you do not apply a voltage greater than the power rail to any device.
So no do not pull it up to 5V.

chriskner

zmatek,

The DUE has a 100k resistor pull-up to 3.3V on the reset line.  The reset input is Active-Low.

There are marginal, silly, circuit tricks to get it to work.  However, I would still would not recommend applying an external active-low reset signal that already has it's own pull-up to 5V.

If the external circuit's pullup is very big, then it 'probably' wouldn't do any damage.. But it's still a foul, and bad form.  Add proper voltage translation, and don't look back.

For a kinder/friendly version of the schematic, look here:
http://forum.arduino.cc/index.php/topic,146574.0.html

-Chris

zmatek

Hello,

it is clear, no pullup resistor needed, because It has built in an internal.
I still need to merge 5V logic with arduino reset pin, so just rectifier diode could do the job.
Please take a look on attached schematic draft, does it seems ok to you guys?

chriskner

zmatek,

IMO, I'd characterize that as a 'silly circuit trick'.

Be wary of the forward voltage of the diode (is it a schottky?).

-Chris

zmatek

no it is not, should be?
But I did not realize, that reset voltage would be going forward through diode...so I updated "silly circuit".
See attachment, does it seems better now?

chriskner

#6
Feb 14, 2014, 07:44 pm Last Edit: Feb 14, 2014, 10:45 pm by chriskner Reason: 1
The "forward voltage" of the diode must be less than the requirements for the "low voltage threshold" on the reset input.

The forward voltage on any-old-diode may be near a 1 volt (~0.7V).  A Schottky will be less (often ~0.3V).  However, Schottky's can have a higher "leakage current" that sometimes you need to deal with.

See attached picture.

-Chris





zmatek

I see, diode keep voltage level equal to forward voltage....
Does schematic you posted work well with Due?
Means is voltage treshold low enough?
It would be pretty useful for my app,however I can see pullup in the circuit you posted.
Qurstion is also, is that pull up resistor essential? If yes, desired value would be 10k?

Thanks a lot in advance for your help. Miro

chriskner


I see, diode keep voltage level equal to forward voltage....

Yes.
Quote

Does schematic you posted work well with Due?
Means is voltage treshold low enough?

I don't know. 
You would need to look at the datasheet for the SAM3 micro on the DUE, as well as the ATMEGA16U2 chip (because the reset line runs there as well - see attached schematic) for the reset pins specs.  I believe that in addition to the on-board 100k pull-up on the DUE pcb, the Sam3X has an internal 15k pull-up on this line as well.  I do not know the internal setup, or requirements, for the ATMEGA chip.
Quote

It would be pretty useful for my app,however I can('t) see pullup in the circuit you posted.
Question is also, is that pull up resistor essential? If yes, desired value would be 10k?

On the DUE side, you would not need to add any additional pull-ups, because they already exist on the DUE's pcb.
In any case, ~10k-15k is a typical pull-up value in this role.
As far as I can tell, you won't hurt anything on the DUE; and it will either work, or not work...

Good luck,

-Chris

zmatek

#9
Feb 15, 2014, 02:12 pm Last Edit: Feb 15, 2014, 02:13 pm by zmatek Reason: 1
Hello,

actually I want to use WDT ( MCP1316M ) it is an open-collector and operating up to 7V supply.

I prepared draft circuit for my application, I have to interface WDT with MCP23017 pin expander which has 5V supply.
My arduino is DUE so 3.3V logic. I choosed MCP1316M, and it should be ok for 3.3V logic, because it has 2.9V trip-point for supply voltage drop.
MCP will be used for "software reset" activated by push button, when jumper closed, puss button will initiate Arduino "hard reset".
Does is seems correct to you now?

BTW: I guess reset pin pull-up resistor is needed because of possible current leak in open-collector transistor.
So Is used to eliminate undesired reset?

Thank you in advance for your help. Miro

Go Up