Unwanted Resistance from Reset Pin to ground!!

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Hello everyone,
I'm new to this Forum but not to Electronic community!

I designed a PCB based on an ATmega128 and it was working fine until recently where I had unwanted resets!
First I thought it's a problem with my code but then I checked the connections and measured ohmic resistance and it turned out there is an unwanted resistance from RESET pin to ground! something around 9kΩ and it makes a voltage divider circuit with the 10k pull-up resistor!! the voltage on the reset pin flows around 3 V and it causes unwanted reset!

here is the schematic

the reset pin is also connected to IDC connector for programmer as well as a connector to interface.

the pull-up resistor circuit connected to Reset pin

 

This topic has a solution.
Last Edited: Fri. Sep 10, 2021 - 02:09 PM
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Welcome to AVRFreaks!

 

You can not measure pin resistance with any meter. What you can do, is to connect A-meter from the Reset pin to ground to measure around 0.1mA /without Reset external pullup resistor/.

Last Edited: Wed. Sep 8, 2021 - 07:10 AM
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How do you know you haven't got a dodgy switch ?

 

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ashoori1993 wrote:
measured ohmic resistance

but a microcontroller input is not "ohmic"

 

ashoori1993 wrote:
here is the schematic

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Last Edited: Wed. Sep 8, 2021 - 07:41 AM
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Well, I'm suggesting there might be an internal resistance which made this situation!
The resistance between Reset pin and ground is a fact, not my assumption!

I will do what you said! I measure the current flowing throught 10 K resistor. How should I deal with that in case I know the current? should I increase the Resistor?

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There are 3 connections to Reset:
a connection from the push button & pull-up resistor to reset
a connection from the programmer IDC connector
a connection from the interface IDC connector

I removed both programmer and interface but no difference!

I also earased the chip and when I powered the pcb up again, it started to reset MCU rapidly!!!

I know there is something with MCU but I can't figure out!

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ashoori1993 wrote:
the voltage on the reset pin flows around 3 V

That is sick, and that is all you can measure. Next, do check whether at the same time Vcc remains 5V, or is dropped to 3V.

Start with removing of external 10k, if possible. Voltage on Reset pin must be close to Vcc, or else, your hardware is problem, not the chip.

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Sorry I can't upload the image directly! I get this message :  You are not authorized to access this page

Last Edited: Wed. Sep 8, 2021 - 08:07 AM
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So did you miss the big red note about that?

 

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do check whether at the same time Vcc remains 5V, or is dropped to 3V

Well I already did that! it remains at 5 V. there are also other pull-up resistors for other Push buttons. they all work properly!

There is also another thing to point out! 
there was a short circuit on my pcb due to an unwanted connection from vcc to gnd which caused some tracks to burn on my pcb!! I cleaned the burned area with brush and contact spray and managed to fix the burned tracks! I doubled checked the tracks and also earased the chip and programed it again to make sure it works properly!!

Do you think it might be the reason??! 

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Allow me...

 

Ross McKenzie, Melbourne Australia

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Is the soldering of the reset pin "perfect"? No leakages?

Ross McKenzie, Melbourne Australia

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When the MCU resets, the relay clicks rapidly, however it is not supposed to turn on at the startup!!

The "logger_rst" which turns on the relay is connected to PB2 (Mosi) in case you ask.

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there was a short circuit between these two pins which burned the tracked to the VCC via hole!

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I would suggest a 10K resistor from the base side of R29 tied to ground.

 

And just to satisfy my curiosity, try making R3 1K temporarily.

Ross McKenzie, Melbourne Australia

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Is the soldering of the reset pin "perfect"? No leakages?

Well, to the extent I can see it's OK, however there are connections passing by each other under the MCU which I can't see!

 

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10K resistor from the base side of R29 tied to ground

try making R3 1K temporarily

Sure I will!

I will let you know if it makes changes.

Thanks in advance wink 

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By the way, is it possible that the abovementioned short circuit caused some slight damages which increases the internal resistance between connections?

I want to make sure whether it's something with PCB or the MCU!
I pray to gods it's the MCU! I'm close to the deadline of the project and ordering another pcb costs me a lot of time and money!

Last Edited: Wed. Sep 8, 2021 - 08:39 AM
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Do you have a spare unloaded pcb?

Ross McKenzie, Melbourne Australia

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No, unfortunately! sad

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ashoori1993 wrote:
is it possible that the abovementioned short circuit caused some slight damages

I'd say that any PCA that's suffered such catastrophic damage (burned tracks!) would have to be considered highly suspect. surprise

 

I'm close to the deadline of the project and ordering another pcb costs me a lot of time and money

did you not order some spares?

 

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ashoori1993 wrote:

There is also another thing to point out! 
there was a short circuit on my pcb due to an unwanted connection from vcc to gnd which caused some tracks to burn on my pcb!! I cleaned the burned area with brush and contact spray and managed to fix the burned tracks! I doubled checked the tracks and also earased the chip and programed it again to make sure it works properly!!

Do you think it might be the reason??! 

Are you using a vitro-pcb-board? If you allow frying for prolonged time, it can develop nice low-value-resistors whenever the color is changed.

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I'd say that any PCA that's suffered such catastrophic damage would have to be considered highly suspect!

So you are suggesting that I have to change the MCU?
I think I can increase the confidence of proper connections by soldering wire wrapp through the burned tracks! that would fix the problem for now, however it is still working! (I will send some images as soon as I get home for you to see the extent of damage!) 
 

did you not order some spares?  

Well, I usually order one PCB and the company gives me as much PCBs as they can fit in a single sheet with the price of one! I did the same, but this time they only gave me a single PCB and I couldn't complain as I paid for one board! 

This reply has been marked as the solution. 
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valusoft wrote:

Do you have a spare unloaded pcb?

 

Or, lift the 2560 /RESET pin from the PCB and now measure the resistance from the PCB pad to GND.

 

Note also that several people have pointed out that pins cannot be measured as resistors. This is why...

 

...depending on the polarity of your resistance measurement you might in fact be measuring the forward voltage drop of that lower diode.

#1 Hardware Problem? https://www.avrfreaks.net/forum/...

#2 Hardware Problem? Read AVR042.

#3 All grounds are not created equal

#4 Have you proved your chip is running at xxMHz?

#5 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

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Are you using a vitro-pcb-board?

I'm not sure about the material! Here we call it Fiber-glass.

The tracks are of copper with final cover of "tin and hot hair"





 

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depending on the polarity of your resistance measurement you might in fact be measuring the forward voltage drop of that lower diode.

Thank you for sharing this information! I didn't know that.

once I check every external connection and make sure they won't lead to the solution, I consider replacing the MCU and I can measure the resistance. 

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ashoori1993 wrote:
So you are suggesting that I have to change the MCU?

I'm suggesting that I'd consider the entire assembly as "suspect".

 

you could waste an awful lot of time (=money) chasing things that are actually just down to damage.

 

Probably best to bite the bullet and get some new boards.

 

 

Well, I usually order one PCB and the company gives me as much PCBs as they can fit in a single sheet with the price of one! I did the same, but this time they only gave me a single PCB and I couldn't complain as I paid for one board! 

So maybe a lesson learned - be sure to order some spares ?

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you could waste an awful lot of time chasing things that are actually just down to damag!

What you said makes 100% sense, but I want to take all my chances and then let go of this PCB!

 

 

So maybe a lesson learned - be sure to order some spares ?

I'm sure it's a lesson for life! yeswink 

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Vitro-pcb-board and Fiber-glass are synonyms.

It is a good practice to have 5V voltage regulator for sensitive chips, and it is also an current-regulator when needed.

Did you lift up Reset pin, #25 is a brilliant suggestion to save the project.

Last Edited: Wed. Sep 8, 2021 - 09:53 AM
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It is a good practice to have 5V voltage regulator for sensitive chips, and it is also an current-regulator when needed.

I already have placed a LM2596-based regulator module on my PCB and Electrolytic capacitor before and after the module and the output is fixed at 5V.

 

 

Did you lift up Reset pin, #25 is a brilliant suggestion to save the project.

I'm about to do that, as soon as get home! 

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LM2596 should be a pre-regulator only. You may check the current regulation setting, actual 2-3A is enough for problems.

Do you need that much current? What I am telling is: use LM7805 or, better: a LDO regulator.

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ashoori1993 wrote:
a connection from the push button & pull-up resistor to reset
Some tactile switches have a ground ring for discharging contact ESD.

ashoori1993 wrote:
a connection from the interface IDC connector
An antenna for air discharge ESD (ESD within a room, motion of coins in one's pocket)

ESD is common and the damage is typically cumulative (ESD's power is greatly variable, AVR don't have a stated ESD spec though ESD is tested for wafer fab qualification)

 

ESD and Transients | AVR040: EMC Design Considerations

Reset Pin Protection | AVR040: EMC Design Considerations

Connection of RESET Pin on AVR Devices | AVR® Microcontroller Hardware Design Considerations (AVR042)

Series resistance into RESET is effective (current division); best is R-TVS-R though R-C-R (100ns) may be good enough (dv/dt reduction, current division)

The Art of Electronics 3rd Edition | by Horowitz and Hill

Download a sample chapter

[page 16, middle of right column]

12.1.5 Input protection 804

Destroying Electronic Components from Across the Room With ESD | Technical Tidbit - May-June 2012 by Douglas C. Smith

2: Finding PCB Layout Defects with the Fischer EFT Pulser - YouTube (2m25s, 40s for air discharge ESD)

 

Tempe Fab 2, Gresham Fab 4 | AVR Freaks (qual report page 9 for ESD, 4KV for HBM is good though 15KV is a somewhat common test, MM is a form of contact ESD wrt PCBA manufacturing)

How to search for Microchip PCNs

 

Absolute Maximum Ratings | AVR® DB Family

IK

  • usually isn't a part of an AVR's spec
  • comparable to CMOS logic IC

Reason :

Migration from the megaAVR® to AVR® Dx Microcontroller Families

 

Edit : a supervisor likely has a stated ESD spec

MCP1316 | Microchip Technology (4KV)

Voltage Supervisors | Microchip Technology

 

"Dare to be naïve." - Buckminster Fuller

Last Edited: Wed. Sep 8, 2021 - 02:50 PM
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ashoori1993 wrote:
I think I can increase the confidence of proper connections by soldering wire wrapp through the burned tracks!
Magnet wire is another; when neither then Ethernet cable (unsheath, four twisted solid pair)

ashoori1993 wrote:
however it is still working!
Well done!  (relief)

 

"Dare to be naïve." - Buckminster Fuller

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One can have too many spares.

brak'lul | Memory Alpha | Fandom

cheeky

 

"Dare to be naïve." - Buckminster Fuller

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I designed a PCB based on an ATmega128 and it was working fine until recently where I had unwanted resets!

The cost for 1 PCB is "high".

The cost for each additional board gets lower and lower.

I consider the cost for "extra" PCBs to be part of the cost of the project.

How many "extra" PCBs I get depends upon the project, the PCB cost break points, shipping and handling for the PCB package, etc.

Having a spare PCB is helpful, as you have now learned.

 

I've not used the M128.

Perhaps the old timers can comment on whether or not the M103 compatibility fuse setting could screw things up, depending upon what is connected to the I/O pins or not.

I assume that you have the chip NOT in M103 mode.

 

 Frequent resets could also be a Watchdog problem, but I think the M128 is too old to have the watchdog module?

 

How long was the PCB working?  

Minutes, hours, months?

 

Was it working for a while after the fried PCB incident?

Presumably that was found when you were initially testing the new board.

 

How much current does the PCB draw?

 

Do you have a "spare" I/O pin and and LED on the PCB?

 

Can you read the chip's Signature?

Can you reprogram a simple flash - the - LED program?

If so, does it work?

 

Can you post a FULL schematic diagram, including the power supply?

 

Did the board get zapped by EMI or a static electrical discharge?  (Edit: Cross post with above...)

 

There is a big difference between a PCB that has never worked, and one that worked for hours/days/months and then quit working.

 

Although it might be very reasonable to build a second PCB, the goal, obviously, is to try to determine WHY the first one failed.

Otherwise history is likely to repeat itself!

 

JC

 

Last Edited: Wed. Sep 8, 2021 - 03:18 PM
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I would suggest a 10K resistor from the base side of R29 tied to ground.

Yes, please also fix this. 

You can add the pull-down resistor with a little creative PCB re-work.

 

The issue is during startup, before the micro sets the I/O pin driving the relay to an output and makes it high or low, the pin is in input mode.

That means that the signal is floating, (briefly) during power up.

That means that the transistor isn't driven hard on, or hard off, and can be in its linear operating range, where it can get very hot, and burn itself up.

That, then, can also draw a lot of current and brown out your power supply, causing (appropriate) resets.

 

Speaking of hardware issues, do you have a by-pass cap across each Vdd/Ground pair of pins on the micro?

 

JC

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ashoori1993 wrote:
LM2596-based
4.5A typical current limit (ouch)

ashoori1993 wrote:
... and the output is fixed at 5V.
Not during a short as the LM2596's error amplifier likely won't keep up (though "might" if the compensator is complete, precise, and correct)

Recommended is a crowbar or clamp; some of those will also handle an under-voltage (reverse current)

AVR have a power clamp.

Hypothesis : over and/or under-volt then a leaky AVR VCC clamp causing die offset voltage

 

P.S.

Some PMIC have an output clamp; use case - load step and compensation isn't a match for that load step.

 


7.9 Electrical Characteristics – All Output Voltage Versions | LM2596 datasheet | TI.com

 

The Art of Electronics 3rd Edition | by Horowitz and Hill

Download a sample chapter

[page 120, bottom of left column]

9.13.1 Overvoltage crowbars

PolyZen Devices for Overvoltage-Overcurrent Protection - Littelfuse (up to 15Vdc)

A zener diode on VCC is usually enough for a transient like a short, mild injection current, or ESD/EMI/EFT/lightning.

 

40V, 2.2MHz, Synchronous Buck Converters Deliver High Efficiency Across All Loads (Diodes)

[AP64100, datasheet, bottom of page 12]

6 Output Overvoltage Protection (OVP)

 

"Dare to be naïve." - Buckminster Fuller

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grohote wrote:
... or, better: a LDO regulator.
If the PMIC can adjust for the voltage drop then some alternatives are

  • capacitance multiplier
  • Ripple BlockerTM

 

The Art of Electronics 3rd Edition | by Horowitz and Hill

Download a sample chapter

[page 49, bottom of right column]

Prefiltering

Ripple Blocker™ (Micrel)

Ripple Blocker (Microchip)

MIC943x0 Ripple Blocker ICs | Microchip | Mouser

Ripple BlockerTM 200mV drop might be reasonable given tolerances (PMIC, MCU brown-out)

 

"Dare to be naïve." - Buckminster Fuller

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ashoori1993 wrote:
I cleaned the burned area with brush and contact spray

That's the very last thing I'd have turned to for PCB cleaning. It leaves behind some oily slime.

Clean the PCB again thoroughly using IPA (that's Isopropyl Alcohol by-the-way, not India Pale Ale)

 

https://uk.rs-online.com/web/p/electronics-cleaners/2274427/

 

 

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N.Winterbottom wrote:
(that's Isopropyl Alcohol by-the-way, not India Pale Ale)
I'd drink either - not too fussy! ;-)

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clawson wrote:

N.Winterbottom wrote:
(that's Isopropyl Alcohol by-the-way, not India Pale Ale)
I'd drink either - not too fussy! ;-)

Desperate?

Ross McKenzie, Melbourne Australia

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I'd drink either - not too fussy! ;-)

It should probably be said (for the newbies/uniformed)...that you should never during this alcohol---it will blind you or maybe kill you. 

In prison, it is a big problem with prisoners drinking hand sanitizer.

 

 

due to an unwanted connection from vcc to gnd ​​​​​​​

You mean by soldering mistake?  Or the layout?  How would that happen? The layout will flag an error if it doesn't match the schematic.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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avrcandies wrote:

It should probably be said (for the newbies/uniformed)...that you should never during this alcohol---it will blind you or maybe kill you. 

In prison, it is a big problem with prisoners drinking hand sanitizer.

 

When people distill their own booze, the initial 'head' when the distilling process begins can have high amounts of methanol that initially makes it through the distillation process. It is notorious for making people very sick and/or going blind. That is why distillers generally throw the 'head' and 'tail' away when plying their processes.

 

Methanol toxicity

 

On the farm where I live we have someone who makes his own rum and peach brandy, and I have sampled his produce. Very nice and tasty but I would not just trust anyone making distilled alcohol!

Wayne

East London
South Africa

 

  • No, I am not an Electronics Engineer, just a 54 year old hobbyist/enthusiast
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WayneZA wrote:
distillers generally throw the 'head'

Should not do that, it is a valuable medicinal alcohol, we do use it for home-made aftershave (with lavender flowers).

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Hello dear friends, and thanks for your helpful comments!


The problem is still available!!


here is the image of how bad the PCB burned during the short circuit!!

 

after cleaning the board with contact cleaner and a brush, the gnd pin detached!! Surprisingly it still works as it has for gnd pins!



I did some of your ideas:


-Removing the reset pin from its pad and checking the connection: I did that and the resistance was still there!!


after removing the pull-up resistor there was a 100K resistance from reset pin to VCC

 

-changing the resistor: I did this with other resistor values but awkardly there was always a voltage division of 1/2 ratio no matter how big was the resistor!!


-connecting a 10K pull-down resistor to the base of transistor: I did it, It's generally a good idea



The last thing I did was to put a 1K resistor as Pull-up for reset and also a 100nf cap from reset to ground!

The voltage still floats somewhere between 2.4 to 3 V but this time it doesn't reset the chip however it keeps another pin (which is connected to a Key) down!!

The other key also has a 10K pull-up resistor and I checked the key physically, nothing is pushing the key down! and there is always a voltage around 0.4 V on the key pin!!

I can manage the program without that specific pin but it's just earasing the problem! I want to solve it!

 

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Here is the PCB project and corresponding schematic file!

 

Attachment(s): 

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I assume that you have the chip NOT in M103 mode.

Yes, I unchecked the M103 compatability in fuse bits, So it's not in M103 mode!

 

  I think the M128 is too old to have the watchdog module

It has watchdog timer and I'm using it.

 

  How long was the PCB working?

for few hours!!

At first it seemed fine but no longer than few hours it started to deteriorate!
But I took the Chip from another board and it's not new!! 

 

Presumably that was found when you were initially testing the new board.

Yes, before I connect other peripherals!

 

Do you have a "spare" I/O pin and and LED on the PCB?

 

Yes, it has an interface which has alphanumeric LCD and some led indicators. they all work fine.

 

does it work?

 yes it works, untill it plays reset games with me! 

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By the way, I had my PCB electrically checked by the company, so if you think the schematic and PCB files show no problems, there shouldn't be problems on the PCB as well!

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ashoori1993 wrote:
there was a 100K resistance from reset pin to VCC

Please, stop talking nonsense. Read our posts, you can measure only with A-meter, it should show 0.1mA from Reset to ground.

And, the Reset pin voltage, now, when disconnected will be something less than Vcc.

Which meter you are using to measure Voltage. Not A-V-O 20k/V, I presume. It should be 10MOhm-all-V-ranges.

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ashoori1993 wrote:

here is the image of how bad the PCB burned during the short circuit!!

Yeah - After seeing your photo, I think you're being a bit silly now. Give up and change the Micro its at death's door after overheating so severe as to detach a pin.

 

Put this episode down to experience and next time:

  1. Use a current limited power supply for testing.
  2. Add decoupling capacitors immediately adjacent to the micro-controller power pins on your next board.

 

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