Stability issue

13 Feb 2012

Hi Guys,

I am experiencing stability issues on the MBED and would like your input.

Sometimes when saving my code it reboots when I touch the reset button without pressing it.

Is this normal?

Kind regards

Jesper

13 Feb 2012

I'm having the same issue but it happens all the time. If I lightly run my thumb over or near the reset button the mbed resets? No way is the button being pressed. I need to find a way to stop this as the board gets handled in the service it's in...

Tim

13 Feb 2012

happens all the time for both of my mbeds.. Haven't really tried to prevent it. Try putting some thick tape on it? I think its ESD related.

13 Feb 2012

I think it's ESD related too... I have the Mbed mounted (plugged in) to a custom board. I then put the whole thing in a clear shrink wrap tube trimmed at both ends and around the reset button. The shrink wrap is more of a plastic then rubber.

When I remove the wraping the condition pretty much goes away. I was hoping to find a way to stabilize the situation. I need to wrap the mebed to keep it from shorting out on something but still want access to the board...

I read in another post about adding a resistor to the nR pin. But I'm not sure what to connect the resistor to? I'm worried to connect it to a Voltage > 3V and hurt the mbed...

Tim

14 Feb 2012

This worries me a bit, as it basically makes the platform unusable for our purposes.

We are prototyping a datalogger product, and have just had 15 units returned from field trials because of a stability issue, which I suspct is related.

Tim: do you think you can dig up the post.

Thank you for your input.

Jesper

14 Feb 2012

Here it is Jesper...

http://mbed.org/forum/mbed/topic/1035/?page=1#comment-4986

If you see that this helps please post back with exactly what you did OK.

Thanks

Tim

14 Feb 2012

I have noticed the spurious resets when I am about to press the resetswitch. Didnt bother me much sofar and never fixed it.

The mbed schematic shows that the resetbutton and the external nReset are two independent reset inputs. Both have a separate 10K pullup resistor to 3V3. The resetbutton connects one of these inputs to ground.The nReset external input should be pulled to ground externally when you want to reset mbed.

The earlier post recommends a 1K resistor in parallel to the existing 10K resistors. That makes the pullup stronger and means that you need a stronger pulldown to reset mbed, which helps to prevent spurious noise or ESD to cause an unintentional reset. An external 1K pull up to 3V3 (pin40) is easy for nReset (pin 4). It wont be that easy for the resetbutton. The 10K is an SMD device that may be hard to replace or bridge with a second 1K. Best option would be to solder the R directly to one of the resetswitch connection pins. You need to connect the 1K R to either one of the 2 contacts at the bottom of the switch (ie the contacts closest to the LPC1768 IC).

14 Feb 2012

Hi All,

I have just been notified of this thread by Jesper, so I'll take to the lab tomorrow and see what we can come up with.

The first challenge will of course be in reliably reproducing it. Jesper suggested an mbed in an ABS box rubbed on denim a few times provokes the issue. If anyone else has an top tips on reproducing it, please post them here. I will pack my woolly jumpers and nylon vests for work tomorrow!

Thanks, Chris

20 Feb 2012

Hi guys,

We have been exploring throughout the weekend and have figured out that our main problem is related to a bug in a Lithium battery step up converter circuit. We have a floating pin that was triggered randomly when the box was exposed to real life use of our product. At the end we could reproduce the problem by touching the chip in question with a bare finger.

I therefore assume that the reset problem during development is unrelated.

Kind regards

Jesper

20 Feb 2012

Jesper,

Thank you for the update. I'm a bit confused by this statement however... "I therefore assume that the reset problem during development is unrelated"

The thread starter, and myself, stated the problem was related to handeling / touching the board trigging a reset. Which is what you found also. So how is this "Unrelated" in any way?

Based on what I read, I'm hearing that the development board has a design defect related to the step up converter circuit causing unexpected resets when handeling the board...

Tim

21 Mar 2012

I hit a related issue tonight with a mbed based project that I was working on. I had placed the mbed inside of a plastic enclosure with an external reset switch wired into the nR and GND pins of the mbed. Whenever I would have an ESD event within a few feet of the device, it would reset the device. This would happen if I had the mbed powered from the USB programming cable or a battery (with nothing else connected to the mbed.) If I removed the cable that connected my external switch to nR then the problem was much less likely to happen but it still would happen occasionally. I asked a couple of EEs on a local robotics IRC channel and they first recommended that I make sure that it was the nR signal that was causing my reset issues by shorting it to Vout. This completely eliminated the resets and they had the following suggestions to fix the problem:

  • Place a 0.01uF ceramic capacitor between the nR and GND pins.
  • Add an additional 10k ohm resistor in parallel to the existing 10k pull-up to halve the resistance and strengthen the pull-up as was recommended in this thread as well.
  • Place a small series resistor (100 ohm) inline with the switch.

I only ended up adding the capacitor and didn't continue with the other recommendations as that remedy alone completely solved my problem.

I don't have a solution for the built-in switch yet though. I might try a cap across the switch on the mbed which lives in my breadboard and see if it eliminates the resets I see there.

Thanks to David and Richard for their help on this one.

19 Jun 2012

I have tacked caps and 1K pullups on both reset lines which help a lot, but still have minor problems with ESD discharges. I live in a dry area (single digit humidity much of the time).

I have ground points in multiple places on my benches and always discharge to them before touching any electronics. I also use a wrist strap and anti-static mats. But when I walk into the room and sit down I always have large discharges when I first contact a ground point.

I find that the mbeds (I have three of them modified) do not reset the running program but the USB drive often disconnects and immediately reconnects, in response to the ESD discharges.

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