Arduino Micro creates spikes and significant drop on 12V supply

When connecting an Arduino Micro to a 12V battery, spikes at a frequency of about 7.6kHz are observed on the supply voltage.
First is a negative spike of more than 1V. After about 6 usec, there is a positive spike of about the same magnitude, followed by a ring-down. There is about 40cm of 18 gauge wire between the Arduino and the battery. The battery is a fully charged 10Ah sealed lead acid battery.

Sometimes half the frequency is observed, sometimes double; seems to be dependent on the current draw on the 5V supply of the processor. For the test, only the Arduino was connected and its steady-state current draw is about 40mA. The 5V supply is relatively stable, however, a small (maybe 100mV) increase followed by a decrease is observed.

This is reproducible on my two recently purchased Micro boards.

It almost seems as if the supply voltage is momentarily shorted.

Has anybody else seen this? Any possible explanations?

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Do you see this only once after you connect the Arduino micro? Or is this a repeating pattern?
How did you connect the 12V battery to the Arduino Micro?

It's consistent. The battery is connected to Vin and Ground using 18 gauge wire.

What do you have connected to the Micro?

Is anything else connected to the SLA battery?

[ And the leads to the battery are twisted pair I hope - otherwise you've created a magnetic loop antenna ]

For testing this, nothing else connected to the Micro or battery.

And yes, the cable is twisted pair.

I am suspecting there are glitches in the circuitry to select Vin, USB or external 5V.

If somebody could try this and reproduce the problem, that would be interesting.

It might be due to the inductance of the long power leads, which in some cases can lead to spikes that are quite destructive. See these test results and discussion by the Pololu engineers: http://www.pololu.com/docs/0J16/1

Well I check the Arduino Micro page and it claims that board only has linear regulators, but perhaps its changed - can you tell if yours has just linear regulators - or have they added a DC-DC converter? That could account for the pulses if it was in a low current mode.

Such converters keep efficient at low average currents by converting at high power for brief pulses till the output cap voltage has risen 50 or 100mV or so, then idling until it falls back down. At higher currents they switch continuously with duty cycle changing to match the current drain (the duty cycle at low power can be too small for continuous operation, and it inefficient anyhow).

Hi, how are you connecting the oscilloscope to measure this pattern, at the battery posts, or at the Vin and gnd of the Micro. A diagram of how you are connected would be appreciated, and are you using a x10 probe?

Have you tried another power supply?

Tom... :)

I have tried the following connected to Vin of the Micro (nothing else connected) - 12V battery - 12V lab power supply - 9V regulator downstream of 12V power supply I have also tried an Uno. Only the Micro creates these massive spikes on whatever is connected to Vin The problem is, they are so big that if you have other components connected, such as opamps, you see the frequency everywhere in the circuit. If somebody has a Micro, can they try to repeat this?

Did you try the fix (high ESR electrolytic capacitor) described in the Pololu article?

Hi, I looked at the schematic for the Micro.

http://arduino.cc/en/uploads/Main/arduino-micro-schematic.pdf

There doesn't appear to be any decent filtering in my opinion on the input circuitry. Apart from a 100nF on the input of the 7805 linear reg, thats it. The application sheet I have recommends 0.33uF in its basic circuit.

I'd be putting say 100uF electro at the Vin and ground pins of the Micro board. It has a SMPS on board to develop the 3.3V. So this could be the source.

Tom..... :)