Adding an inductor keeps my AVR from turning on.

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So I'm trying to protect a circuit board that is receiving 5V from a wire that passes by some noisy relays. If I pass the wire several times through a ferrite core and then into my circuit board, the AVR doesn't start up. If I remove the ferrite core, starts up just fine. There's voltage on the board as there is an LED that lights up just fine that taps into the 5V bus on the board but no dice on the AVR.

Anyone know why an inductor would do this? I thought it would just delay turnon my a few ms or something.

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Note sure if this is part of the issue or not, but if you have a decent O'scope watch the V+ line on power up.

The Power On Reset circuit watches the V+ bus when the device is powered up and if the V+ meets certain criteria then an internal reset signal is generated to initiate the micro "running".

In the Mega168, for example, the Vponsr, (Power-On Slope Rate), must be between 0.01 to 4.5 V/mSec.

I believe, but I can't find it right now, that the V+ signal must also be monotonic.

If the added inductor puts the V+ signal outside the above range, and perhaps if the signal is non-monotonic, then the POR circuit won't trigger the internal startup.

Just a thought...

JC

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Wow. That's interesting. Didn't have a scope on me at the time but I'll bring it next time to have a look.

Never heard of a "Power-On Slope Rate" requirement. Very interesting.

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That monotonic requirement involves, I think, multiple passes into the zone where power on reset detects a power problem. If it is ringing when the voltage gets near the target 5V, there should not be of a problem (unless thing ring amplitude is quite high.

I've used snap-on ferites with lots of MCU devices and never had a problem.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ka7ehk wrote:
That monotonic requirement involves, I think, multiple passes into the zone where power on reset detects a power problem. If it is ringing when the voltage gets near the target 5V, there should not be of a problem (unless thing ring amplitude is quite high.

I've used snap-on ferites with lots of MCU devices and never had a problem.

Jim

Hmm. Snap-on ferites look pretty easy. I have a 16-gauge 5V wire going into the circuit board along with a common 16-gauge wire as well of course. Do both go into the snap-on ferite?

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It's all about how stable the 5V is when reset is released.
I bet if you not all ready have done it, a BOD of 4.3V will solve your problem.
How is your reset hooked up?

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Did you add a large enough capacitor after the filter, right before the avr?

No such chip is supposed to work ok with an inductor right on the supply, as its current requirements are not stable but fluctuate hugely. (i.e. the inductor causes voltage drops when the mcu tries to draw more current).
I have put large filters on supply lines without problems, but always with large enough capacitors on the mcu side. An oscilloscope on the mcu side will surely show the problem but my guess is that an array of a few 10uF mlcc's will keep the avr happy.

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fizgig wrote:

Anyone know why an inductor would do this? I thought it would just delay turnon my a few ms or something.

Maybe it slows down how fast voltage rises. Maybe voltage rises so slowly that it goes out of reset but voltage is still rising but too low to actually run at the clock speed you have.

What MCU you are using? What is the clock frequency? Are you using BOD or other reset circuitry? How much capacitance you have after the inductor?

If the voltage input can be understood as step response into an LC filter, there should be ringing at the resonant frequency?

Adding capacitance will just slow down voltage rise more, although it will bring down the resonant frequency. If you have too little capacitance, maybe the LC resonance is near the frequency of AVR clock so the power just oscillates?