Brown out detection

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I use an ATMega328AU a lot usually with the 8MHz internal RC oscillator

Now I know what brown out detection is, I know what it does i.e it holds the processor in a reset state if the voltage drops below the threshold

I am reading this

http://www.atmel.com/Images/doc1...
Which seems to recommend external circuitry but I am not clear if this is essential?

My first question is

Can I just use the BOD fuse set to the relevant value and not need any circuitry except the standard pull up on reset with a 0.1uF cap to ground?

if this is fine then I wonder why anyone would bother with external circuitry? what does it bring?

Now my main question is what level should I go for if I am running at 5VDC 8MHz

I have seen a graph somewhere displaying the safe operating area based on clock speed and voltagebut I can't find it at the minute!, I havent looked very hard yet

Looking at the fuses I have three options

4V3
2V7
1V8

I am sure the 4.3V would be fine but what happens if I choose 1.8V? or 2.7?

What do the freaks use?

thanks

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AVR180: External Brown-out Protection.

Mega328 has an internal BOD.
Why would you want an external then?

Quote:
what happens if I choose 1.8V? or 2.7?
Nothing tragical.
But if it works with 4.3V then stay with it.

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Welcome to the forum.
Generally if the chip has internal BOD then no external circuit is needed, but there can be exceptions.
I usually select the highest BOD level below the normal operating voltage, so for 5v, I would use 4v3, and for 3.3v, I would use the 2v7, and for 2.5 I would use the 1v8 setting. Again there can be exceptions,
but those are good starting points.

JC

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Note that the reason why AVRs don't just come with BOD turned on all the time (apart from the fact that Atmel can't predict what your Vcc will be) is that it costs power to operate it. So you may want to weigh this into the equation. But most of the time it's wise to enable it at the level Doc JC says.

Some modern AVR (esp the "picopower" ones) have an added featured called "sleeping BOD" which recognises the fact that BOD protection is not really required if the chip is asleep.