AVR (FLASH) unable to reset if supply voltage falls to 1V...

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pieterc
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Posted by pieterc: Tue. Aug 29, 2006 - 01:31 PM
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I have a circuit where the AVR's supply voltage is kept at 1V (when switched off) by drawing parasitic power from an external source (+5V to 1k resistor, through protection diode of AVR input pin).

When power is restored (VCC = 5V), the AVR fails to return out of reset. The BOD or external reset has no effect. The issue was solved by using a 10k resistor, which limited the parasitic power voltage to 0.2V.

Has anyone had a similar experience? Is this a known issue with FLASH?

Regards,
Pieter
http://www.piconomic.co.za

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AVR Microcontrollers, megaAVR and tinyAVR
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Bronze Marktheusch
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Posted by theusch: Tue. Aug 29, 2006 - 01:58 PM
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This can be a tricky situation. First, which AVR model are you using? Those with AVcc need to have both Vcc and AVcc drop below the BOD hysteresis level before a restart will be done. This can be a problem in systems with lightly-loaded analog supplies.

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MegaUSBFreak
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Posted by MegaUSBFreak: Wed. Aug 30, 2006 - 08:43 AM
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Hi,

maybe this is some kind of latch up effect.

I had a similar(?) problem with a simple mosfet. Once powered in reverse the gate was not high ohmic anymore.
(The fet was not dead, but it needed some 100microamperes to the gate to get low impedance. This effect was reproducible.
Driving the FET with >100kOhms failed while driving the gate with <100Ohms worked fine)

The solution to avoid this effect was to place an external schottky diode in parallel with the internal body diode.

Maybe in your application an external schottky diode in parallel to the AVR internal protection diode helps....

Klaus
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sgomes
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Posted by sgomes: Wed. Aug 30, 2006 - 01:22 PM
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Yep, Klaus got it. We had a similar problem in one of our products. Same solution. Turns out that if you don't have enought current (which it sounds like you won't) you won't develop the full 0.2V so this will result in an even better situation for your reset!

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