Chicken & Egg Low power AVR question

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I want to power an 88pa from a supercap. What I would like is to have a way to "hold" the AVR from starting until the VCC reaches something "comfortably" over 1.8V (like, 2).

The problem is that the cap takes a long time to charge (Induction charging, takes "forever"), so I don't want the AVR to try to boot, dip the VCC by doing so under the necessary voltage (or ripples in the rail), halt again because of that, start again etc etc and thus is fact make matters worse on the charging cap.

What I'd like is to be able to have the AVR start "cleanly" and use the first cycles to put itself into sleep mode and monitor the rail from there, in low power.

So I've been wondering how to either, use a transistor to hold the whole AVR rail offline until the VCC reaches 2V, possibly with a voltage divider on a transistor... of course I don't like the voltage divider, since it itself sucks uAs...

Alternatively, I could try to hold the AVR in reset until that condition is reached, but I've been wondering how to do that cleanly, and cheaply in terms of power.

Any suggestion ?

Author of simavr - Follow me on twitter : @buserror

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Lock into the datasheet where they talk about brown-out detection (BOD).

Stealing Proteus doesn't make you an engineer.

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Yeah I know that, I also look at the datasheet for low power applications where they explicitly suggest that the BoD be turned off.

Author of simavr - Follow me on twitter : @buserror

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Maybe you can use a second smaller capacitor that charges up much quicker?

The POR prevents the AVR from trying until VCC reaches 1.1-1.5V.

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Quote:

Yeah I know that, I also look at the datasheet for low power applications where they explicitly suggest that the BoD be turned off.

Quote:

an 88pa

??? So much for all the talk of picopower, and the sleeping BOD?

Considering the alternatives, you may well find the few uA of even the non-P BOD will be the most efficient method.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Looking at the Vdd versus current graphs for the brown out detection, it would appear that at voltages below the threshold the current increases rapidly. While the sleeping BOD might help once the system is up and running, that's not going to help much for the first start-up. I see there are P*Cs that claim to have very low BOD current draws, but my gut feeling(based on next-to-nothing) is that the figure referred to is the current drawn when the Vdd is above the BOD threshold.

Four legs good, two legs bad, three legs stable.

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Quote:

Looking at the Vdd versus current graphs for the brown out detection, it would appear that at voltages below the threshold the current increases rapidly.

I poked at the graphs and didn't see that one. Gotta look again... [edit] Found it--had to search for "brown" instead of "bod".

But anyway, let's flip the question around. The AVR is already in reset 'cause there wasn't any power and it died. Let's say it is because the BOD kicked in.

If the supercap is already [near] dead, then who cares if it keeps drawing some uA until it is dead-dead? (Well, if it was being trickle-charged from a small solar cell then it might matter.) Now, when it is being recharged per the description there is some good current flow. If that is from an outside source, who cares if the BOD draws some uA during that charging?

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Quote:
Now, when it is being recharged per the description there is some good current flow.

Well I can't see that one! I was going on the 'takes "forever"' statement from the first post, and assuming that the charging current is small. Anyway, I'm sure the OP can set us straight on that.

Four legs good, two legs bad, three legs stable.

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I was mostly going by our super-caps apps. We usually (always?) limit the charging current because of the inrush shock to the "charger" as well as the resulting possible "bouncing" effects noted by the OP. So I was assuming.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Something like this ?

You can add many diodes in series with D1 to adjust the voltage for start up. Also the resistors values are informative only.

George.

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Being basically a ones and xeros person, I'm intrigued by what sort of circuitry is used in BODs that would exhibit that "negative resistance" type voltage/current curve.
Any smart analog type care to enlighten me?

Four legs good, two legs bad, three legs stable.

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John_A_Brown wrote:
Quote:
Now, when it is being recharged per the description there is some good current flow.

Well I can't see that one! I was going on the 'takes "forever"' statement from the first post, and assuming that the charging current is small. Anyway, I'm sure the OP can set us straight on that.

Thanks for everyone input so far.

I'm charging the supercap by induction. I have an AVR pumping inverted PWM thru a (tuned) coil, and I have a "slave" coil, also tuned, that passes thru a rectifier and gets about 1.2V DC, this is then passed into a "joule thief" with minor improvements to raise the voltage, and that goes thru a diode to charge a cap. I get nice 6V steady on a 100uF cap after a few seconds. The supercap however takes /hours/ at the moment (mostly due to ti's resistance that "bounce" back power spikes and lowers efficiency), and of course I need to improve the supply (in progress) but it will still takes several minutes to load the cap.

After reading the datasheet properly, it might be that:
+ The Power on Reset might be what I need to solve my startup dilemna. It needs 0.2V "steady" for a reasonable amount of time before it starts. If I put the AVR in low power/sleep mode in the first few cycles it might be OK...

+ It might be a good idea to use the BOD anyway, because it's price in terms of power is rather low, compared to other "reset management ICs" I've seen, and I get the advantage of being able to use that source back into the ADC block (comparator) and monitor it.

So I think I will attempt to run the AVR "naked" first, and see if there's any real life problems.

Author of simavr - Follow me on twitter : @buserror

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Quote:

So I think I will attempt to run the AVR "naked" first, and see if there's any real life problems.

In general, running an AVR without BOD--internal or external--is scary. Especially with dropping supply V, any particular subsystem can "fail" first while the others are struggling onward. Anything can happen, but a quite common occurrence is EEPROM corruption.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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buserror wrote:

I'm charging the supercap by induction. I have an AVR pumping inverted PWM thru a (tuned) coil, and I have a "slave" coil, also tuned, that passes thru a rectifier and gets about 1.2V DC, this is then passed into a "joule thief" with minor improvements to raise the voltage, and that goes thru a diode to charge a cap.

I fear this couple of many conversions has only bad efficiency.

Why you not use a coil with enough windings or a voltage multiplier as rectifier?

http://en.wikipedia.org/wiki/Vol...

Peter

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thanks danni, great link! I'm a newb, i was anaware of that configuration. The "cascade" mode with like 3 stage should do it and relegate the joule thief bit to the led driving stage, when the voltage of the cap discharging goes under their Vf!

I'll go and play with that and report. I'll post my final schematic.

Author of simavr - Follow me on twitter : @buserror

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OK I ripped the bridge, and cast asside the joule thief (I'm going to keep it to do the led blinking stage of that project). I also re-spun the slave coil and tuned it.
Now using one of these magic BAT754S NXP 2*diodes, and 2 1uF 0805 caps, I get 5.6V DC steady out of the slave coil, with nothing else needed!

So now the supercap charges in about 20 minutes. Still slow, but getting there.

Thanks for the tip Lee ! next adding the mega and see how it starts/stops...

Author of simavr - Follow me on twitter : @buserror

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If you're not afraid of partsCount++, you could use a voltage supervisor IC. They draw very little juice when sleeping. MN13812-C may be exactly what you want?

I use them in solar circuits all the time, with regular caps.