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jman

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Joined: 12/06/2011
Location: New Zealand
Posts: 711

Posted: 01:41pm 30 May 2014

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Hi

Does anybody know or have a method to calculate the capacitor value mentioned
in the Micromite manual used to wake the Micromite up.
A table with Sleep time and capacitor values would be a great addition to the manual
(For those like me anyway)

I have a battery powered circuit that would benefit greatly from
the power saving

Quote
WAKEUP pin could also be connected to a capacitor which would slowly discharge.

Thanks
Jman

Grogster

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Joined: 31/12/2012
Location: New Zealand
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Posted: 01:57pm 30 May 2014

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Although not the best method, perhaps, how about trial-and-error?

You could start with something along the lines of 1uF and see if that does what you need, and then change the cap up or down running the same test till it does work how you want.

Smoke makes things work. When the smoke gets out, it stops!

crez

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Joined: 24/10/2012
Location: Australia
Posts: 152

Posted: 02:37pm 30 May 2014

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If you are already using the RTC chip, check my post MICROMITE RTC WAKEUP. This allows accurate wakeup times with only 3uA extra current draw while sleeping. This arrangement would suit something like a data logger that takes readings at regular intervals.

David

Goeytex
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Joined: 12/05/2014
Location: United States
Posts: 74

Posted: 06:28pm 30 May 2014

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Posted by jman:

Quote
Does anybody know or have a method to calculate the capacitor value mentioned
in the Micromite manual used to wake the Micromite up.
[/quote]
Assuming an input impedance of ~30Meg ohms and a low level change @ .66v a rough formula might be.

Time = 30,000,000 * C * 1.66 (Untested)

Goeytex
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Posted: 04:51pm 31 May 2014

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UPDATE:
It seems that when the pin is changed to an input and the cap discharges, that the voltage will never drop low enough to change the state of the pin without some kind of added load. Probably due to internal leakage current.

So I added a 10Meg resistor from the Sleep Pin to ground. Now the pin changes states and this resulted in:

1.1 uf = 14 seconds (tant)

2.2 uf = 28 seconds (tant)

10 uf = 127 seconds (Elect)

45 uf = 563 seconds (tant)

Looks fairly linear ... @ about 13 seconds per microfarad with a 10Meg pulldown

Edited by Goeytex 2014-06-02

jman

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Posted: 05:43pm 31 May 2014

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Thanks for the tip guys
Goeytex your cap/resistor idea works like a charm I never had 10M resistor in the junk box so I used a 2.2M and a 100uf (electrolytic) and this gives me about 190 seconds and that's perfect for the current task

Regards
Jman

TassyJim

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Joined: 07/08/2011
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Posted: 05:51pm 31 May 2014

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Using a large value capacitor uses up a lot of energy recharging it. The power has to come out of the micromite pin and could be overloading it for a bit too long.

I would try a 0.1uF or smaller without any bleed resistor to see what times you achieve.

Jim

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Goeytex
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Posted: 07:05pm 31 May 2014

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TassyJim said Using a large value capacitor uses up a lot of energy recharging it. The power has to come out of the micromite pin and could be overloading it for a bit too long.

I would try a 0.1uF or smaller without any bleed resistor to see what times you achieve.

Jim

It will likely never drop low enough to change the pin state. It won't on mine. Not without some kind of minimal load. Of course you can't read it with a scope because the scope has enough impedance to bring it low and make it appear to work.

To prevent surge current to a large cap a, 220 ohm resister can be placed between the pin and the + side of the cap. Just make sure that the Micromite Pin is high long enough to fully charge the cap before going to sleep.

Edited by Goeytex 2014-06-02

TassyJim

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Posted: 07:47pm 31 May 2014

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Goeytex said
It will likely never drop low enough to change the pin state. It won't on mine. Not without some kind of minimal load. Of course you can't read it with a scope because the scope has enough impedance to bring it low and make it appear to work.

To prevent surge current to a large cap a, 220 ohm resister can be placed between the pin and the + side of the cap. Just make sure that the Micromite Pin is high long enough to fully charge the cap before going to sleep.

After a few tests, I agree. With 5pF it takes more than 10 minutes and we are getting into the unpredictable areas. The time would depend on the leakage currents on the circuit board so would be all over the place!

Stick to 10 meg ohms or less to get consistent times.

For long times it would be OK to have short times and a software counter to know if it's time to do the main events or just increment the timer and go back to sleep.

Jim

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PicFan
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Joined: 18/03/2014
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Posted: 09:53am 06 Jul 2014

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I have tried the following and it works well. 220 ohms in series to a 1uF tantalum capacitor. Anti-parallel to the capacitor, a diode 1N4148. The leakage current of the diode compensates the leakage current of the uP Pin. Delay: 1uF = 240 sek.

bigmik

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Joined: 20/06/2011
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Posted: 12:35pm 06 Jul 2014

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Hi PicFan,

Have you got a schematic (even a photo of a sketch) as I am a bit confused as to how this is hooked up?

What do you mean by ANTI-PARALLEL? Do you mean reverse BIASED?

Regards,

Mick

Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<<

PicFan
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Joined: 18/03/2014
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Posted: 11:38pm 06 Jul 2014

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Hi bigmik,

Best regards from Austria - Tirol !

Wolfgang

bigmik

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Posted: 12:47am 07 Jul 2014

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Perfect,

Thanks Wolfgang,

I understand what you mean now.

Regards,

Mick

Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<<

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