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'[EE:] Supercap - how long will it backup my RTC?'
2004\08\09@155750 by

Hi all,

I need to calculate how long a supercap will keep my Dallas RTC backed up.

The cap is 0.22F, my circuit is 5V.  The RTC draws 1uA when powered off.

Anyone know the equation(s) to turn capacitance into micro-amp hours?

Thanks!

matt redmond

--

Matt Redmond wrote:
> I need to calculate how long a supercap will keep my Dallas RTC
> backed up.
>
> The cap is 0.22F, my circuit is 5V.  The RTC draws 1uA when powered
> off.
>
> Anyone know the equation(s) to turn capacitance into micro-amp hours?

It depends on the minimum voltage it is allowed to drain to.  Let's say the
RTC works down to 4V.  That means the question is how long does it take a
220mF cap to drop 1V at 1uA.  Just do the math:

1V * 220mF / 1uA = 220Ksec = 3.7Khours = 152 days

However, the leakage of the cap is probably much higher than 1uA.  The cap
leakage is really the limiting factor in this case.

*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--

> I need to calculate how long a supercap will keep my Dallas RTC backed up.
> The cap is 0.22F, my circuit is 5V.  The RTC draws 1uA when powered off.
> Anyone know the equation(s) to turn capacitance into micro-amp hours?

Since the voltage on a capacitor will slope, you need to decide what
cutoff voltage you want.

But if you assume it will be 4 volts, everything gets easy, since a 1
Farad cap will lose 1 volt in one second with a 1 amp load.

So in your case, you will drop 1 volt in 220000 seconds, or about 61
microamp hours.

But you should probably calculate based on the specified Dallas part's
lower voltage.

Cheerful regards,

Bob

--

<MAJOR SNIPS>

Can't really offer much advice, but I can tell you that the use of a
Supercap for the RTC in the Xbox is a major PIA! Why Microsoft didn't just
go with a Lithium cell is beyond me...

-marc

--

2004\08\09@181311 by
Discharge (1000, 100, 10uA) curves for Elna's supercaps

http://www.elna-america.com/discharge-dxdb0.htm

15,000 hours at 10uA, 5V down to 2V

All supercaps (no leakage figures)

http://www.elna-america.com/PDF/DYNACAP.PDF

=======================================

Mouser 555-DX5R5H334

http://www.mouser.com/catalog/619/466.pdf

http://www.elna-america.com/PDF/DX.PDF

--

At 02:49 PM 8/9/04, you wrote:
>Matt Redmond wrote:
> > I need to calculate how long a supercap will keep my Dallas RTC
> > backed up.
> >
> > The cap is 0.22F, my circuit is 5V.  The RTC draws 1uA when powered
> > off.
> >
> > Anyone know the equation(s) to turn capacitance into micro-amp hours?
>
>It depends on the minimum voltage it is allowed to drain to.  Let's say the
>RTC works down to 4V.  That means the question is how long does it take a
>220mF cap to drop 1V at 1uA.  Just do the math:
>
>1V * 220mF / 1uA = 220Ksec = 3.7Khours = 152 days

Olin - woops, you missed a division there

>1V * 220mF / 1uA = 220Ksec = 3.7Kminutes =  61hours = 2.55 days

Scott

--

Sorry, I fudged up - the Elna 0.2FF graph doesn't say

> 15,000 hours at 10uA, 5V down to 2V

It's 15,000 seconds = 250 hours / 41 days

--

Check your math! 86,400 seconds in a day.

On Tue, 10 Aug 2004, Jinx wrote:

{Quote hidden}

--

> > It's 15,000 seconds = 250 hours / 41 days

> Check your math! 86,400 seconds in a day.

Ah well, see, you're probably working in American days. I was

Yes, sigh, I knew that. Extrapolating for Matt, he could probably
expect 150,000 seconds (41.7 hours) to reach 2V at 1uA

--

> Elna's graph, http://www.elna-america.com/discharge-dxdb3.htm ,
> looks to me like 60000 sec from 5 down to 2 volts @ 10uA

That's the one I looked at and mis-read it terribly (or mis-read it
rather well). Of course the log graph goes 10k 20k 30k, not 10k,
11k, 12k....

Think I'll go and have some quiet time and stop trying to be helpful ;-)

--

Scott Fraser wrote:
>> 1V * 220mF / 1uA = 220Ksec = 3.7Khours = 152 days
>
> Olin - woops, you missed a division there
>
>> 1V * 220mF / 1uA = 220Ksec = 3.7Kminutes =  61hours = 2.55 days

Right, sorry.

However, leakage is still probably the limiting factor and needs to be
investigated.

*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--

> >> 1V * 220mF / 1uA = 220Ksec
>
> However, leakage is still probably the limiting factor and needs
> to be investigated

Elan say 60,000 secs for a 0.22F to drop 3V at 10uA. Assuming
600,000 (167hr) at 1uA

using the formula ;

3 * 0.22 / 1uA = 660,000

which implies

3 * 0.22 / 600,000 = 1.1uA, or 1uA load + 100nA leakage

Note that the caps have -20% to +80% tolerance, so the actual
value could be anywhere from 0.176F (480k sec, 133hr) to 0.396F
(1080k sec, 300hr)

--

I fully agree with Marc. Using a lithium battery (like 2032) in my opinion is a much better design than using the super cap. The volume of space occupied is also less.
Recently, I was looking for an RTC chip to be used with a PIC. I found the Philips PCF8563 ideal in many aspects.

Its specs are really impressive.

- Provides year, month, day, weekday, hours, minutes and seconds based on 32.768 kHz quartz crystal
- Century flag
- Clock operating voltage: 1.8 V to 5.5 V
- Low backup current; typical 0.25 mA at VDD = 3.0 V and Tamb = 25 °C
- 400 kHz two-wire I2C-bus interface (at VDD = 1.8 V to 5.5 V)
- Programmable clock output for peripheral devices (32.768 kHz, 1024 Hz, 32 Hz and 1 Hz)
- Alarm and timer functions
- Integrated oscillator capacitor
- Internal power-on reset
- Open-drain interrupt pin.

Regards,
Bala

> {Original Message removed}
> Recently, I was looking for an RTC chip to be used with a PIC.
> I found the Philips PCF8563 ideal in many aspects.

> Low backup current; typical 0.25 mA at VDD = 3.0 V
> and Tamb = 25 °C

Make that 0.25 uA (fortunately)
(I've never used it but some friends who do speak highly of it).

http://www.semiconductors.philips.com/pip/PCF8563T_F4.html

____________________________________________________________________________
____
The PCF8563 is a CMOS real time clock/calendar optimized for low power
consumption. A programmable clock output, interrupt output and voltage-low
detector are also provided. All address and data are transferred serially
via a two-line bidirectional I²C-bus. Maximum bus speed is 400 kbit/s. The
built-in word address register is incremented automatically after each

a.. Provides year, month, day, weekday, hours, minutes and seconds based
on 32.768 kHz quartz crystal
b.. Century flag
c.. Clock operating voltage: 1.8 V to 5.5 V
d.. Low backup current; typical 0.25 uA at VDD = 3.0 V and Tamb = 25 Cel.
e.. 400 kHz two-wire I²C-bus interface (at VDD = 1.8 V to 5.5 V)
f.. Programmable clock output for peripheral devices (32.768 kHz, 1024 Hz,
32 Hz and 1 Hz)
g.. Alarm and timer functions
h.. Integrated oscillator capacitor
i.. Internal power-on reset
k.. Open-drain interrupt pin.

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Hi Russell,

Thanks for pointing out.
I merely copied the specs from the data sheet of 8563 to a Word file, inserted bullet marks and pasted in Outlook mail. But surprisingly, the change from uA to mA has happened while pasting the text in Outlook. I repeated the entire process and discovered this. Even if I directly take the text from the pdf file, the same thing happens (the mu character gets changed to m).

Any explanations?

Regards,
Bala

> {Original Message removed}
> I merely copied the specs from the data sheet of 8563 to a Word file,
inserted bullet marks and pasted in Outlook mail. But surprisingly, the
change from uA to mA has happened while pasting the text in Outlook. I
repeated the entire process and discovered this. Even if I directly take the
text from the pdf file, the same thing happens (the mu character gets
changed to m).

Any explanations?
/>

All together now ...

"Bill Gates. Bill Gates. B ..... "

:-)

RM

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On Tue, Aug 10, 2004 at 12:24:56PM +0530, Bala.ChandarAVENTIS.COM wrote:
> I merely copied the specs from the data sheet of 8563 to a Word file,
> inserted bullet marks and pasted in Outlook mail. But surprisingly,
> the change from uA to mA has happened while pasting the text in
> Outlook. I repeated the entire process and discovered this. Even if I
> directly take the text from the pdf file, the same thing happens (the
> mu character gets changed to m).
>
> Any explanations?

Yes, the letter 'mu' is the Greek version of 'm'.

Chris

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>> I merely copied the specs from the data sheet of 8563 to a Word file,
>> inserted bullet marks and pasted in Outlook mail. But surprisingly, the
>> change from uA to mA has happened while pasting the text in Outlook. I
>> repeated the entire process and discovered this. Even if I directly take the
>> text from the pdf file, the same thing happens (the mu character gets
>> changed to m).
>>
>> Any explanations?

> All together now ...
>
>         "Bill Gates. Bill Gates. B ..... "

Rather than the standard answer, you might look at actual standards... :)

E.g. transliteration from Greek into Latin characters, as in ISO 843
http://www.biology.uoc.gr/gvd/contents/databases/01c.htm

Probably this wouldn't happen if you used HTML mail, as I suspect OE would
then use (and specify in the headers) an extended character set that
includes Greek characters, and continue to use the Greek mu.

The u for mu substitution is limited to a small niche.

Gerhard

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Someone (I can't tell who) wrote:

> I merely copied the specs from the data sheet of 8563 to a Word
> file, inserted bullet marks and pasted in Outlook mail. But
> surprisingly, the change from uA to mA has happened while pasting
> the text in Outlook. I repeated the entire process and discovered
> this. Even if I directly take the text from the pdf file, the same
> thing happens (the mu character gets changed to m).
>
> Any explanations?

Sure.  There are two ways to generate a mu character using "standard"
Windows fonts:

1.  Switch to Symbol font, type "m".
2.  In your current font, hold down the ALT key while typing 0181 on

If you do it the first way (like the data-sheet authors did), the mu
becomes an "m" as soon as someone changes its font from Symbol to
anything else.  Microseconds become milliseconds, microamps become
milliamps, etc... And there's no way for a reader to detect the
change.  This is a catastrophe; people who use the Symbol font to
generate mu characters should be lined up and shot.

If you do it the second way, the mu may STILL become something else
if you change its font, but at least it won't become an "m".  Since
it'll change to something that doesn't look like a valid unit prefix,
a reader may have a chance to detect that there's a problem.

Still, the safest way (aside from writing out the "micro-" prefix) is
to use a lowercase "u"; it looks close enough to a mu, everyone
understands what it means, and it'll NEVER get changed to some other
character just because someone changes its font or converts it to
plain ASCII or sends it to a text-only printer.

As an aside, I see the "mu turned into 'm'" error ALL THE TIME in
trade-magazine articles, press releases, appnotes, BOMs, datasheets,
etc; it's almost as common as the "omega turned into 'W'" error that
happens for the same reason.  The difference -- which makes the "W"
error more benign -- is that if you see a 47 kW resistor called out
on a schematic, you can't accidentally put one on your board.

A few months ago, I got fed up with seeing the mu-to-m error in
published Cypress documents, so I started refusing to approve any
documents that used mu; engineers who need my signature have to
either use a lowercase "u" or spell the word out.  Omega's next;
"ohm" doesn't take up THAT much more room than the Greek symbol...

-Andy

=== Andrew Warren -- aiwcypress.com
=== Principal Design Engineer
=== Cypress Semiconductor Corporation
===
=== Opinions expressed above do not
=== necessarily represent those of
=== Cypress Semiconductor Corporation

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Andrew Warren wrote:

>happens for the same reason.  The difference -- which makes the "W"
>error more benign -- is that if you see a 47 kW resistor called out
>on a schematic, you can't accidentally put one on your board.
>
>
>
Heck, you'd have a hard time mounting a 47 kW resister on your board on
purpose, nevermind accidently!

"Yes, I'd like to know if you can layer fr4 circuitboard, oh, say 12
layers?  I need to mount a huge component..."

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> Sure.  There are two ways to generate a mu character using "standard"
> Windows fonts:
>
> 1.  Switch to Symbol font, type "m".
> 2.  In your current font, hold down the ALT key while typing 0181 on

There are actually a few more options, one of them being using the
US-International keyboard layout and pressing Right-Alt + m (which is
equivalent to your second option), and another one being opening the
character map, scrolling down to the Greek section, and selecting the
Unicode lower case mu -- which is defined as actually the Greek lower case
mu, whereas the 181 character is defined as the "micro" prefix.

This would indicate that any automatic conversion into ASCII should convert
the 181 character to "u" and the Unicode Greek character to "m". (Which of
course assumes that nobody used the Symbol font -- and that the converting
program knows what character set has been used :)

Maybe it's time for application developers to get an understanding of
Unicode, character sets and font standards and write their apps
accordingly...

Gerhard

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What, and pretend a world exists outside of American English???  Don't
you think you're asking a little bit too much?

:-)

I was involved many years ago in converting a VB app to work correctly
on the Japanese version of windows.  Thankfully I didn't have to worry
about the labels (although it convinced me to never hard code any text
strings in the app - always use a language file) but the trick was that
the string handling functions we double byte character strings - not
even unicode, though there were other function to convert back and
from/to the serial port and all the data handling functions since the
engineer (not programmer) who wrote the app used string functions
throughout for processing data.  There were a few other issue, but the
primary issue was treating binary data as strings.

I agree that it would be nice if programmers stuck to the standards,
instead of doing the same way they've always done it.  Unfortunately
many (if not most) programs start out as proof of concepts, and rather
than starting over after it's been proven they simply add on the
remaining requirements, conveniently forgetting that their proof of
concept was a quick hack...

Gerhard Fiedler wrote:

{Quote hidden}

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> I agree that it would be nice if programmers stuck to the standards,
> instead of doing the same way they've always done it.  Unfortunately
> many (if not most) programs start out as proof of concepts, and rather
> than starting over after it's been proven they simply add on the
> remaining requirements, conveniently forgetting that their proof of
> concept was a quick hack...

I guess the trick is to start doing proofs of concept already using Unicode
strings and string functions. Most higher level languages support that
(here we're back to Java as a good example... :)

I think just thinking of it would already do some good.

Gerhard

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