Post by thread:[EE:] Supercap - how long will it backup my RTC?

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 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> 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 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> >> 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) -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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 - I2C-bus slave address: read A3H and write A2H - 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 written or read data byte. 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 j.. I²C-bus slave address: read A3H and write A2H k.. Open-drain interrupt pin. -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> 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? /> Standard answer. All together now ... "Bill Gates. Bill Gates. B ..... " :-) RM -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

On Tue, Aug 10, 2004 at 12:24:56PM +0530, spam_OUTBala.ChandarTakeThisOuTAVENTIS.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 -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

>> 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? > Standard answer. > 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 -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

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 the numeric keypad. 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 -- .....aiwKILLspam@spam@cypress.com === Principal Design Engineer === Cypress Semiconductor Corporation === === Opinions expressed above do not === necessarily represent those of === Cypress Semiconductor Corporation -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

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..." -Adam -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> 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 > the numeric keypad. 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 -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

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 forth. I had to change everything that had to do with reading/writing 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... -Adam Gerhard Fiedler wrote: {Quote hidden}>>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 >> the numeric keypad. >> >> > >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 > >-- >http://www.piclist.com hint: The PICList is archived three different >ways. See http://www.piclist.com/#archives for details. > > > > > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> 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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body