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DS1307 RTC and Supercap — Parallax Forums

DS1307 RTC and Supercap

ArchiverArchiver Posts: 46,084
edited 2004-03-16 17:29 in General Discussion
I'm attempting to add on a DS1307 RTC (real time clock) with a
supercap power backup.

Seems the cap isn't charging from the pin as I expected it would,
that some other means of keeping the charge must be supplied.

Does anyone know of an app note for doing this? Thanks.

Comments

  • ArchiverArchiver Posts: 46,084
    edited 2004-03-14 18:58
    In a message dated 3/14/2004 1:43:04 PM Eastern Standard Time,
    ernie@c... writes:


    > Seems the cap isn't charging from the pin as I expected it would,
    > that some other means of keeping the charge must be supplied.
    >
    > Does anyone know of an app note for doing this? Thanks.
    >
    >

    If I could make a suggestion, replace the supercap with a CR2032 Kithium
    button. Has a capacity of 220 maH, which will run the clock for several years.

    I'm using one in my setup.


    [noparse][[/noparse]Non-text portions of this message have been removed]
  • ArchiverArchiver Posts: 46,084
    edited 2004-03-14 19:13
    According to the docs, the DS1307 is designed for a 3v Lithium battery
    (good for 10 years without power), not for a cap like other devices..
    Switch to a 3v coin cell for back-up power.

    -- Jon Williams
    -- Applications Engineer, Parallax
    -- Dallas Office


    Original Message
    From: erniem1957 [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=1a0XDffjKjv3EXljvNI61KAN3d-oTFFPh5-jjexO-f55jY0pOZLXhnWscgWsA2Mo4ZCQQYObX4ljpQ]ernie@c...[/url
    Sent: Sunday, March 14, 2004 12:27 PM
    To: basicstamps@yahoogroups.com
    Subject: [noparse][[/noparse]basicstamps] DS1307 RTC and Supercap


    I'm attempting to add on a DS1307 RTC (real time clock) with a
    supercap power backup.

    Seems the cap isn't charging from the pin as I expected it would,
    that some other means of keeping the charge must be supplied.

    Does anyone know of an app note for doing this? Thanks.
  • ArchiverArchiver Posts: 46,084
    edited 2004-03-14 20:30
    --- In basicstamps@yahoogroups.com, "erniem1957" <ernie@c...> wrote:
    > I'm attempting to add on a DS1307 RTC (real time clock) with a
    > supercap power backup.
    >
    > Seems the cap isn't charging from the pin as I expected it would,
    > that some other means of keeping the charge must be supplied.
    >
    > Does anyone know of an app note for doing this? Thanks.


    I had to read this twice. you are trickle charging the SuperCap from
    the pin ?

    Are you sure the RTC is not seeing the current and preventing
    charging from the anti-Lithyum charing circuit, or is it lithyum
    anti-charging.... the part of the circuti that looks at the charge
    and prevents charging ?

    Also, you should be able to just charge the SC with a diode from any
    power supply when it is connected. Maybe a voltage divider to get the
    proper voltage to charge it, remember the diode drop.

    My curosity is how long of a back-up you get when you have a full
    charge ?

    And, of course I assume the only thing on Vbat (pin 3) of the chip ?

    I would expect the charge life of the SC would be the limiting factor
    as the chip has such a low use.

    But, I used a button with a battery instead of a SC as the holder and
    battery from DigiKey was less than a buck.

    Dave
  • ArchiverArchiver Posts: 46,084
    edited 2004-03-14 21:34
    Dave,

    Hold up time is computed directly from I = C * dV / dT, solving for
    dT. From the DS1307 spec sheet, I is 500nA. Range of Vbatt is 2 to
    3.5V, so let's assume we'll discharge from 3 to 2 volts for a dV of
    1V. I'm picking supercaps capable of holding the full VCC of 5 volts,
    such as a Panasonic EEC-S0HD223H ($2.70 each, .91 in quantity)
    with .022 *FARADS* of capacitance.

    This is the smallest supercap Digi-Key has in that series, and it
    should give me a hold up time of 12 hours. For another 6 cents I can
    get a full tenth of a farad (.1F) that brings the hold up time to
    over 2 days.

    Of course, this ignores any self-discharge of the cap, and any
    leakage back into the cap's recharging supply.

    The advantage of a SC is the device self-charges and should have a
    very long life. Of course, since even a 48mAhr battery is spec'ed for
    10 years of continious hold-up, I probably don't have much to worry
    about.

    I just don't like a limited shelf life that starts from the day a
    product is assembled, not used.


    --- In basicstamps@yahoogroups.com, "Dave Mucha" <davemucha@j...>
    wrote:
    > --- In basicstamps@yahoogroups.com, "erniem1957" <ernie@c...> wrote:
    > > I'm attempting to add on a DS1307 RTC (real time clock) with a
    > > supercap power backup.
    > >
    > > Seems the cap isn't charging from the pin as I expected it would,
    > > that some other means of keeping the charge must be supplied.
    > >
    > > Does anyone know of an app note for doing this? Thanks.
    >
    >
    > I had to read this twice. you are trickle charging the SuperCap
    from
    > the pin ?
    >
    > Are you sure the RTC is not seeing the current and preventing
    > charging from the anti-Lithyum charing circuit, or is it lithyum
    > anti-charging.... the part of the circuti that looks at the charge
    > and prevents charging ?
    >
    > Also, you should be able to just charge the SC with a diode from
    any
    > power supply when it is connected. Maybe a voltage divider to get
    the
    > proper voltage to charge it, remember the diode drop.
    >
    > My curosity is how long of a back-up you get when you have a full
    > charge ?
    >
    > And, of course I assume the only thing on Vbat (pin 3) of the
    chip ?
    >
    > I would expect the charge life of the SC would be the limiting
    factor
    > as the chip has such a low use.
    >
    > But, I used a button with a battery instead of a SC as the holder
    and
    > battery from DigiKey was less than a buck.
    >
    > Dave
  • ArchiverArchiver Posts: 46,084
    edited 2004-03-14 23:40
    In a message dated 3/14/2004 6:37:24 PM Eastern Standard Time,
    ernie@c... writes:


    > The advantage of a SC is the device self-charges and should have a
    > very long life. Of course, since even a 48mAhr battery is spec'ed for
    > 10 years of continious hold-up, I probably don't have much to worry
    > about.
    >

    You can not use a supercap with a DS1307. If you will look at the data sheet
    is specifically says 3V battery. The chip has no provisions for a trickle
    charge like the DS1302.

    Sid


    [noparse][[/noparse]Non-text portions of this message have been removed]
  • ArchiverArchiver Posts: 46,084
    edited 2004-03-15 00:14
    The DS1307 uses the backup power source as a reference for switching
    from main power to backup power, and that really works best with a
    lithium coin cell as the backup.

    When the main power drops below 3.75 volts (or 1.25 times the backup
    --1.25*3=3.75), the interface shuts off and the Stamp can no longer
    read the clock (it gets back all $FF if it tries!). At lower
    voltages the clock oscillator operates off of the backup source.

    Note that the main power for the I2C interface draws about 200
    microamps, but the clock oscillator itself draws only 0.5 microamp.

    A 200 milliamp-hour lithium cell has a projected life of 400000
    hours, or over 45 years based on milliamp hours alone. Crank that
    down to the shelf life of 10 years. In micro-power systems, it is
    standard practice to run the DS1307 most of the time off of the
    lithium cell, and only turn on the main interface power when it is
    necessary to read the chip.

    You could use a supercap, but you would have to provide means to
    charge it up to no more than 3.3 volts. You can't use diodes alone
    to drop the voltage from 5 to 3, because at those low current levels
    diodes do not have much voltage drop. The theoretical backup time
    from a 1 farad supercap is huge, but in practice the self discharge
    rate of the capacitor limits it to hours or days. As others have
    said, you will be much better off using the lithium coin cell with
    the DS1307.

    -- regards,
    Tracy Allen
    electronically monitored ecosystems
    mailto:tracy@e...
    http://www.emesystems.com
  • ArchiverArchiver Posts: 46,084
    edited 2004-03-15 22:46
    --- In basicstamps@yahoogroups.com, "erniem1957" <ernie@c...> wrote:
    > Dave,
    >
    > Hold up time is computed directly from I = C * dV / dT, solving
    for
    > dT. From the DS1307 spec sheet, I is 500nA. Range of Vbatt is 2 to
    > 3.5V, so let's assume we'll discharge from 3 to 2 volts for a dV of
    > 1V. I'm picking supercaps capable of holding the full VCC of 5
    volts,
    > such as a Panasonic EEC-S0HD223H ($2.70 each, .91 in quantity)
    > with .022 *FARADS* of capacitance.
    >
    > This is the smallest supercap Digi-Key has in that series, and it
    > should give me a hold up time of 12 hours. For another 6 cents I
    can
    > get a full tenth of a farad (.1F) that brings the hold up time to
    > over 2 days.
    >
    > Of course, this ignores any self-discharge of the cap, and any
    > leakage back into the cap's recharging supply.
    >
    > The advantage of a SC is the device self-charges and should have a
    > very long life. Of course, since even a 48mAhr battery is spec'ed
    for
    > 10 years of continious hold-up, I probably don't have much to worry
    > about.
    >
    > I just don't like a limited shelf life that starts from the day a
    > product is assembled, not used.


    I have to agree about the Lithium batteries. I have seen surplus go
    cheap, AND have date manufactured listed. with a 4 year old battery,
    the time is numbered.

    For the 220mAh lithium batt from digi-Key and a cost of about 39
    cents, it is not a concern for many projects, even if you know it
    will be dead in 5 or 10 years.

    I have no experiance with the actual long term voltage holding of a
    super cap so can't comment. I figure that it would not be hard to
    charge one in a circuit that occasionally sees any type of power
    supply.


    Dave
  • ArchiverArchiver Posts: 46,084
    edited 2004-03-15 23:17
    --- In basicstamps@yahoogroups.com, "erniem1957" <ernie@c...> wrote:
    > Dave,
    >
    > Hold up time is computed directly from I = C * dV / dT, solving
    for
    > dT. From the DS1307 spec sheet, I is 500nA. Range of Vbatt is 2 to
    > 3.5V, so let's assume we'll discharge from 3 to 2 volts for a dV of
    > 1V. I'm picking supercaps capable of holding the full VCC of 5
    volts,
    > such as a Panasonic EEC-S0HD223H ($2.70 each, .91 in quantity)
    > with .022 *FARADS* of capacitance.
    >
    > This is the smallest supercap Digi-Key has in that series, and it
    > should give me a hold up time of 12 hours. For another 6 cents I
    can
    > get a full tenth of a farad (.1F) that brings the hold up time to
    > over 2 days.
    >
    > Of course, this ignores any self-discharge of the cap, and any
    > leakage back into the cap's recharging supply.
    >
    > The advantage of a SC is the device self-charges and should have a
    > very long life. Of course, since even a 48mAhr battery is spec'ed
    for
    > 10 years of continious hold-up, I probably don't have much to worry
    > about.
    >
    > I just don't like a limited shelf life that starts from the day a
    > product is assembled, not used.



    I dug a little deeper on the Dallas site and found the app note on
    enabling the trickle charger. It is shipped OFF and must be turned
    on.

    However, the note is written for the 1302, and does not explicitly
    list the 1307 chip, so I do not know if it works for that one.

    http://pdfserv.maxim-ic.com/en/an/app82.pdf


    Charging for a rechargable battery or SuperCap can be selected in
    software, but it is required that you do that.

    Also, I have not tried to use the trickle charger, but it appears
    that it defaults to the no-charge on start-up and may need to be told
    to trickel charge after a complete battery drain or change of both
    power supplies.

    All the calculations for charging the Super Cap are on that sheet.

    Dave
  • ArchiverArchiver Posts: 46,084
    edited 2004-03-16 16:44
    Actually, as mentioned by someone earlier. The 1307 doesn't include a
    trickle charger. I assumed it had one as well but, check the datasheet. Not
    a single peep about any form of charger on the vbat circuit.

    Not really an issue for me since I'd intended to go with a 3v coin cell from
    the start anyway.

    >
    Original Message
    > From: Dave Mucha [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=MPYvBZnsZiEApcQCznMsQolkWfUyW5HeUGE0MjcUAdqsM46apm3Ye_XN7q8n3BEXhvbCs_uYTv8]davemucha@j...[/url
    > Sent: March 15, 2004 3:17 PM
    > To: basicstamps@yahoogroups.com
    > Subject: [noparse][[/noparse]basicstamps] Re: DS1307 RTC and Supercap
    >
    > I dug a little deeper on the Dallas site and found the app note on
    > enabling the trickle charger. It is shipped OFF and must be turned
    > on.
    >
    > However, the note is written for the 1302, and does not explicitly
    > list the 1307 chip, so I do not know if it works for that one.
    >
    > http://pdfserv.maxim-ic.com/en/an/app82.pdf
    >
    >
    > Charging for a rechargable battery or SuperCap can be selected in
    > software, but it is required that you do that.
    >
    > Also, I have not tried to use the trickle charger, but it appears
    > that it defaults to the no-charge on start-up and may need to be told
    > to trickel charge after a complete battery drain or change of both
    > power supplies.
    >
    > All the calculations for charging the Super Cap are on that sheet.
    >
    > Dave
  • ArchiverArchiver Posts: 46,084
    edited 2004-03-16 17:29
    --- In basicstamps@yahoogroups.com, "Dave Mucha" <davemucha@j...>
    wrote:
    > --- In basicstamps@yahoogroups.com, "erniem1957" <ernie@c...> wrote:
    > > Dave,
    > >
    > > Hold up time is computed directly from I = C * dV / dT, solving
    > for
    > > dT. From the DS1307 spec sheet, I is 500nA. Range of Vbatt is 2
    to
    > > 3.5V, so let's assume we'll discharge from 3 to 2 volts for a dV
    of
    > > 1V. I'm picking supercaps capable of holding the full VCC of 5
    > volts,
    > > such as a Panasonic EEC-S0HD223H ($2.70 each, .91 in quantity)
    > > with .022 *FARADS* of capacitance.
    > >
    > > This is the smallest supercap Digi-Key has in that series, and it
    > > should give me a hold up time of 12 hours. For another 6 cents I
    > can
    > > get a full tenth of a farad (.1F) that brings the hold up time to
    > > over 2 days.
    > >
    > > Of course, this ignores any self-discharge of the cap, and any
    > > leakage back into the cap's recharging supply.
    > >
    > > The advantage of a SC is the device self-charges and should have
    a
    > > very long life. Of course, since even a 48mAhr battery is spec'ed
    > for
    > > 10 years of continious hold-up, I probably don't have much to
    worry
    > > about.
    > >
    > > I just don't like a limited shelf life that starts from the day a
    > > product is assembled, not used.
    >
    >
    >
    > I dug a little deeper on the Dallas site and found the app note on
    > enabling the trickle charger. It is shipped OFF and must be turned
    > on.
    >
    > However, the note is written for the 1302, and does not explicitly
    > list the 1307 chip, so I do not know if it works for that one.
    >
    > http://pdfserv.maxim-ic.com/en/an/app82.pdf
    >
    >
    > Charging for a rechargable battery or SuperCap can be selected in
    > software, but it is required that you do that.
    >
    > Also, I have not tried to use the trickle charger, but it appears
    > that it defaults to the no-charge on start-up and may need to be
    told
    > to trickel charge after a complete battery drain or change of both
    > power supplies.
    >
    > All the calculations for charging the Super Cap are on that sheet.
    >
    > Dave


    Dave,

    Awesome find, thanks. I must have read the DS1302 sheet and thought
    it applied to the DS1307. I'll be switching to the '02 now.

    Ernie
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