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DS1307 Object — Parallax Forums

DS1307 Object

NewzedNewzed Posts: 2,503
edited 2006-09-17 17:12 in Propeller 1
Beau, in your DS1307 Object, you have it running off of 3.3VDC.· Accoring to the data sheet, the 1307 requires 4.5VDC minimum for proper operation.· Am I reading the data sheet wrong?

Sid

Comments

  • Beau SchwabeBeau Schwabe Posts: 6,559
    edited 2006-09-17 04:20
    Sid,

    I think you are reading it correct... I will change it to use 5V instead of 3.3V



    from page 2 of the datasheet...

    Vpf (Power fail voltage) = 3V ... So 3.3V is cutting it close.
    From the same page, the DS1307 "sees" a logic "1" as 2.2V so we are OK here.

    from page 6 (Pin 8 description)...

    "Primary Power Supply. When voltage is applied within normal limits, the device is fully
    accessible and data can be written and read. When a backup supply is connected to the
    device and VCC is below VTP, read and writes are inhibited. However, the timekeeping
    function continues unaffected by the lower input voltage."

    Although I am stretching the "normal limits" with the power supply, I interpret the
    statement...
    "...the timekeeping function continues unaffected by the lower input voltage"
    ...to mean that even though I am supplying the DS1307 with 3.3V, it's not going to
    impact the clock oscillator. (In this DEMO, I am also driving the 32.768kHz from the
    Propeller) so this could make a difference if you are comparing this to free running
    with a crystal.

    from page 7 (CLOCK ACCURACY)

    Here it does not indicate crystal accuracy correlating to supply voltage so I think we are OK here.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
    pdf
    pdf
    DS1307.pdf
    875K
  • Tracy AllenTracy Allen Posts: 6,660
    edited 2006-09-17 17:12
    The DS1338-33 is an alternative chip, the same pinout and command structure, but meant for a 3.3 volt supply. It is harder to come by than the '1307 though.

    The old DS1307 uses the backup battery as the reference for low voltage detection, and it goes into backup mode when the power supply voltage drops below Vbat+1.5. With a 3 volt backup, that puts the threshold at 4.5 volts. When the power drops below that, the I2C interface goes dead, and the clock operation switches over to the backup battery. If you don't use the backup battery and hook that pin to ground instead, then the I2C interface can usually work to quite low voltages, even at 3.3 volts, although it is speced for operation over Vcc= 4.5 to 5.5 volts.

    The DS1338-33 uses an internal 2.8 volt reference instead of the batttery for power switching between Vcc and Vbat. It can use a 3 volt backup battery. The I2C interface goes dead below 2.8 volts.

    Note that on either of these chips, the backup mode is not an ultra-low power mode for Vcc. Even in backup mode, the DS1307 draws ~200 microamps from Vcc, and the DS1338 draws ~60 microamps. To get full power down, it is necessary to bring Vcc to zero (and also be sure that the I2C lines are at zero to avoid parasitic power). Then the clock operates at <1 microamp off the backup battery.

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    Tracy Allen
    www.emesystems.com
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