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Real Time Clocks?

Discussion in 'Electronic Design' started by simon, Apr 23, 2004.

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  1. simon

    simon Guest

    Hello. Can anyone advise please?

    I have to design a product around an RTC that generates up to 25
    programmable alarms per day. I can handle all necessary programming by
    way of a micro controller (inc. EEPROM) and 2x16 LCD display. Ideally, I
    would like to set the alarm times, then put all active devices into
    sleep mode until the RTC enters an alarm condition which, it turn,
    generates an interrupt to wake-up the micro controller. Power saving is
    a premium condition - operating voltage is not a consideration.

    Can anyone suggest a suitable RTC that may go some way to achieving the

    Thank you,
  2. Any RTC with a hardware alarm output will do. DS1371 but there are
    many others. You keep the list of 25 alarms on the controller, and
    when the RTC wakes up the controller, your software loads the RTC
    with the next alarm, and goes back to sleep again. You don't need
    an RTC with 25 alarms inside the RTC itself, and you won't find
    any, for 99.99% sure ;)
  3. simon

    simon Guest

    I feel like a real idiot now - why didn't I think of that! Probably too
    many beers last night!

    Cheers Frank,
  4. Joerg

    Joerg Guest

    Hi Simon,

    Another option would be to design this around an MSP430. These run on a
    clock crystal and the internal higher frequency oscillator is sync'd to
    that. When running only the clock crystal oscillator with the high frequency
    osc turned off (kind of a "semi sleep mode") they consume only a few micro

    Regards, Joerg
  5. Tim Wescott

    Tim Wescott Guest

    There are other micros that do this, but I don't know how they stack up
    against the TI part.
  6. Joerg

    Joerg Guest

    Hi Tim,

    I haven't seen any yet that are this low in power. Some of the 80C51 family can
    but I had to completely shut down their clocks to bring the power down this low
    and then there is no realtime capability.

    The TI parts are really nice. My only concern is that they are single sourced
    whereas much of the 51 family isn't. Or at least wasn't some years ago.

    Regards, Joerg
  7. Tim Wescott

    Tim Wescott Guest

    I can believe that. I remember that TI was pretty proud of their
    low-power performance when they came out with the things.

    I suspect that anything, 8051-based or not, is going to have it's own
    unique peripherals, so you're going to be stuck with that manufacturer's
    product. If you just go with a plain-jane 8051 (assuming you could find
    one!) you'd have to add peripherals at the expense of mondo board space.
  8. Joerg

    Joerg Guest

    Hi Tim,

    Yes, if it has to be multi source you have to be frugal with peripheral stuff. Only
    dual slope AD and so on. The 89C51 did pretty well and you could buy it almost
    anywhere. But only if run at slow clock speeds or the number of available mfgs goes
    down rapidly. However, I think they rang the last order bell on that nice chip.

    Unfortunately many designs aren't really possible with single source parts. The
    client just doesn't want that or maybe got burned badly some time ago. I bet that is
    one reason for the success of the 51 architecture and the huge number of firmware
    programmers that are familiar with it. Sometimes I wish more uC mfgs would realize
    that and negotiate a deal with the competition about 2nd source. At least for the
    run-of-the-mills versions of their part.

    Regards, Joerg
  9. simon

    simon Guest

    Thanks Joerg, I'll look this part up.

  10. Michael

    Michael Guest

    I second what Frank said. I use a DS1305 RTC alarm interrupt to wake up
    a data logger every 5 minutes, 24/7. After servicing interrupt, uC
    computes time of next alarm, loads that into RTC, goes to sleep.
  11. simon

    simon Guest

    Michael. I'm going to follow your advice - thank you.

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