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8051 On/Off Circuit

Discussion in 'Electronic Design' started by mpm, Dec 28, 2007.

  1. mpm

    mpm Guest

    This is general enough to be of use to several folks.

    Anybody got a simple momentary keypad-switchable ON-OFF circuit for a
    8051 (or similar)?
    Figure about 100 mA MAX, and will be battery operated (9-Volt).

    Program will run for a minute or so, then the device should shutdown
    (under uPC control) until the next time it's needed. Probably do
    this about 20 times a day. Battery lifetime is important.

    MINIMUM cost, and minimum parts count would be nice too! :)


    The only thing I can think of is a FET controlling the input to a
    switch mode power supply / regulator. (But admittedly, I don't do
    much battery-only stuff, and my solar design are higher current and
    not easily adaptable for low cost.)

    Thanks.
     
  2. Henry Kiefer

    Henry Kiefer Guest

    A self-holding thyristor in series to the 8051.


    - Henry
     
  3. Jamie

    Jamie Guest

    P channel enhanced high side switch.
    The remote signal will pull the gate low. Use one output from the uC
    as a open collector config that also connects to the gate. When the Uc
    boots, it shall pull this output low. when the uC wants to turn it self
    off, just lift this output to high.
    if you want to manually turn the device off, use another input that
    you simply pull low which will have the uC turn off the output line.
    etc..
    that's just as basic run down.
    you can use a low voltage logic level P channel.
     
  4. Nico Coesel

    Nico Coesel Guest

    If battery lifetime is important, the MSP430 from TI is definitely the
    way to go. Since it has interrupts based on its I/O pins it is very
    easy to have the device to wake-up when a key is pressed. The MSP430
    alone can consume less than 1mA while running. An MSP430 running at
    6MHz will beat a 40MHz 8051 hands down when it comes to processing
    power.
     
  5. Frank Buss

    Frank Buss Guest

    A nice part is the MAX4715 digital switch:

    http://datasheets.maxim-ic.com/en/ds/MAX4715-MAX4716.pdf

    The key can switch it on and decoupled with 2 diodes the CPU can hold
    itself on. And an additional pull-down at the input: When the CPU releases
    the output, it turns off itself, with very low quiescent current.

    I've used this part in a customer product and works great. Only drawback is
    that it is from Maxim, but at least the Maxim store has it in stock and the
    customer uses this part for some years without delivery problems.

    But for minimum price a FET solution might be better. Part count might be
    better with the MAX4715.
     
  6. I think it's non-trivial. The micro will do undefined things as its
    supply voltage drops, and there are protection diode networks that can
    come into play. 8051s have those awful pseudo-bidirectional ports
    (sometimes).

    The easiest solution, assuming a modern CMOS micro with sleep mode and
    "wake on port pin change" or "wake on interrupt" function is to power
    the micro continuously and shut down the internal clock oscillator.
    Usually you can leave port pins and so on in such a state as to draw
    microamps or better during sleep. There are potential safety issues
    with this approach, and you might need to hide a "hard" reset switch
    somewhere.


    Best regards,
    Spehro Pefhany
     
  7. Eeyore

    Eeyore Guest

    Why use an 8051 ?

    A monostable driving a power device would seem to be all you need.

    Graham
     
  8. What he said. :) Seems to me that most modern parts should have sleep
    modes that draw negligible amounts of current, yet can wake up instantly on
    an input pin state change. IOW, use a PIC. ;-)
     
  9. Henry Kiefer

    Henry Kiefer Guest

    Think about the thyristor idea a second time.
    Start it by pressing a button parallel to the thyristor. Stop it by let
    the CMOS MCU going in sleep state. Reducing the current let the
    thyristor go open.

    Of course, there are other possible solutions too.


    - Henry
     
  10. Eeyore

    Eeyore Guest

    Actually you need to feed some gate current to initially turn the thyristor
    on. Shorting it out ought to take it out of conduction too as it won't any
    longer have its holding current.

    OK as long as the OP doesn't mind the supply voltage varying by ~ 0.7V.

    The sleep idea sounds good too.

    Graham
     
  11. mpm

    mpm Guest

    Agreed, but in this particular design (data collection & storage),
    it's unlikely undefined startup conditions or flake-y power would
    actually cause a problem. The device just won't appear to operate...

    But back to the issue:
    The device is battery powered (single 9-volt) and the 8051 is NOT the
    only thing consuming power. So while "sleep" mode and/or power down
    mode will work for that, it won't necessarily power down everything
    else. (LCD display, serial EEPROM, communciations circuits, etc..)

    Additional note: You know, I really did want to use the LCD
    backlight, but at 160mA it does seem like you'd be replacing the 9V
    battery pretty often. Bummer.

    And I did not mention it but the same circuit board will power both a
    wall-mount version AND a portable, handheld version. The former being
    powered by a wall transformer, so no problem there obviously. (I
    could just leave it running constantly).

    But on the handheld unit, power conservation is really rather
    important.
    I was already thinking along the lines of switching P/S, etc...

    Ideally, I'd just like to turn the device completely OFF, until the
    user presses a momentary keypad switch to turn it ON. Then let the
    8051 device do it's thing and shut off.

    I will look at the Thyrister and high-side suggestions. Also the
    MAX4715 idea.
    I am aware of several regulators that have an "enable" line, and that
    would of course solve the problem.

    I was just wondering how everyone else does this, and as I suspected,
    there are lots of good ideas out there.

    -mpm
     
  12. It might not shut off, and drain the batteries for you, or just annoy
    the user because they have to hit the key multiple times to get it to
    shut down. You may be able to design it so that a hardware supervisory
    circuit holds the micro in a defined /RESET state to low enough supply
    voltage (eg. 1V) that nothing bad can happen below that, if your
    on/off circuit is designed to work thusly.
    You have control over those things if it's your circuit.
    You could use AAs and maybe a SEPIC, but 160mA is still pretty
    thirsty. More $$ = a more efficient backlight.
    I don't think that's necessarily ideal.
    Would it? Only with some careful design.

    Best regards,
    Spehro Pefhany
     
  13. MooseFET

    MooseFET Guest

    The switching is done with a P-MOSFET

    The logic is only a little tricky.

    You need a comparitor or something on the reset to keep the micro from
    running when the voltage is low.

    The button has a direct method of turning on the P-MOSFET and can be
    read by the micro. Usually this involves two small N-MOSFETs or NPN
    transistors. One pulls down the gate of the P-MOSFET and the other
    goes to an IO pin.

    There is also a flip-flop driving another NPN or small N-MOSFET. This
    flip-flop is set by an RC when the voltage rises because the button
    was pressed. This is what latches the power on. To switch off the
    power, the micro polls the power button to make sure it isn't pressed
    and clears the flip-flop.
     
  14. Henry Kiefer

    Henry Kiefer Guest

    Oh sorry. Yes, the gate must be triggered by the button.

    Sleeping is best solution if the circuit draws almost nothing in this state.


    - Henry
     
  15. Henry Kiefer

    Henry Kiefer Guest

    My next idea woild be a OptoMOS relais in series to the voltage
    regulator/mcu. The LED driver side is controlled by an mcu output pin.
    To start the circuit just press one button parallel to the relais.

    It is more efficient than the thyristor.


    - Henry
     
  16. Nico Coesel

    Nico Coesel Guest

    You should avoid using switchers since these use quite a lot of power
    even when disabled. Powering your device from a 3.6V battery makes
    much more sense; you won't need a regulator (especially when using the
    MSP430 I proposed before). The LCD backlight can be controlled by a
    pulsating current (perhaps a small inductor in series) rather than a
    switching regulator.

    IMHO any solution with a switching regulator or an external power
    supply switch element is going to draw more power than a (sleeping)
    microcontroller which is directly powered from the battery.
     
  17. Eeyore

    Eeyore Guest

    There's no shortage of low-voltage 8051 family members.

    The 89C51 goes as low as 2.7V for example.

    Graham
     
  18. "mpm" <> schreef in bericht
    \
    \Agreed, but in this particular design (data collection & storage),
    \it's unlikely undefined startup conditions or flake-y power would
    \actually cause a problem. The device just won't appear to operate...
    \
    \But back to the issue:
    \The device is battery powered (single 9-volt) and the 8051 is NOT the
    \only thing consuming power. So while "sleep" mode and/or power down
    \mode will work for that, it won't necessarily power down everything
    \else. (LCD display, serial EEPROM, communciations circuits, etc..)
    \
    \Additional note: You know, I really did want to use the LCD
    \backlight, but at 160mA it does seem like you'd be replacing the 9V
    \battery pretty often. Bummer.
    \
    \And I did not mention it but the same circuit board will power both a
    \wall-mount version AND a portable, handheld version. The former being
    \powered by a wall transformer, so no problem there obviously. (I
    \could just leave it running constantly).
    \
    \But on the handheld unit, power conservation is really rather
    \important.
    \I was already thinking along the lines of switching P/S, etc...
    \
    \Ideally, I'd just like to turn the device completely OFF, until the
    \user presses a momentary keypad switch to turn it ON. Then let the
    \8051 device do it's thing and shut off.
    \
    \I will look at the Thyrister and high-side suggestions. Also the
    \MAX4715 idea.
    \I am aware of several regulators that have an "enable" line, and that
    \would of course solve the problem.
    \
    \I was just wondering how everyone else does this, and as I suspected,
    \there are lots of good ideas out there.
    \
    \-mpm
    \

    I don't like the idea of a processor switching itself off, but the principle
    looks not too difficult. The circuit below can be considered standard.

    +9V----+--------- ------------
    | \ ^
    | ---
    | ___ |
    +---|___|--+
    | ___ |
    +---|___|--)----+
    | |
    | o |
    | |=|
    | o |
    | | to
    \| | ___ processor
    |--+-|___|-- pin
    <|
    |
    |
    GND--------------+-------------
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    Problems may arise when the switch is pressed too short so you will need
    some additional circuitry. Starting a micro is always surrounded by unwanted
    side effects anyway. I'd make the processor switch off all components but
    itself by a circuit like the one above and then go to sleep. This way the
    micro keeps control.

    petrus bitbyter
     
  19. Henry Kiefer

    Henry Kiefer Guest

    There is always a transitional problem, so any scheme must be tested at
    least several hundreds time for reliable switching the state.


    - Henry
     
  20. MooseFET

    MooseFET Guest

    On Dec 29, 12:08 pm, "petrus bitbyter"
    [....]

    I think you wanted a PNP as the pass device. As drawn it doesn't
    work.
    (org. drawing)

    I also think you may need a pull up on the processor pin. The 8051s
    don't pull up very well.
    Modified:
     
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