power on reset circuits

Discussion in 'Digital Circuit Design' started by shortbus, Dec 14, 2017.

  1. shortbus

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    What does everyone do for power up circuits, to put flip flops or other chips in the correct mode at start up? Ones that will set or reset more than one chip at the same time. Would like to see some proven circuits that you guys use for this, both high going and low going. Care to show what you use?
     
  2. MrChips

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  3. dl324

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    I use a spare inverter (schmitt preferred), with an RC on the input to give a brief HIGH or LOW pulse at power on.
     
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  4. shortbus

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    After posting and going offline, I remembered I didn't specify the logic I'm using. I know to you guy's it is obsolete, but I have a ton of it. I'm using CD4000 series logic, running on 12V.

    I looked at the available chips and didn't see any that work for the old Cmos logic.
     
  5. OBW0549

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    A lot of my PIC designs use an LP2951 LDO voltage regulator chip, which contains an internal comparator driving an open-collector, active-low ERROR output that pulls down when the output voltage falls to 92-95% of its programmed voltage. I connect this output, along with a 10 kΩ pullup resistor, to the RESET input of my MCU (and any other chips that require a POR input).

    The LP2951 can be programmed to an arbitrary voltage using an external voltage divider, or it can use its internal laser-trimmed voltage divider (it's made in 5.0V and 3.3V versions).
     
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  6. shortbus

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    Maybe the terms "power on reset" isn't what I'm looking for. I need to set - 2 CD4013 d flip flops, 2 CD4098 timers and two half H bridge drivers(low side to charge the caps for the high side) on start up.
     
  7. ElectricSpidey

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    I have always used a small cap, maybe .01uf or .1uf and a pull up/down resistor when using the 4000 series with no problems, this config also leaves the input available for use by other outputs.

    The cap connects to whichever rail provides the reset level, and the resistor connects to the rail that provides the chip active level. (yes usually opposite the reset level)

    I admit I have used this method mostly for starting 4017s at output 0 and only a few other chips, but I see no reason it wouldn’t work on most.

    I have never had the need to use additional logic to provide this function, but that could be the nature of the simpler circuits I mostly use.
     
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  8. ScottWang

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    Show the schematic then we can discuss more practical.
     
  9. shortbus

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    First thank you Scott for moving it to a more appropriate forum.

    Was this answer directed at me? I can't show my schematic yet because not knowing how to or what to use is a hold up on making the schematic. I'm a total newbie to drawing/designing circuits.
     
  10. shortbus

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    Last edited: Dec 16, 2017
  11. ScottWang

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    I fixed the error so the picture is shows up now.
    The component of circuit is different with what you said in #6, why?

    What is the ic number?
     
  12. AnalogKid

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    Without knowing where the input comes from, it's hard to say. If you define a POR signal as something that can be triggered *only* by the rising edge of applied power to the circuit, then you don't need a full-blown monostable, just a boxcar circuit (old RCA term).
    [​IMG]
    The right-hand circuit delivers a positive going pulse. Yes, the trailing edge is slow, but you're driving reset inputs, not an analog comparator the might produce a noise burst. Add an inverter to speed up the edges, swap the R and C for a negative-true output, etc.

    ak
     
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  13. ScottWang

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    You missed the good part is in the #6.
     
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  14. AnalogKid

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    The 4098's need a low to reset.
    The 4013's need a high to set or reset.
    The H bridge drivers need one or the other.

    The circuit in 13 will work if followed by two inverters in series.

    ak
     
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  15. shortbus

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    I don't understand what you asking/meaning. I just need to switch the low side on long enough to initially charge the bootstrap capacitor, so when the circuit starts to actually run/work it will switch on the high side. I'm using it as a half bridge not in a "H" bridge type configuration.

    The gate driver is a FAN7390.
     
  16. shortbus

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    Thanks for that. The input will be from a toggle switch.
     
  17. shortbus

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    I thought they had two resets, a rising edge or a falling edge. And you tied the unused one to the appropriate rail to disable it.
     
  18. AnalogKid

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    The 4098 has two *triggers* (one pos edge, one neg edge) and one reset that is negative true. The trigger inputs clock an internal flipflop, and thus cannot force the Q output low. The reset is an asynchronous clear to the flipflop and terminates a timing cycle.

    ak
     
  19. ScottWang

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    How are the two output sides, are they connected together or separated and how is the load?
     
  20. shortbus

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    Still not clear to me what your asking. I'm just wanting to "pre-charge" the bootstrap capacitors of two separate half bridges so the circuit will work when called on. This circuit will be totally sequential, no clock signal involve, so the bootstrap caps need to be pre-charged.
     
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