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PICList Thread
'power-on reset problems'
1996\10\31@001452 by Matthew Mucker

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I have a prototype circuit that, about 75% of the time, does not do a
proper power-on reset.  The power supply is a wall-wart transformer with a
10uF cap across the leads ffeding a 7805 regulator with a 100uF cap across
+5V and gnd. (The capacitor values were chosen by the scientific method of
looking into the junkbox and picking out what was in there)

Is there a small, cheap external circuit I can build to guarantee a
power-on reset?

Thanks,
       -Matt


 "DOS Computers manufactured by companies such as IBM, Compaq, Tandy, and
millions of others are by far the most popular, with about 70 million
machines in use wordwide. Macintosh fans, on the other hand, may note that
cockroaches are far more numerous than humans, and that numbers alone do
not denote a higher life form."

1996\10\31@012352 by tjaart

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Matthew Mucker wrote:
>
> I have a prototype circuit that, about 75% of the time, does not do a
> proper power-on reset.  The power supply is a wall-wart transformer with a
> 10uF cap across the leads ffeding a 7805 regulator with a 100uF cap across
> +5V and gnd. (The capacitor values were chosen by the scientific method of
> looking into the junkbox and picking out what was in there)
>
> Is there a small, cheap external circuit I can build to guarantee a
> power-on reset?
>
> Thanks,
>         -Matt

The databooks contain a number of circuits, ranging from cheap to
effective.

--
Friendly Regards

Tjaart van der Walt
______________________________________________________________
|  Another sun-deprived R&D Engineer slaving away in a dungeon |
|WASP International GSM vehicle tracking and datacomm solutions|
|           +27-(0)11-622-8686 | http://wasp.co.za             |
|______________________________________________________________|

1996\10\31@012803 by Clyde Smith-Stubbs

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Matthew Mucker <spam_OUTmmuckerTakeThisOuTspamAIRMAIL.NET> wrote:

> Is there a small, cheap external circuit I can build to guarantee a
> power-on reset?

Yes, but if you tell us what you have hanging off the /MCLR pin
right now, it might be easier to suggest what to do next. Also,
why not put the bigger (100uF) cap on the *input* to the 7805 and
the smaller one on the output? There's no reason why you should need
100uF on the 5V rail.

Clyde

--
Clyde Smith-Stubbs       | HI-TECH Software,       | Voice: +61 7 3354 2411
.....clydeKILLspamspam@spam@hitech.com.au      | P.O. Box 103, Alderley, | Fax:   +61 7 3354 2422
http://www.hitech.com.au | QLD, 4051, AUSTRALIA.   |
---------------------------------------------------------------------------
For info on the World's best C cross compilers for embedded systems, point
your WWW browser at http://www.hitech.com.au, or email infospamKILLspamhitech.com.au

1996\10\31@062418 by dontronics

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Matthew Mucker wrote:
>
> I have a prototype circuit that, about 75% of the time, does not do a
> proper power-on reset.  The power supply is a wall-wart transformer with a
> 10uF cap across the leads ffeding a 7805 regulator with a 100uF cap across
> +5V and gnd. (The capacitor values were chosen by the scientific method of
> looking into the junkbox and picking out what was in there)
>
> Is there a small, cheap external circuit I can build to guarantee a
> power-on reset?
>
> Thanks,
>         -Matt

There are lots of devices designed for doing just this. Some cheap, some
not so cheap.

Try the Panasonic MN1381-S. This is possibly the cheapest version in a
TO-92 package. It rates a mention in the Oct-96 edition of Nuts and
Volts, and basic data can be found in the current Digi-Key catalogue. A
3 wire cct. VCC, GND and reset. Forces a low reset if the voltage is
detected in a specified range.


Don McKenzie .....donmckKILLspamspam.....labyrinth.net.au
DonTronics Tullamarine, Australia
http://www.labyrinth.net.au/~donmck

EASY PIC'n Beginners Guide to using PIC 16/17 MicroChip products.
Picosaurus(tm) 40 pin FED Basic with 8 channels of A-D, and real Uart.
MEL PicBasic Compiler. Programmers from 15 USD.  Pic-Axe(tm) A New Tool.

1996\10\31@070159 by Michael Yano

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I've had similar problems with all of my pic projects, the hardware solution
is a Motorola part MC33064 reset chip, three leads in a transistor package.
This is fairly reliable, 98% of the time. Using the watchdog timer seems to be
the software solution and is reliable. The power supply is not the issue, I
use $200
Hammond power supplies and the problem still occurs. If the POR fails then the
watchdog will bring it up after the timeout.

At 18:57 10/30/96 +0500, you wrote:
{Quote hidden}

Thanks.
    __           Micheal Yano
   ----          Internet:     EraseMEmiyspam_OUTspamTakeThisOuTcimtek.on.ca
 -CIMTEK-        IBMMAIL:      i1320285spamspam_OUTIBMMAIL.COM
------------      Voice:        (905)847-8811 x22
                   FAX:        (905)847-8822 x22

1996\10\31@110554 by myke predko

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>Matthew Mucker <@spam@mmuckerKILLspamspamAIRMAIL.NET> wrote:
>
>> Is there a small, cheap external circuit I can build to guarantee a
>> power-on reset?
>
>Yes, but if you tell us what you have hanging off the /MCLR pin
>right now, it might be easier to suggest what to do next. Also,
>why not put the bigger (100uF) cap on the *input* to the 7805 and
>the smaller one on the output? There's no reason why you should need
>100uF on the 5V rail.
>
>Clyde
>

Matthew,

I typically use your type of circuit, without brownout protection, so the
problem is soluable.

I agree with Clyde, you should not have a 100 uF cap on your Vdd Rail - this
will slow down your power up, which will cause restart problems.  I use 10
uF caps on both sides of the 7805.

As Clyde said, you should also check how _MCLR is connected.  I usually
attach it to Vdd through a 4.7K resistor.  This way, to reset the circuit, I
just have to short _MCLR and Vss (adjacent) pins with a screw driver.

Lastly, have you turned on the PWRTE Bit in the configuration file?  This
will delay powerup by 70 ms (approx) for the Power to stabilize.

Good Luck,

myke


>--
>Clyde Smith-Stubbs       | HI-TECH Software,       | Voice: +61 7 3354 2411
>KILLspamclydeKILLspamspamhitech.com.au      | P.O. Box 103, Alderley, | Fax:   +61 7 3354 2422
>http://www.hitech.com.au | QLD, 4051, AUSTRALIA.   |
>---------------------------------------------------------------------------
>For info on the World's best C cross compilers for embedded systems, point
>your WWW browser at http://www.hitech.com.au, or email RemoveMEinfoTakeThisOuTspamhitech.com.au
>
>

Avoiding precedents does not mean nothing should ever be done.  It only
means that nothing should ever be done for the first time - Sir Humphrey
Appleby K.C.B.

1996\10\31@151448 by Matthew Mucker

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>> Is there a small, cheap external circuit I can build to guarantee a
>> power-on reset?
>
>Yes, but if you tell us what you have hanging off the /MCLR pin
>right now, it might be easier to suggest what to do next. Also,
>why not put the bigger (100uF) cap on the *input* to the 7805 and
>the smaller one on the output? There's no reason why you should need
>100uF on the 5V rail.

!MCLR is tied to +5V.  I only have the condensed data sheets that Parallax
sends with their stuff, so I don't really know all that I should here.
Obviously.

-Matt


 "DOS Computers manufactured by companies such as IBM, Compaq, Tandy, and
millions of others are by far the most popular, with about 70 million
machines in use wordwide. Macintosh fans, on the other hand, may note that
cockroaches are far more numerous than humans, and that numbers alone do
not denote a higher life form."

1996\10\31@165609 by Clyde Smith-Stubbs

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Matthew Mucker <spamBeGonemmuckerspamBeGonespamAIRMAIL.NET> wrote:

> !MCLR is tied to +5V.  I only have the condensed data sheets that Parallax
> sends with their stuff, so I don't really know all that I should here.

Ok, then all you probably need is to take a 100K resistor from VCC,
paralleled with a diode, connected to a 1uF tantalum cap to ground. Connect
/MCLR to the junction, e.g.


                                 +| |
       VCC--+--/\/\/\/\/\---+-----| |-----GND
            |               |     | |
            |     |/        |
            ------| --------+
                  |\        |
                            /MCLR

The diode can be a 1N914 or equivalent. You should find this will give a solid
power-on reset. It won't give total brownout protection, but it should be
quite good enough. The diode is reverse-biased while operating, but dischages
the cap quickly when power is removed.

Clyde

--
Clyde Smith-Stubbs       | HI-TECH Software,       | Voice: +61 7 3354 2411
TakeThisOuTclydeEraseMEspamspam_OUThitech.com.au      | P.O. Box 103, Alderley, | Fax:   +61 7 3354 2422
http://www.hitech.com.au | QLD, 4051, AUSTRALIA.   |
---------------------------------------------------------------------------
For info on the World's best C cross compilers for embedded systems, point
your WWW browser at http://www.hitech.com.au, or email RemoveMEinfospamTakeThisOuThitech.com.au

1996\10\31@192455 by Dwayne Reid

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>I have a prototype circuit that, about 75% of the time, does not do a
>proper power-on reset.  The power supply is a wall-wart transformer with a
>10uF cap across the leads ffeding a 7805 regulator with a 100uF cap across
>+5V and gnd.


Just for fun, try putting a 220 ohm resistor accross the input filter cap.
PICs can lock up if the power supply voltsge does not collapse below 0.6 Vdc
or so.  Your filter cap can hold at that voltage for days.  Thanks to Andrew
Warren for helping me out with exactly that problem a couple of years ago.

Dwayne

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