[SI-LIST] Re: Reset problem with power supply

I did try with large caps, it did not seem to make difference. I still do not 
know what caused the difference reset behaviour when turning the system on with 
the on of switch of the power supply Vs plugging the cable.

I do seem to have solved the problem using a small micro controller to generate 
a reset ( double reset). 

I am going to make it robust by making the small micro controller ( we could 
have used a small CPLD for that matter) act as watch dog. If it does not get a 
signal from the main processor for a specified time, it will generate a reset.

I will investigate more to ensure that it was not caused by any power noise / 
signal integrity issue.

Vikas Shukla  

http://referencedesigner.com
--- On Sat, 12/6/08, steve weir <weirsi@xxxxxxxxxx> wrote:

> From: steve weir <weirsi@xxxxxxxxxx>
> Subject: [SI-LIST] Re: Reset problem with power supply
> To: olaney@xxxxxxxx
> Cc: hmurray@xxxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
> Date: Saturday, December 6, 2008, 4:39 PM
> Electrolytic caps come along with some engineering
> requirements.  The 
> first is long term product life:  electrolytics have a
> typical shelf 
> life of 5-10 years for cheap to very high quality.  This is
> a function 
> primarily of the seals.  However it is not that unusual to
> find a box 
> with large body computer grade electrolytics going for more
> than 20 
> years.  The big issues: are  initial conditioning, adequate
> temperature, 
> voltage derating, and absolutely : NO EXPOSURE TO HALOGENS.
>  A 10 year 
> life is readily attainable, and with a lot of derating 20
> years can be 
> had in large body parts.  Miniatures are really constrained
> to about 10 
> years no matter what is done.  Consumer products contain
> only miniatures 
> these days, and are so fiercely price sensitive that no one
> pays for the 
> kind of derating needed to see long service lives.  As a
> result, the 
> electrolytics rank #1 to #2 for failure rates in consumer
> electronics 
> ahead or behind of the power semiconductors.
> 
> Steve
> olaney@xxxxxxxx wrote:
> > Greetings, Hal.  As you have noted, the Schmitt
> trigger properly handles
> > slow power risetimes.  A proper design will include an
> RC time constant
> > at the input so that it also handles fast risetimes by
> including a
> > minimum delay before reset releases.  As I mentioned,
> there are decent
> > ICs to do this that are lots more convenient than
> rolling your own
> > design.  Reset circuits are a specialty in their own
> right.  It's easy to
> > slap together a lousy one, tougher to design a
> bullet-proof one from
> > scratch.
> >
> > The issue with electroytic caps isn't whether they
> are "modern" but
> > whether they are cheap.  There are decent Al
> electrolytics to be had for
> > a modest premium (OK, sometimes a big one), but the
> aluminum caps
> > actually used in consumer gear seem as crummy as ever.
>  I've had
> > motherboards fail after several years because the caps
> in the switching
> > supply weren't properly rated for the high
> frequency current and ran too
> > hot.  When it's really bad you get a brown tint to
> the circuit board in
> > the area around where the caps are mounted because the
> internal losses
> > increase as they age.  At the end they run hot as a
> pistol. 
> >
> > The last time this happened I metered the caps and
> found that 100uF caps
> > had dropped to less than 2uF!  Replacing them did not
> bring the board
> > back from the dead because the excessive ripple
> voltage peaks from
> > inadequate filtering had already done their dirty
> work.  For machines you
> > really care about, checking the power supply caps once
> in awhile might be
> > a reasonable preventive maintenance procedure.  A
> finger touch while
> > under power for a quick temperature check is probably
> good enough (but
> > I'd touch the plastic insulation rather than the
> bare metal).  Tantalum
> > caps generally do not have the aging issue, though
> they still need to be
> > rated for the current demand.
> >
> > Orin
> >
> > On Sat, 06 Dec 2008 01:34:44 -0800 Hal Murray
> <hmurray@xxxxxxxxxxxxxxx>
> > writes:
> >   
> >> How does a Schmitt trigger solve the problem? 
> I'd expect it to make
> >>     
> > things cleaner if the input signal has a slow rise
> time, but that's not
> > going to solve the problem of reset going away before
> the power is
> > stable.
> >   
> >> Are electrolytics getting old really a serious
> problem with modern
> >>     
> > caps?  I've seen lots of credible reports of very
> old gear being brought
> > back to life by replacing all the old filter caps.  I
> hadn't thought much
> > about the  implications on gear I was designing.  How
> should I think
> > about modern filter caps getting old?
> >
> >   
> >>> Cable inductance absolutely should not be an
> issue.  If it 
> >>>       
> >> actually
> >>     
> >>> is, be grateful that you found a design flaw
> before you reached
> >>> production! This might be related to the power
> risetime.  Your POR
> >>> circuit needs to operate properly even if you
> slowly bring the 
> >>>       
> >> voltage
> >>     
> >>> up manually.  Some circuits depend on fast
> power risetime to 
> >>>       
> >> operate
> >>     
> >>> properly, and work OK... most of the time. 
> The worst ones 
> >>>       
> >> capacitor
> >>     
> >>> couple the voltage bus into a transistor.  Not
> only do these fail 
> >>>       
> >> to
> >>     
> >>> reset on a slow rise, if an aluminum
> electrolytic cap is used, the
> >>> value will drop with age as it dries out and
> eventually the 
> >>>       
> >> circuit
> >>     
> >>> won't work even for normal supply
> risetime.  A Schmitt trigger is 
> >>>       
> >> a
> >>     
> >>> good basis for a bullet-proof approach, as are
> any number of
> >>> supervisor ICs that also check for
> overvoltage, brown-out, and the
> >>> like. 
> >>>       
> >
> ____________________________________________________________
> > Take a break - you deserve it.  Click here to find a
> great vacation.
> >
> http://thirdpartyoffers.juno.com/TGL2141/fc/PnY6rw2gGUymLyXCUuqFPiRkgWMzRlFZD0pHTeyutmKSJLoJ92e4X/
> >
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> 
> -- 
> Steve Weir
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