By "on the edge" I meant that is the reset or brownout protection level set
very close to the operating voltage. If so then minor dips in the supply
could cause a reset. A probe on the reset line should tell you what is
happening here and you've probably already done that.

Richard


On 24 June 2014 21:53, Jahn <jana1972@centrum.cz> wrote:

> Thank you  all for the replies
>
> > Does the CPU start, if you reset it while keeping the power on? If so,
> > most likely the problem is slow rise time.
> I tried to reset CPU several times, but still does not start.But when I
> use the correct power
> supply, it starts when resetting.
>
> > Could be some sort of inrush current effect. Also, if there is a
> > secondary power supply downstream, it may require the input to have a
> > fast risetime.
> Yes, there are more powers but I reset CPU while keeping power on, so the
> risetime should
> be OK.
>
> >Especially if power supply sequencing is important.
> Not sure if  the sequencing is important or not, but when I reset
> CPU(keeping power on and
> using the working voltage supply), it started. So, I do not think, in my
> case, the sequence is
> important.
>
> > Check that your crystal is operating OK.
> Do you mean the crystal of the mainboard? There is NO crystal that uses
> the Asynchronous
> Step-Down Regulator
>
> >Dust/dirt providing a
> > leakage path & preventing correct startup?
> I hotair reflowed the power supply part already
>
> > Another thing to check
> > would be the reset/brownout arrangements - could be on the edge?
> Can you explain a little more?
> Thanks
> >
>
> Do you have any other idea?
>
>
>
>
>
>
> >
>
> > >
> > > I have a Linux embeded board that  uses
> > > FP6116 Asynchronous Step-Down Regulator
> > > a datasheet  is here
> > > http://www.micro-bridge.com/data/Feeling-tech/FP6116.pdf
> > >
> > > In this design , that Asynchronous Step-Down Regulator  should
> > > supply 1,2V for  CPU  on this board. It worked for some time but now
> > > it stopped. So, I measured voltage and that Asynchronous Step-Down
> > > Regulator  still supply 1.2V, yet CPU does not start. But when I use
> > > a different source of 1.2V, the CPU starts. So, I am rather
> > > confused. I can measure the same voltages, and more or less the same
> > > spikes( using oscilloscope), yet different results.
> > >
> > > Can anyone suggest any solution to that problem?
> > > Thanks
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