Reset on ATMEGA8535 oddity

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  I've used the simple cap/resistor RC circuit on the reset line of my
AVR boards forever quite reliably to hold the AVR in reset until the power
was up.  However I'm finding that it is highly unreliable on the newer
ATMEGA8535.  Has anyone run across an appnote or errata that specifies
the need to do something different between the 90S8535 and the MEGA8535?
If you have can you post a reference or your experience anyway?  If this
doesn't work I guess I'll have to break down and put a real reset device
in there like a DS1233 or something.  

Thanks,
DLC
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* Dennis Clark         snipped-for-privacy@frii.com                www.techtoystoday.com   *
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Re: Reset on ATMEGA8535 oddity
...
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You can always enable internal BOD.
I know, it is not answer for your question, but maybe can help.

Regards,
--
Artur Lipowski


Re: Reset on ATMEGA8535 oddity
: ...
:> If you have can you post a reference or your experience anyway?  If this
:> doesn't work I guess I'll have to break down and put a real reset device
:> in there like a DS1233 or something.  
: You can always enable internal BOD.
: I know, it is not answer for your question, but maybe can help.

Artur,

  Thanks, that was the first thing that I did, but it didn't help a bit.
I've also found that when fiddling with the fuse settings on the AVR one
should be rather careful.  I twiddled one bit (slow rise on resonator) that
rendered the chip non responsive.  Since the device is in a board using
ISP programming this presented a bit of a problem.  I solved it by grabbing
the clock out crystal pin signal from one board and injecting it into the
clock in crystal pin on that board to reset the fuses.  Whew.

thanks anyways though,
DLC

: Regards,
: --
: Artur Lipowski


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* Dennis Clark         snipped-for-privacy@frii.com                www.techtoystoday.com   *
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Re: Reset on ATMEGA8535 oddity
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I've found that the Mega devices are much more sensitive to the fuse
settings than the AT90S... devices were. I've also had the above problem
of losing ISP by changing one of the startup fuse settings, I had to get
hold of a parallel programmer to rescue the device.

I would guess that this is causing your problem. The mega parts have an
enormous range of power-up fuse settings which only seem to work in
limited conditions and maybe the startup characteristics of your board
fall in between them. While testing out the various fuse settings I
found that devices would sometimes not start up the first time, then
next time and all subsequent times would start, which made testing a bit
unpredictable.
--
Tim Mitchell

Re: Reset on ATMEGA8535 oddity
Quoted text here. Click to load it

 Not nice.
 When facing this type of problem, it can help to cycle through
a tube of (ideally spread date codes) devices. That way, you have
devices
of long un-powered times to check. It's surprising for how long
unpowered CMOS can behave a little like EEPROM, especially when you
don't want it to :)
-jg

Re: Reset on ATMEGA8535 oddity
A bleeder resistor on the circuit board can be requisite at times.

Hul


: Tim Mitchell wrote:
:>
:> >: Dennis Clark wrote:
:> >: ...
:> >:> If you have can you post a reference or your experience anyway?  If this
:> >:> doesn't work I guess I'll have to break down and put a real reset device
:> >:> in there like a DS1233 or something.
:> >: You can always enable internal BOD.
:> >: I know, it is not answer for your question, but maybe can help.
:> >
:> >Artur,
:> >
:> >  Thanks, that was the first thing that I did, but it didn't help a bit.
:> >I've also found that when fiddling with the fuse settings on the AVR one
:> >should be rather careful.  I twiddled one bit (slow rise on resonator) that
:> >rendered the chip non responsive.  Since the device is in a board using
:> >ISP programming this presented a bit of a problem.  I solved it by grabbing
:> >the clock out crystal pin signal from one board and injecting it into the
:> >clock in crystal pin on that board to reset the fuses.  Whew.
:> >
:> I've found that the Mega devices are much more sensitive to the fuse
:> settings than the AT90S... devices were. I've also had the above problem
:> of losing ISP by changing one of the startup fuse settings, I had to get
:> hold of a parallel programmer to rescue the device.
:>
:> I would guess that this is causing your problem. The mega parts have an
:> enormous range of power-up fuse settings which only seem to work in
:> limited conditions and maybe the startup characteristics of your board
:> fall in between them. While testing out the various fuse settings I
:> found that devices would sometimes not start up the first time, then
:> next time and all subsequent times would start, which made testing a bit
:> unpredictable.
:
: Not nice.
: When facing this type of problem, it can help to cycle through
: a tube of (ideally spread date codes) devices. That way, you have
: devices
: of long un-powered times to check. It's surprising for how long
: unpowered CMOS can behave a little like EEPROM, especially when you
: don't want it to :)
: -jg

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Re: Reset on ATMEGA8535 oddity
[snip]
:>  Thanks, that was the first thing that I did, but it didn't help a bit.
:>I've also found that when fiddling with the fuse settings on the AVR one
:>should be rather careful.  I twiddled one bit (slow rise on resonator) that
:>rendered the chip non responsive.  Since the device is in a board using
:>ISP programming this presented a bit of a problem.  I solved it by grabbing
:>the clock out crystal pin signal from one board and injecting it into the
:>clock in crystal pin on that board to reset the fuses.  Whew.
:>
: I've found that the Mega devices are much more sensitive to the fuse
: settings than the AT90S... devices were. I've also had the above problem
: of losing ISP by changing one of the startup fuse settings, I had to get
: hold of a parallel programmer to rescue the device.

  I'd have been SOL there, my part is PLCC and my trusty STK500 is strictly
DIP, which makes it useless with the new MEGA parts being in those tiny
pitch SMT only formats...

: I would guess that this is causing your problem. The mega parts have an
: enormous range of power-up fuse settings which only seem to work in
: limited conditions and maybe the startup characteristics of your board
: fall in between them. While testing out the various fuse settings I
: found that devices would sometimes not start up the first time, then
: next time and all subsequent times would start, which made testing a bit
: unpredictable.

  I've decided NOT to fiddle with the fuses any more beyond the simple
stuff and add a 80 cent reset chip.  My MEGA8535 starts fine every time
with my cockroached DS1233 on it.  That was enough to decide me - I went
back and modded the schematics and added the part to the PCB layout, fin.

DLC
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* Dennis Clark         snipped-for-privacy@frii.com                www.techtoystoday.com   *
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