what kind of capacitor to use on reset pin?

On Mon, 04 Jul 2005 12:04:35 GMT, the renowned Fred Bloggs
<nospam@nospam.com> wrote:

Not to mention that a SOT-23 reset chip will probably be smaller
than
the RC network.

Jim

Can you reccomend such a chip? I've never used anything except for an RC
circuit on the reset pin.


You don't need an external supervisory chip- the ATMEGA162 contains an
internal power-on reset, an internal programmable brown-out protection
reference and comparator, and the external /reset. The external /reset
is most often used to bring the chip out of sleep mode, although it can
also be used as a general hardware reset.
Click to expand...
You really have to look carefully at how well the internal reset
circuitry actually works. Some AVR chips are (in)famous for EEPROM
corruption during power cycling, and some PICs with BOR seem to
occasionally get themselves into a state that can't be reset without
power cycling. It was clearly stated in data sheet in the former case
that the BOR tolerance is outside the guaranteed range of operation of
the chip, so you'd only have yourself to blame for trusting the big
print.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
Any type of cap will work because there are no constraints. The rc constant
is not critical and polarity is always positive.

Harold




"Michael Noone" <mnoone.uiuc.edu@127.0.0.1> wrote in message
news:Xns9688744675491mnooneuiucedu127001@216.196.97.136...
Hi I was just wondering - what kind of capacitor should be used on the
reset pin of a microcontroller? I've always used a 100K resistor and a .1
micro farad film capacitor. But is film what should be used here? When I
first was connecting a microcontroller the capacitor reccomended was a
film
one - but is it being film important at all? Or would a ceramic,
electrolytic, or anything else work just as well? To me it seems like they
would - but maybe I'm forgetting something...

Thanks!

-Michael
Click to expand...
 
Spehro Pefhany wrote:
On Mon, 04 Jul 2005 12:04:35 GMT, the renowned Fred Bloggs
nospam@nospam.com> wrote:


Not to mention that a SOT-23 reset chip will probably be smaller
than
the RC network.

Jim

Can you reccomend such a chip? I've never used anything except for an RC
circuit on the reset pin.


You don't need an external supervisory chip- the ATMEGA162 contains an
internal power-on reset, an internal programmable brown-out protection
reference and comparator, and the external /reset. The external /reset
is most often used to bring the chip out of sleep mode, although it can
also be used as a general hardware reset.


You really have to look carefully at how well the internal reset
circuitry actually works. Some AVR chips are (in)famous for EEPROM
corruption during power cycling, and some PICs with BOR seem to
occasionally get themselves into a state that can't be reset without
power cycling. It was clearly stated in data sheet in the former case
that the BOR tolerance is outside the guaranteed range of operation of
the chip, so you'd only have yourself to blame for trusting the big
print.


Best regards,
Spehro Pefhany
Click to expand...
The ATMEGA162 BOD is based on an internal 1.1V bandgap reference and
comparator. The datasheet is screwed- my download is missing page 46 and
some other critical stuff relating to reset times. Oh well- "most of it"
is there and that is good'nuf for the programming world.
 
Spehro Pefhany wrote:
On Mon, 04 Jul 2005 06:38:02 GMT, the renowned PaulCsouls
paulcsouls@worldnet.att.net> wrote:

It's just like decoupling caps. Above a certain frequency the lead
inductance dominates and the cap acts inductively. That's why they add
those 0.1uF or 0.01uF film caps in parallel with the electrolytics in
power supplies. I've seen it happen. Evey time the relay clicks the
microprocessor resets because of some high frequency spike and I had
to go back and add filtering to kill the spike.

Paul C


Okay, so say we've got a 1uF electrolytic in series with a 100K
resistor to Vdd and maybe 5pF of capacitance from the input.

In what frequency range(s) do you think that the impedance of the
e-cap becomes significant compared to the 100K?



Best regards,
Spehro Pefhany
Click to expand...
See that's just it- the e-cap leakage lowered his noise margin with 20uA
leakage current..../reset was hanging near Vdd/2:)
 
On Mon, 04 Jul 2005 11:15:48 GMT, the renowned Fred Bloggs
<nospam@nospam.com> wrote:

BUT- I strongly advise against using RC circuits for reset in all but
the most non-critical AND (not OR) cost-sensitive applications, for
reasons that have already been mentioned. An open-collector level
detector (a few discretes) and an RC will be reliable in many cases,
but the reset chip is smaller, will probably use less power, and maybe
even will be cheaper.



Hmmm- tons of LDOs with logic compatible dropout indicator signals
available these days...
Click to expand...
Sure that's another solution if you need such a regulator and can find
a fit that doesn't cost a fortune. National has a bunch. But you
generally want the type with a timer as well as a comparator and
reference to avoid the need for the RC network. You want time for the
oscillator/PLL etc. to stabilize before removing the reset both on
power up and in any other case where Vdd has gone out of spec. Some
micros may provide this function.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
Spehro Pefhany wrote:
On Mon, 04 Jul 2005 11:15:48 GMT, the renowned Fred Bloggs
nospam@nospam.com> wrote:




BUT- I strongly advise against using RC circuits for reset in all but
the most non-critical AND (not OR) cost-sensitive applications, for
reasons that have already been mentioned. An open-collector level
detector (a few discretes) and an RC will be reliable in many cases,
but the reset chip is smaller, will probably use less power, and maybe
even will be cheaper.



Hmmm- tons of LDOs with logic compatible dropout indicator signals
available these days...


Sure that's another solution if you need such a regulator and can find
a fit that doesn't cost a fortune. National has a bunch. But you
generally want the type with a timer as well as a comparator and
reference to avoid the need for the RC network. You want time for the
oscillator/PLL etc. to stabilize before removing the reset both on
power up and in any other case where Vdd has gone out of spec. Some
micros may provide this function.


Best regards,
Spehro Pefhany
Click to expand...
That's what's nice about the open drain /LBO type outputs- couple with
an RC on /reset and you slow on/ fast off. This is all battery
application stuff anyway- so the Vdd holds up well above the minimum
operating voltage after /LBO has gone low. The RC takes over on power
up. I don't really see where the external reset chip should have any
advantage over the exact same circuit internal to the AVR- both should
behave in exactly the same way.
 
On Mon, 04 Jul 2005 12:34:42 GMT, the renowned Fred Bloggs
<nospam@nospam.com> wrote:
The ATMEGA162 BOD is based on an internal 1.1V bandgap reference and
comparator. The datasheet is screwed- my download is missing page 46 and
some other critical stuff relating to reset times. Oh well- "most of it"
is there and that is good'nuf for the programming world.
Click to expand...
It seems to (now) be properly specified. See doc2513.pdf PDF page 49
Note 1. They say they test the micro operation at a voltage below the
(otherwise) specified range to guarantee reset if the BOD level falls
below the minimum voltage. *Presumably* (????) they add a factor to
allow for BOR and minimum operating voltage changes with temperature
at the *maximum* clock frequency.

Minimum reset time of a couple of us could conceivably be a problem
since it's only set by hysteresis. If there's a lot of supply current
available it could charge the capacitance on the supply rails up in
less than that time if power is interrupted then restored just after
falling below Vbot.



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
On Mon, 04 Jul 2005 08:21:09 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

On Mon, 04 Jul 2005 06:38:02 GMT, the renowned PaulCsouls
paulcsouls@worldnet.att.net> wrote:

It's just like decoupling caps. Above a certain frequency the lead
inductance dominates and the cap acts inductively. That's why they add
those 0.1uF or 0.01uF film caps in parallel with the electrolytics in
power supplies. I've seen it happen. Evey time the relay clicks the
microprocessor resets because of some high frequency spike and I had
to go back and add filtering to kill the spike.

Paul C

Okay, so say we've got a 1uF electrolytic in series with a 100K
resistor to Vdd and maybe 5pF of capacitance from the input.

In what frequency range(s) do you think that the impedance of the
e-cap becomes significant compared to the 100K?



Best regards,
Spehro Pefhany
I see what you're saying. I'd need a pretty big ESL to cause a
Click to expand...
problem. Still I had a spike coupling in from 12V circuit into a 5
volt circuit I thought I had isolated and one of the changes I made
was to change a 10uF electrolytic to 1uF tantalum in the reset
circuit. It worked.

Paul C
 
On Mon, 04 Jul 2005 16:39:23 GMT, the renowned PaulCsouls
<paulcsouls@worldnet.att.net> wrote:

I see what you're saying. I'd need a pretty big ESL to cause a
problem. Still I had a spike coupling in from 12V circuit into a 5
volt circuit I thought I had isolated and one of the changes I made
was to change a 10uF electrolytic to 1uF tantalum in the reset
circuit. It worked.

Paul C
Click to expand...
Could have been that the smaller physical size of the tantalum was
coupling less of the interference to the reset input. EMI can be a
PITA. An 0402 ceramic sitting right over a ground plane is nice.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
news:69fhc1ppp4l26ng86r961c810e10ib3m2n@4ax.com:
MAX809, and about a million clones from TI and National and such. The
low-end parts cost $0.19 or such.

John
Click to expand...
Hi - if I'm reading all the datsheets properly (for the MAX809 and for the
ATMEGA162, the uC I'm using) - the MAX809L is the chip that I want, as it's
threshhold voltage is 4.63V, and the ATMEGA162 can't be run at 16Mhz (the
speed I'm running it at, at anything below 4.5V, according to page 265 of
the datasheet.

But what I'm confused about is the extrnal resistor that is drawn in. On
page 6 of the MAX809 datasheet it says that this resistor is needed for
high impedance CMOS inputs - is that what the ATMEGA162 RESET' pin is? It
says that's only needed for if the supply voltage dips below 1V, which I
suppose should never happen (the AVR won't even run at that low of a
voltage anyways).

But is all this even necessary? The AVR has built in brown out protection
at three voltages - 4.3, 2.7, and 1.8.

Thanks for the all the help,

-Michael
 
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