what kind of capacitor to use on reset pin?

M

Michael Noone

Guest
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
 
"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...
Click to expand...

What criteria did you use to decide between a mylar
reset cap and a polypropalene or polystyrene cap?
Once you answer that, you can likely answer your
own posted question.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
 
"Larry Brasfield" <donotspam_larry_brasfield@hotmail.com> wrote in
news:pqWxe.36$jB5.7441@news.uswest.net:

What criteria did you use to decide between a mylar
reset cap and a polypropalene or polystyrene cap?
Once you answer that, you can likely answer your
own posted question.
Click to expand...
To be entirely honest - I was just told "film capacitor" - so I found a
film capacitor of the right value that looked like the one in the picture -
a small yellow box - and got it. I've been using the same part ever since.

I should mention that the reason for my question is this: I'm converting a
circuit I have breadboarded to an entirely surface mount board - and I
can't find a film capacitor in a 0805 package of the right value (Digi-Key
doesn't carry any at least). There were plenty of ceramics of the right
package and value though - so I'm wondering if it would be OK to use one of
those. Instinct tells me yes - but it also tells me I should ask to be sure
before I do something silly.

-Michael
 
Either ceramic or mylar will work not electrolytic

Dan

--

Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
QuickScore@USSailing.net
www.QuickScoreRace.com


"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...
 
Michael Noone wrote:
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...
Click to expand...
A reset capacitor is a non-critical time delay application. You don't care
if the reset pulse is 5% longer or shorter on a warm day, you're not
concerned with series resistance or high-frequencies. You can select a
capacitor for its physical size and price.
 
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...
Click to expand...
The only engineering consideration in this case would be the capacitor
leakage current. It must be negligibly small so as not to induce a
voltage drop across the 100k and thereby prevent complete pull-up. Film
and ceramics are good, electrolytics go from risky to bad.
 
On Sun, 03 Jul 2005 13:26:19 -0500, Michael Noone
<mnoone.uiuc.edu@127.0.0.1> wrote:

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...

If the uP has a schmitt type input, then any cap type will work. But
some processors need a clean, fast edge, so a uP reset chip is the
best way to start them up.

The other problem with a simple RC is that you may be able to tease
the power supply (apply various brownout conditions) and hang things
up or crash the code. A good reset chip is immune from being fooled
like that.

Which uP are you using?

We just did a new product that wouldn't start up. Seems we went from a
5 volt CPU on the old design, to a 3.3 volt one on the new, but forgot
to replace the 5-volt reset chip. Oops.

John
 
On Sun, 03 Jul 2005 19:45:48 GMT, "Dan Hollands"
<dhollan3@rochester.rr.com> wrote:

Either ceramic or mylar will work not electrolytic

Dan
Click to expand...
Why not?

John
 
Hello Michael,

Most of the uC circuits I came across use a simple ceramic SMT cap. But
I am always suspicious when it comes to such simple resets. Unless the
uC has a proper (and that's easier said than done) reset function or
better yet a brown-out reset it may be better to build you own. Or use a
reset chip but in that case you'd have to trust that the designers of
that chip knew what they were doing. They usually do but not always.

Regards, Joerg

http://www.analogconsultants.com
 
On Sun, 03 Jul 2005 19:54:50 GMT, Fred Bloggs <nospam@nospam.com> wroth:

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...

The only engineering consideration in this case would be the capacitor
leakage current. It must be negligibly small so as not to induce a
voltage drop across the 100k and thereby prevent complete pull-up. Film
and ceramics are good, electrolytics go from risky to bad.
Click to expand...
Of course, one could always *engineer* a solution. I know that's a
novel concept here, but it's true.

The micro's data sheet will specify the voltage levels that constitute
valid ones and zeros. They are seldom Vcc and ground.

They also specify the minimum time that the reset level must be
asserted.

Once you have that data, you can choose an RC product that meets the
time and choose a maximum capacitor leakage and minimum capacitor value over the
expected temperature range.

Show me the calculations that say that almost any tantalum capacitor can
not be used.

Jim
 
On Sun, 03 Jul 2005 13:27:07 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wroth:

If the uP has a schmitt type input, then any cap type will work. But
some processors need a clean, fast edge, so a uP reset chip is the
best way to start them up.
Click to expand...
Not to mention that a SOT-23 reset chip will probably be smaller than
the RC network.

Jim
 
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
news:eek:4igc15hcblj9qbsbl6ptofnfv7qkg8rv0@4ax.com:

If the uP has a schmitt type input, then any cap type will work. But
some processors need a clean, fast edge, so a uP reset chip is the
best way to start them up.

The other problem with a simple RC is that you may be able to tease
the power supply (apply various brownout conditions) and hang things
up or crash the code. A good reset chip is immune from being fooled
like that.

Which uP are you using?

We just did a new product that wouldn't start up. Seems we went from a
5 volt CPU on the old design, to a 3.3 volt one on the new, but forgot
to replace the 5-volt reset chip. Oops.

John
Click to expand...
I'm using an Atmel ATMEGA162. (an AVR)

It does have built in brownout protection I believe, though the supply
should be fairly constant so I don't think that will be an issue.
 
On Sun, 03 Jul 2005 22:50:12 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

On Mon, 04 Jul 2005 01:48:43 GMT, the renowned jmeyer@nowhere.net
wrote:


Of course, one could always *engineer* a solution. I know that's a
novel concept here, but it's true.

The micro's data sheet will specify the voltage levels that constitute
valid ones and zeros. They are seldom Vcc and ground.

They also specify the minimum time that the reset level must be
asserted.

Once you have that data, you can choose an RC product that meets the
time and choose a maximum capacitor leakage and minimum capacitor value over the
expected temperature range.

Show me the calculations that say that almost any tantalum capacitor can
not be used.

Jim


Even the crappiest aluminum electrolytic series should be able to
handle a 100msec time constant with typical levels.

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.


Best regards,
Spehro Pefhany
Click to expand...
You have to think about the frequency response of the caps otherwise
high frequency glitches will give you unwanted resets. A crappy
electrolytic won't handle as high a frequency as the uP will respond
to. I agree, go with a reset chip. It should help get the power
supply rejection you need.

Paul C
 
On Mon, 04 Jul 2005 03:03:10 GMT, PaulCsouls
<paulcsouls@worldnet.att.net> wrote:

You have to think about the frequency response of the caps otherwise
high frequency glitches will give you unwanted resets. A crappy
electrolytic won't handle as high a frequency as the uP will respond
to.
Click to expand...
Sorry, but that's just plain silly.

John
 
jmeyer@nowhere.net wrote in
news:dd5hc1hfqea079jadfvo8f6hj9ddebbhvv@4ax.com:

On Sun, 03 Jul 2005 13:27:07 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wroth:


If the uP has a schmitt type input, then any cap type will work. But
some processors need a clean, fast edge, so a uP reset chip is the
best way to start them up.


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

Jim
Click to expand...
Can you reccomend such a chip? I've never used anything except for an RC
circuit on the reset pin. Thanks,

-Michael
 
On Sun, 03 Jul 2005 23:21:22 -0500, Michael Noone
<mnoone.uiuc.edu@127.0.0.1> wrote:

jmeyer@nowhere.net wrote in
news:dd5hc1hfqea079jadfvo8f6hj9ddebbhvv@4ax.com:

On Sun, 03 Jul 2005 13:27:07 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wroth:


If the uP has a schmitt type input, then any cap type will work. But
some processors need a clean, fast edge, so a uP reset chip is the
best way to start them up.


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. Thanks,

-Michael
Click to expand...
MAX809, and about a million clones from TI and National and such. The
low-end parts cost $0.19 or such.

John
 
jmeyer@nowhere.net wrote:
On Sun, 03 Jul 2005 19:54:50 GMT, Fred Bloggs <nospam@nospam.com> wroth:


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...

The only engineering consideration in this case would be the capacitor
leakage current. It must be negligibly small so as not to induce a
voltage drop across the 100k and thereby prevent complete pull-up. Film
and ceramics are good, electrolytics go from risky to bad.


Of course, one could always *engineer* a solution. I know that's a
novel concept here, but it's true.

The micro's data sheet will specify the voltage levels that constitute
valid ones and zeros. They are seldom Vcc and ground.

They also specify the minimum time that the reset level must be
asserted.

Once you have that data, you can choose an RC product that meets the
time and choose a maximum capacitor leakage and minimum capacitor value over the
expected temperature range.

Show me the calculations that say that almost any tantalum capacitor can
not be used.

Jim
Click to expand...
The original question dealt with a 10ms time constant- that is 69% of
Vdd if you recall. Why don't you show us how to calculate the minimum
power supply turn-on time required to reliably reset the uc when the
minimum reset duration is say T- that is duration of time for voltage
across C to be less than 0.2Vdd for Vdd > Vdd,min. Obviously, there will
be no reset when the voltage across C essentially follows the
instantaneous Vdd.
 
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.
Click to expand...
Hmmm- tons of LDOs with logic compatible dropout indicator signals
available these days...
 
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.
Click to expand...
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.
 
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
Click to expand...
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
--
"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
 
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