Serial Port 0.1uF 100V!! caps??
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Hi,
I have the BS2P24 Basic Stamp and I see "most" of the schematics have
two 0.1uF caps on the ATN pin (3) of the stamp. I thought it odd
that they have 0.05uF to ground and teh series/ground combination
seems "odd". If they wanted to delay the signal, a
resistor/capactior combination would be better. Maybe they want DC
isolation. I see the caps are rated at 100V!! =[noparse]:D[/noparse]
I note there are a few "odd" things like this running around in basic
stamp circuits that seem ill thought out, but get copied over a lot...
So I was wondering if anyone knew the "true story" behind the serial
port caps and exactly what they are for???
Kind thanks for your thoughts on this one that has been bugging me a
few days. I can't find the answer searching the docs...
Cheers,
Terry
I have the BS2P24 Basic Stamp and I see "most" of the schematics have
two 0.1uF caps on the ATN pin (3) of the stamp. I thought it odd
that they have 0.05uF to ground and teh series/ground combination
seems "odd". If they wanted to delay the signal, a
resistor/capactior combination would be better. Maybe they want DC
isolation. I see the caps are rated at 100V!! =[noparse]:D[/noparse]
I note there are a few "odd" things like this running around in basic
stamp circuits that seem ill thought out, but get copied over a lot...
So I was wondering if anyone knew the "true story" behind the serial
port caps and exactly what they are for???
Kind thanks for your thoughts on this one that has been bugging me a
few days. I can't find the answer searching the docs...
Cheers,
Terry
Comments
name for "random RF (radio frequency) noise generator".
Capacitors like these (.1 uF and similar values ) are used to sent
this noise the shortest way possible to the ground and are
called "(RF) bypass capacitors".
Not to worry, your stamp does not generate 100 Volts by itself.
The signal (voltage) generated is very very low, but sometime can be
heard even by the cheapest AM radio sitting next to the processor,
try it!
Voltage rating of the capacitor - 100 Volts - is generally the most
manufatured component (size), therefore the cheapest to use.
Vaclav
The PC to BS2 link uses the ATN line to reset
the chip in preparation of programming. This
is wired to the PC's 'DTR' line.
When the line goes 'active', it resets the
chip. If you don't have those caps, then as
long as the line is 'active' it HOLDS the chip
in reset. Since 'active' is the default state
of the DTR line on most serial port programs
(Hyperterm being one), without the caps
Hyperterm will hold the chip in reset.
With the caps, Hyperterm holds the DTR signal
active, but as you point out, the caps remove
the DC component of the signal, and so the
BS2 is allowed to come out of reset and run.
Yes, it is 'odd', but since ATN is only used
when reset is desired, and that doesn't happen
very often, the AC behavior is not a problem.
--- In basicstamps@yahoogroups.com, "ti_wrex" <ti_wrex@y...> wrote:
> Hi,
>
> I have the BS2P24 Basic Stamp and I see "most" of the schematics
have
> two 0.1uF caps on the ATN pin (3) of the stamp. I thought it odd
> that they have 0.05uF to ground and teh series/ground combination
> seems "odd". If they wanted to delay the signal, a
> resistor/capactior combination would be better. Maybe they want DC
> isolation. I see the caps are rated at 100V!! =[noparse]:D[/noparse]
>
> I note there are a few "odd" things like this running around in
basic
> stamp circuits that seem ill thought out, but get copied over a
lot...
>
> So I was wondering if anyone knew the "true story" behind the
serial
> port caps and exactly what they are for???
>
> Kind thanks for your thoughts on this one that has been bugging me
a
> few days. I can't find the answer searching the docs...
>
> Cheers,
>
> Terry
I now see what they are trying to do. I guess "I" would have used
0.01uF caps and a resistor, but I guess this works fine for everyone,
so cool! ;-))
I checked the waveforms on pin 3 tonight during programming:
http://team-titanium.com/%7Eti-tan/WeAreACyborg/serial1.jpg
http://team-titanium.com/%7Eti-tan/WeAreACyborg/serial2.jpg
Seems slow, and the negative spike is a bother, but I guess it is not
an issue here. So I will just do the same cap thing too...
I am adding microcontroller logic to my little battle bot ;-))
http://team-titanium.com/%7Eti-tan/
http://team-titanium.com
Just a simple addition of the Basic Stamp to carry out some pre-
defined attack maneuvers:
http://team-titanium.com/%7Eti-tan/WeAreACyborg/ControlSchem.jpg
http://team-titanium.com/%7Eti-tan/WeAreACyborg/ControlBoxLayout.jpg
I hope to make the computer control thing more sophisticated over
time, but just learning to crawl first ;-))
I love this Basic Stamp thing!! I could never figure out the C++
style heavy duty microcontroller coding. Having the code as a form
of good ol' basic is a super help to those that are not sharp
programmers 0:-)) I wrote up a little code and it seemed to work
pretty well right off without a bunch of goofy problems :-)))
Cheers,
Terry
--- In basicstamps@yahoogroups.com, "Allan Lane" <allan.lane@h...>
wrote:
> Those two caps serve an important purpose.
> The PC to BS2 link uses the ATN line to reset
> the chip in preparation of programming. This
> is wired to the PC's 'DTR' line.
>
> When the line goes 'active', it resets the
> chip. If you don't have those caps, then as
> long as the line is 'active' it HOLDS the chip
> in reset. Since 'active' is the default state
> of the DTR line on most serial port programs
> (Hyperterm being one), without the caps
> Hyperterm will hold the chip in reset.
>
> With the caps, Hyperterm holds the DTR signal
> active, but as you point out, the caps remove
> the DC component of the signal, and so the
> BS2 is allowed to come out of reset and run.
>
> Yes, it is 'odd', but since ATN is only used
> when reset is desired, and that doesn't happen
> very often, the AC behavior is not a problem.
>
> --- In basicstamps@yahoogroups.com, "ti_wrex" <ti_wrex@y...> wrote:
> > Hi,
> >
> > I have the BS2P24 Basic Stamp and I see "most" of the schematics
> have
> > two 0.1uF caps on the ATN pin (3) of the stamp. I thought it odd
> > that they have 0.05uF to ground and teh series/ground combination
> > seems "odd". If they wanted to delay the signal, a
> > resistor/capactior combination would be better. Maybe they want
DC
> > isolation. I see the caps are rated at 100V!! =[noparse]:D[/noparse]
> >
> > I note there are a few "odd" things like this running around in
> basic
> > stamp circuits that seem ill thought out, but get copied over a
> lot...
> >
> > So I was wondering if anyone knew the "true story" behind the
> serial
> > port caps and exactly what they are for???
> >
> > Kind thanks for your thoughts on this one that has been bugging
me
> a
> > few days. I can't find the answer searching the docs...
> >
> > Cheers,
> >
> > Terry