BS2 Characteristics Questions

Archiver

I'm using a BS2, Rev. F which has a PIC16C57 Microcontroller.

Question 1. What is the minimum bypass capacitor on Vin, with the
battery about 10 inches away.

Question 2. What are the characteristics of the I/O pins when the
power is off(Vdd=0)? Can I apply +5v to the I/O pins from an
external
source without damage to the 16C57? What is the leakage? I've
looked
at the Microchip Technology 16C57 spec sheets but I can't figure out
which parameter is applicable.

Thanks

Dave

Archiver

Dave there is no single correct answer to that question.
First off different families of caps have differing characteristics, namely
internal resistance, which acts as an rc combination.
Secondly is the question of what you are trying to bypass out of the supply,
for example a .001uf ceramic disc will keep out most UHF and higher
interference, but wouldn't help much at keeping 60Hz ripple out.
In general I use anything from a .1uf to a 1uf monolithic ceramic, even
though often times there would be no problems if no bypass cap were used at
all.
Also note that large value electrolytics while good at keeping 60Hz line
ripple down are of little help at bypassing RF (Which most of my stuff has
to deal with, as many of my projects are connected to 2 to 50 watt
transmitters.)
In worst case conditions 470uf will keep the 60Hz ripple below a volt or so,
and a .1uf mono directly across Vin to Vss will block RF.
To sum it up, you could try not using a bypass cap and if trouble arises
connect a scope from Vin to Vss and see if you cap identify the Freq. and
amplitude, If low freq. large cap, high freq small cap.
Or you could make it a standard practice to always use a small monolithic,
just to be safe, after all it also helps keep the processors clock and
switching spikes off the supply where they could cause trouble for other
parts of the circuit.

KF4HAZ Lonnie

---

Original Message

---

From: "barrowman11" <barrowman11@

> I'm using a BS2, Rev. F which has a PIC16C57 Microcontroller.
>
> Question 1. What is the minimum bypass capacitor on Vin, with the
> battery about 10 inches away.
>
> Question 2. What are the characteristics of the I/O pins when the
> power is off(Vdd=0)? Can I apply +5v to the I/O pins from an
> external
> source without damage to the 16C57? What is the leakage? I've
> looked
> at the Microchip Technology 16C57 spec sheets but I can't figure out
> which parameter is applicable.
>
> Thanks
>
> Dave

Archiver

Lonnie,
Thanks for the advice. I've settled on a 1uF monolithic and the
circuit seems to work fine.
Dave

--- In basicstamps@y..., "FalconWireless Tech Support - KF4HAZ"
<techsupport@f...> wrote:
> Dave there is no single correct answer to that question.
> First off different families of caps have differing characteristics,
namely
> internal resistance, which acts as an rc combination.
> Secondly is the question of what you are trying to bypass out of the
supply,
> for example a .001uf ceramic disc will keep out most UHF and higher
> interference, but wouldn't help much at keeping 60Hz ripple out.
> In general I use anything from a .1uf to a 1uf monolithic ceramic,
even
> though often times there would be no problems if no bypass cap were
used at
> all.
> Also note that large value electrolytics while good at keeping 60Hz
line
> ripple down are of little help at bypassing RF (Which most of my
stuff has
> to deal with, as many of my projects are connected to 2 to 50 watt
> transmitters.)
> In worst case conditions 470uf will keep the 60Hz ripple below a
volt or so,
> and a .1uf mono directly across Vin to Vss will block RF.
> To sum it up, you could try not using a bypass cap and if trouble
arises
> connect a scope from Vin to Vss and see if you cap identify the
Freq. and
> amplitude, If low freq. large cap, high freq small cap.
> Or you could make it a standard practice to always use a small
monolithic,
> just to be safe, after all it also helps keep the processors clock
and
> switching spikes off the supply where they could cause trouble for
other
> parts of the circuit.
>
> KF4HAZ Lonnie
>
>

---

Original Message

---

From: "barrowman11" <barrowman11@
>
> > I'm using a BS2, Rev. F which has a PIC16C57 Microcontroller.
> >
> > Question 1. What is the minimum bypass capacitor on Vin, with the
> > battery about 10 inches away.
> >
> > Question 2. What are the characteristics of the I/O pins when the
> > power is off(Vdd=0)? Can I apply +5v to the I/O pins from an
> > external
> > source without damage to the 16C57? What is the leakage? I've
> > looked
> > at the Microchip Technology 16C57 spec sheets but I can't figure
out
> > which parameter is applicable.
> >
> > Thanks
> >
> > Dave