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'[PIC]: MCLR Circuit'
2000\11\12@111534 by tal Signal Processing

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Forgot the first [ in [PIC]:


> Hi,
>
> I'm having problems programming my PICs.
>
> I have connected the MCLR pin of my PIC16F876 through a 10k resistor and a
> BAS16 diode to 5V. The programmer connection is straight to MCLR pin.
>
> I cannot get this to work.
>
> Now I understand is says I need a Schottky diode but can someone explain
> what difference a Shottky diode makes over a normal signal diode? (Sorry
to
> be dumb)
>
> When I say I cannot get this to work I mean with the XTAL taken off the
> board the PIC programs O.K. I place the XTAL back on and now the program
> runs but I cannot get it to program the PIC with the XTAL on and the above
> circuit for MCLR. I suspect it's not putting the PIC into programming mode
> by pulling MCLR low or something? Will a Schottky diode make tat much
> difference? I have no problem buying a Schottky diode (BAT54 or BAT17 ?)
and
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2000\11\12@175546 by Morgan Olsson

picon face
Digital Signal Processing wrote:
>Hi,
>
>I'm having problems programming my PICs.
>
>I have connected the MCLR pin of my PIC16F876 through a 10k resistor and a
>BAS16 diode to 5V. The programmer connection is straight to MCLR pin.
>
>I cannot get this to work.
>
>Now I understand is says I need a Schottky diode but can someone explain
>what difference a Shottky diode makes over a normal signal diode?

About half the forward voltage drop over the diode.

>(Sorry to
>be dumb)

No, that is a relevant question.

But do you really need an diode?
Om my board I have only the 10k resistor.
The ICD i use to program it to take that load without problem.
The cirquitry on the application consumes enough power to keep its +5V nominal.


The rest of question i don´t recognize.

Have you checked LVP pin?
Different opinions there, but i use 10k resistor to GND, nothing else connected so far, no problem.

Dinner time
/Morgan

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2000\11\13@120518 by Olin Lathrop

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> Now I understand is says I need a Schottky diode but can someone explain
> what difference a Shottky diode makes over a normal signal diode?

A normal semiconductor diode is formed by the junction of P and N
semiconductors.  A Shottkey diode is the junction of a semiconductor and a
normal conductor.  This type of junction still works like a diode, but has
less forward drop.  I'm being very loose with the physics here, but the
diode action is less "strong", so Shottkey diodes tend to have more leakage
and can tolerate less reverse voltage, all else being sorta equal.

The main reason for using a Shottkey diode in a circuit is to get the lower
forward voltage drop.  Normal silicon diodes have about 500 to 700 mV drop,
with silicon Shottkey diodes about half that.  Germanium diodes are also in
that range, but are hardly manufactured anymore.  The only kind of diode I
know of that can go to zero forward drop and some cases even negative (yes,
it is possible in some cases to have short circuit current) is a vacuum
tube, but these don't come in surface mount packages ;-)


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olinspamKILLspamcognivis.com, http://www.cognivis.com

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2000\11\14@124036 by jamesnewton

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Also, the leakage on Shottkey diodes is often very, very temperature
sensitive. They tend to turn in to simple resistors as the heat is applied.
User beware.

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