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'[PIC]: ICSP Circuit Question'
2002\09\26@204128 by kben

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Hi,
I am trying ICSP for the first time with a
rats nest circuit, I found this page and
it looks like a decent icsp circuit to me.
www.sprut.de/electronic/pic/icsp/icsp.htm
The circuit is half way down the page, does this
circuit look ok, just wondering about the 200k
resistor ?

Thanks,
Kevin

Please reply to spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU
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2002\09\26@211658 by Doug Hemingway

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Kevin A. Benedict wrote:
> Hi,
> I am trying ICSP for the first time with a
> rats nest circuit, I found this page and
> it looks like a decent icsp circuit to me.
> www.sprut.de/electronic/pic/icsp/icsp.htm
> The circuit is half way down the page, does this
> circuit look ok, just wondering about the 200k
> resistor ?
>
> Thanks,
> Kevin
>

The resistor is there to keep from jamming your 5V supply to Vss when
you hit the reset button and letting all the smoke out of the diode.
Has nothing to do with the ICSP part of the circuit, everything to do
with proper level on MCLR during normal operation.

--Doug

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2002\09\26@224050 by Brent Brown

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Doug Hemingway wrote:
> The resistor is there to keep from jamming your 5V supply to Vss when
> you hit the reset button and letting all the smoke out of the diode.
> Has nothing to do with the ICSP part of the circuit, everything to do
> with proper level on MCLR during normal operation.

Except if this resistor value is too low then your programmer may
have trouble getting Vpp up to 13V or whatever it is.

IMHO 200k is too high and will make the MCLR pin susceptible to noise
spikes. As a rule I like to keep any input impedances below 10k max,
for most environments, and at that keep associated tracks short. Like
I said though, how low you can go with the resistor value may depend
on your programmer. I am using a 10k pullup and no diode on a
PIC16F877 and use an MPLAB-ICD for programming. It works but I think
the ICD might be struggling, might have to do some more testing to
make sure.

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2002\09\26@225435 by Brent Brown

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Brent Brown wrote:
> Doug Hemingway wrote:
> > The resistor is there to keep from jamming your 5V supply to Vss
> > when you hit the reset button and letting all the smoke out of the
> > diode. Has nothing to do with the ICSP part of the circuit,
> > everything to do with proper level on MCLR during normal operation.
>
> Except if this resistor value is too low then your programmer may have
> trouble getting Vpp up to 13V or whatever it is.

Ooops, my mistake, sorry. That's why the diode is there isn't it. My
explanation only suits my dodgy circuit where I only use a resistor
and no diode. Cheapskate.

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16 English Street, Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/txt: 025 334 069
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2002\09\27@075515 by Olin Lathrop

face picon face
> I am trying ICSP for the first time with a
> rats nest circuit, I found this page and
> it looks like a decent icsp circuit to me.
> www.sprut.de/electronic/pic/icsp/icsp.htm
> The circuit is half way down the page, does this
> circuit look ok, just wondering about the 200k
> resistor ?

The 200Kohm resistor is sufficient to keep MCLR high during normal
operation, but still allow the reset switch to short the line to ground.
Personally I would have used a lower value like 10K to reduce the chance of
noise pickup, especially since this line goes to an external connector.

This circuit won't work with all PICs because the normal operating supply
voltage will be one diode drop below 5V, which brings it below the 4.5V
minimum required to get the full operating range of many PICs.  On the other
hand, many PICs do tolerate this at less than full speed.

One outright error is showing Vpp as 12V, although it doesn't effect the
circuit topology.  Most PICs require from 12.5V to 13.5V for Vpp, but this
is a requirement of the external programmer circuit.

> Please reply to .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
> as my ISP always forces my return address.

OK, just this once, but I'm not going to keep doing this.  Get a real ISP.
It's not the rest of the world's job to work around this for you.


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2002\09\27@075917 by Olin Lathrop

face picon face
> Except if this resistor value is too low then your programmer may
> have trouble getting Vpp up to 13V or whatever it is.

No, the resistor is irrelevant when the programmer is driving the line to
Vpp.  The diode will block any reverse current.


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2002\09\27@095441 by Hazelwood Lyle

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part 1 1111 bytes content-type:text/plain; (decoded quoted-printable)


>IMHO 200k is too high and will make the MCLR pin susceptible to noise
>spikes. As a rule I like to keep any input impedances below 10k max,
>for most environments, and at that keep associated tracks short.
From the Microchip Midrange Reference Manual, DS33023A:

Page 3-4, Acrobat page 56 states..

"Just tie the MCLR pin directly (or through a resistor) to VDD as shown in Figure 3-2."


Since a resistor (or some other isolation) is necessary for reset buttons and/or ICSP, A note included in Figure 3-3 goes on with..

"R < 40K is recommended to ensure that the voltage drop
across R does not exceed 0.2V. A larger voltage drop will
degrade Vih level on the MCLR/VPP pin."


Please note: These Quotes are out of context, as I'm too lazy
to copy and paste the entire section of the manual. Also, there
was no mention of the particular PIC in question. This is from the
Midrange reference manual, though I have seen
similar references in the PIC18Fxx8 reference manual.

A link to the midrange reference manual is below.

Lyle


part 2 166 bytes content-type:application/octet-stream; (decode)

part 3 136 bytes
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2002\09\27@234817 by Russell McMahon

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> > I am trying ICSP ...  I found this page ....
> > www.sprut.de/electronic/pic/icsp/icsp.htm
> > ....  just wondering about the 200k
> > resistor ?

> The 200Kohm resistor is sufficient to keep MCLR high during normal
> operation, but still allow the reset switch to short the line to ground.
> Personally I would have used a lower value like 10K to reduce the chance
of
> noise pickup, especially since this line goes to an external connector.

In fact 200k is too high for guaranteed operation - it violates a little
observed specification.
This is a special spec developed by Microchip just so we can say that even
Olin is wrong sometimes. (I know he's been wrong before but I can't think
when :-) ). (As opposed to me who is often enough wrong all by myself
without Microchip having to gang up on me). This largely only applies if the
circuit is used as shown to subsequently run the processor (which is usually
what is implied by ICSP)

It goes like this. The /MCLR (aka /RESET) pin has a specified leakage
current of +/- 5uA worst case. This means that it CAN theoretically sink up
to 5 uA through the pullup. The /MCLR pin has a 0.8Vdd spec for a logical
high. At 5 volts you are therefore allowed to drop no more than 0.2Vdd
across this resistor = 1 volt. For this R = 1v/5uA = 200k. BUT we here have
a series diode which takes part of the available 1v. It will be less than
the typical 0.6v due to the small currents involved. At 0.5v diode drop say
that leaves 0.5v for the resistor or 100k maximum.

As Olin says (showing that even when he's wrong he's still right!) a value
nearer 10k would be more comfortable in practice (assuming that other
factors not discussed here don't come into play.) It's also worth noting
that in the real world the 200k will probably usually work even though it
violates specifications. Just don't blame uChip if it doesn't.


           Russell McMahon

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2002\09\30@232347 by Kevin A. Benedict

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Hey,
Thanks to all who responded, I went with a slightly
modified circuit and also used a 10K resistor.
ICSP works like a charm.

Thanks again,
            Kevin

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