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'[PICLIST] best MCLR setup for ICSP'
2001\02\05@124742 by Mike Mansheim

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What is the best way to wire MCLR for ICSP?
(referring to the permanent interface between pin 1 and the rest of the
circuit, not the temporary interface to MCLR during ICSP)

The ICSP guide is unclear on this, in my opinion.  It says that there is
typically an RC on MCLR, and recommends a diode in that case.  We don't
need the RC (because we use the power up timer), so we've always had
just a diode between MCLR and the 5V supply.  However, I have *finally*
figured out that, for programming to work properly, the chip needs to
be reset by pulling MCLR low, then put into programming mode by raising
MCLR to Vpp (this is undoubtedly painfully obvious to most of you - my
apologies).
With our diode only setup, anything that tries to pull MCLR low will short
its own 5V supply, which explains some odd voltages I've seen when
watching on a scope a promate ICSP one of our existing boards.
Is a resistor in series with the diode the best approach, or a resistor,
or something else?
Why does the ICSP recommend a schottky diode?
I'm now amazed the promates managed to get the job done for a couple of
years with our existing design.  Any thoughts on why?
Other than being able to program properly, is there any way of quickly
checking that the chip has actually entered programming mode?

Thanks for any help.

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2001\02\05@124951 by Mike Mansheim

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[forgot the tag - sorry for multiple posts]

What is the best way to wire MCLR for ICSP?
(referring to the permanent interface between pin 1 and the rest of the
circuit, not the temporary interface to MCLR during ICSP)

The ICSP guide is unclear on this, in my opinion.  It says that there is
typically an RC on MCLR, and recommends a diode in that case.  We don't
need the RC (because we use the power up timer), so we've always had
just a diode between MCLR and the 5V supply.  However, I have *finally*
figured out that, for programming to work properly, the chip needs to
be reset by pulling MCLR low, then put into programming mode by raising
MCLR to Vpp (this is undoubtedly painfully obvious to most of you - my
apologies).
With our diode only setup, anything that tries to pull MCLR low will short
its own 5V supply, which explains some odd voltages I've seen when
watching on a scope a promate ICSP one of our existing boards.
Is a resistor in series with the diode the best approach, or a resistor,
or something else?
Why does the ICSP recommend a schottky diode?
I'm now amazed the promates managed to get the job done for a couple of
years with our existing design.  Any thoughts on why?
Other than being able to program properly, is there any way of quickly
checking that the chip has actually entered programming mode?

Thanks for any help.

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2001\02\05@133059 by Nigel Goodwin

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In message <OF1C298C1E.8BBA2DD6-ON862569EA.005F9F82spamKILLspammsp.graco.com>, Mike
Mansheim <.....Michael_J_MansheimKILLspamspam.....GRACO.COM> writes
>The ICSP guide is unclear on this, in my opinion.  It says that there is
>typically an RC on MCLR, and recommends a diode in that case.  We don't
>need the RC (because we use the power up timer), so we've always had
>just a diode between MCLR and the 5V supply.  However, I have *finally*
>figured out that, for programming to work properly, the chip needs to
>be reset by pulling MCLR low, then put into programming mode by raising
>MCLR to Vpp (this is undoubtedly painfully obvious to most of you - my
>apologies).

I would advise a diode and resistor feeding the MCLR pin, plus modifying
the programmer to use an extra output (using a spare gate in the 740x
buffer) to pull MCLR low.

>With our diode only setup, anything that tries to pull MCLR low will short
>its own 5V supply, which explains some odd voltages I've seen when
>watching on a scope a promate ICSP one of our existing boards.
>Is a resistor in series with the diode the best approach, or a resistor,
>or something else?

A resistor sounds fine to me!.

>Why does the ICSP recommend a schottky diode?

Presumably because it has a smaller voltage drop?.

>I'm now amazed the promates managed to get the job done for a couple of
>years with our existing design.  Any thoughts on why?
>Other than being able to program properly, is there any way of quickly
>checking that the chip has actually entered programming mode?

As far as I know the only way is to try and write a byte, then verify
it, most programmers verify after every byte - so it will fail on the
first attempt.
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2001\02\05@202502 by Roman Black

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Mike Mansheim wrote:
>
> [forgot the tag - sorry for multiple posts]
>
> What is the best way to wire MCLR for ICSP?
> (referring to the permanent interface between pin 1 and the rest of the
> circuit, not the temporary interface to MCLR during ICSP)


Hi Mike, in my apps I use a 68k resistor from MCLR
to +5v. Simple and cheap, never had a problem. I
suppose it depends how bad you need brown-out circuits,
etc.
-Roman

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