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'Best pullup resistor value for /MCLR pin?'
2003\12\10@101520 by

Hi!

What would be the best pullup resistor value to connect +5V to the /MCLR to achieve the most stable operation?  (I'm
using PIC18F452)

I've seen circuit diagrams with straight 5 volts, some with really high resistors, and this given 'ICD ready' circuit
with 47K Ohm...

Regards,
Jean-Pierre

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> What would be the best pullup resistor value to connect +5V
> to the /MCLR to achieve the most stable operation?  (I'm
> using PIC18F452)
>
> I've seen circuit diagrams with straight 5 volts, some with
> really high resistors, and this given 'ICD ready' circuit
> with 47K Ohm...

Unless you are in a very (elctromagnetically) noisy environment it does
not matter much, that's why you see such a wide range used in practice.
You can determine the maximum value from the /MCLR pin current and
required voltage, it used to be ~ 50k on older PICs, IIRC it is much
higher now. For ICSP the programmer will specify the minum value, for my
Wisp628 the recommended value is 33k.

Note that if you also put a capacitor on the /MCLR things will be
different: there must be some resistance between the cap and the /MCLR
pin (see uChip datasheets), and for ICSP that resistor must probably
have the 'recommended' value.

A combination of ICSP and an external brown-out reset chip complicates
the matter even further....

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products

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On Wed, 10 Dec 2003 10:14:35 -0500, you wrote:

>Hi!
>
>What would be the best pullup resistor value to connect +5V to the /MCLR to achieve the most stable operation?  (I'm
>using PIC18F452)
>
>I've seen circuit diagrams with straight 5 volts, some with really high resistors, and this given 'ICD ready' circuit
>with 47K Ohm...
>
>Regards,
>     Jean-Pierre
It is not hugely critical - there is no 'best value'.
If you want to do in-circuit programming, you need to ensure that the resistance is high enough not