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PIC MCLR pin: Can Pullup Resistor to Vdd causes problem?

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eblc1388

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Hi guys, any comments?

The following is a quote from the book, referring to PIC16C73:

Designing Embedded Hardware
By John Catsoulis
Publisher: O'Reilly
Pub Date: May 2005
ISBN: 0-596-00755-8

The reset input, /MCLR, is tied directly to the power supply, such that it is permanently inactive. In this case, we are relying on the processor's internal power-on reset circuitry and don't need to provide an external reset. It is common practice to use a pull-up resistor to tie an unused input, such as MCLR, inactive. However, in this case I have found that a pull-up resistor can affect the activation of the internal power-on reset to the point where it fails to kick in. (The internal capacitance of the pin combines with the resistor to form an RC circuit, which delays from reaching the appropriate level.) Thus, the resistor can actually cause the processor to never start properly. So in this case, it's better to leave it out.
 
Interesting, I didn't work with 16C so I don't have any experience with such behaviour. I suppose this might be a problem with 16C only, so there is no reason to talk about it (seriously, would you design a project with 16C nowadays?), and maybe Author experienced this problem once on a single chip and he made it a general rule?
 
eblc1388 said:
Hi guys, any comments?

The following is a quote from the book, referring to PIC16C73:

I've never heard of such a problem, and considering the low capacitance of the pin you would need a very high value pull-up to potentially cause problems.

It's also a fairly obscure PIC, and not one you're likely to be using?.
 
Yes, I think so too. As those PICs with ICSP would need at least a resistor and a diode connection on MCLR pin, so I think this cannot be a problem on them or else they will not work. But Microchip do specify a schottky and not an oridinay diode.
 
This was a problem with the early PICs like the PIC16C5X family. It is documented in the datasheet. However, I never experienced this problem personally. In fact, they recommend special reset circuitry. This has been eliminated with the latter PICs with powerup timer and built-in brownout circuitry.
 
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