MCLR Pin

[Aussie Barry]

Hi All,

All the PIC documentation I have ever read says to tie the MCLR pin to +5V via a 4k7 resistor. If an application will not use the MCLR function (ie no switch to 0V as a manual reset) is it OK to simply wire the MCLR pin directly to +5V or does it still need to go via the 4k7 resistor?

If the resistor is essential, can someone please explain why?

Cheers
Barry

[dhouston]

My guess would be to limit the current drain to minimal - about 1mA with 4.7K.

[mister_e]

As there's no protection diode on this pin, it's high likely to limit/help in case of ESD.

Could be tied to Vdd, or better, assuming your PIC allows it, disable it, and use it as an I/O.

[Darrel Taylor]

The MCLR circuit varies between PIC families.
You should always consult the datasheet for the PIC you are using.

For instance, the 12F683 has an internal pull-up on MCLR, so you don't need anything on that pin when MCLR is enabled.

But when using external circuitry there are two warnings that apply, 1 for pull-up (red box) and 1 for pull-down (Note: ).

5664

[mister_e]

Can't argue against Datasheet, that's for sure... But (cough) let's have some fun and let's play with words a lil bit.


Voltages applied to the MCLR pin that exceed its
specification can result in both MCLR Resets and
excessive current beyond the device specification
during the ESD event. For this reason, Microchip
recommends that the MCLR pin no longer be tied
directly to VDD. The use of an RC network, as shown inHere's the fun... MCLR IS tied to VDD...

Absolute Maximum Ratings(†)
.....
.....
Voltage on MCLR with respect to Vss .................................................. ............................................. -0.3V to +13.5V

Well OK...max spec for MCLR is set 'round 13 volts... well, if this happen... high likely the PIC will burn way before :D

[/end smarta55]

[boroko]

Found one of those ghost problems that only seems to occur after days of living with it and finally deciding to go to the Forum and seek and answer....

On my 18F4550, I have been living with 30 second start up times after programming or a reset. As I was typing the question, it occured to me to check if I had put a pull-up on MCLR.....NOPE!

Disabled the reset option and tada! it works just the way the datasheet says it should. Imagine!

Another one for the mental library.
Bo