Reset circuit for PIC! Need diode?

pic mclr diode

Hi all,

I just wonder if it's always necessary for me to put a diode in between of the reset switch and the MCLR pin of a PIC? My reset circuit for PIC is by connecting a 33K resistor in series with a 0.1uF capacitor whereby one end of resistor is connected to the Vout of 7805 and one end of the capacitor is connected to ground. The reset switch is connected in parallel to the capacitor and the cathode of diode is connected to the end of reset switch which is not connected to ground but connected to the resistor and capacitor.

My problem is that this reset circuit works fine for PIC16F877 but when I change my PIC to PIC18F452, it doesnt work properly. If I bypass the diode by short-circuiting it, I can reset my PIC18F452, otherwise, I cant reset it at all.

Thanks for any help!

Regards,
Eric

mclr circuit

I dont use diode for PIC 16F877, and I had no problems.

pic mclr circuit

pixel said:

I dont use diode for PIC 16F877, and I had no problems.

Click to expand...

So u just connect directly from one end of reset switch to the MCLR pin?

recommended mclr circuit

Microchip do recommend using a small (few hundred ohm) resistor between the MCLR pin and any external capacitor.

how circuit with diode and resistor work

Why not simply use the recommended (and attached) circuit from microchip 18F452 document?

If you need a reset switch you can connect it in parallel to the cap

best regards

switch capacitor resistor reset circuit

C-Man said:

Why not simply use the recommended (and attached) circuit from microchip 18F452 document?

If you need a reset switch you can connect it in parallel to the cap

best regards

Click to expand...

Wats the other end of the diode connected to? Vdd?

Thanks heaps!

Regards,
Eric

18f452 7805

ericmar said:

C-Man said:

Why not simply use the recommended (and attached) circuit from microchip 18F452 document?

If you need a reset switch you can connect it in parallel to the cap

best regards

Click to expand...

Wats the other end of the diode connected to? Vdd?

Thanks heaps!

Regards,
Eric

Click to expand...

Yes should be VDD.

best regards

pic reset circuit

4700 works very well

pic mclr reset capacitor

mrm said:

4700 works very well

Click to expand...

Hi,

Which resistor do u mean? R or R1?

Thanks..

Regards,
Eric

reset circuitry with diode resistor capacitor

ericmar said:

mrm said:

4700 works very well

Click to expand...

Hi,

Which resistor do u mean? R or R1?

Thanks..

Regards,
Eric

Click to expand...

He means R but you should be careful with the values of R if you use a CCS ICD. I recommend a value of 22 K to 47 K in this case and you can also not use the diode because programming voltage (13V) is applied via the reset pin when ICSP programming.

best regards

pic18 mclr reset circuit

Those circuits seem mainly for solving special situations, and stand a good chance of making things worse.

I have always just run a 10k resistor from MCLR to Vdd and I've never had a problem... at least not one I'm aware of being related to the MCLR circuit, it would be quite inaccurate to say I've never had a problem without being more specific.

I've never even used a reset button. Seems to work fine.

reset circuit for pic

To solve this problem and forget about it you should use proper reset IC such as MAX1232. It has Res-L, Res-H and SW input and you can select voltage threshold and reset time.
I just dont trust this resistor/cap combination, and it doesn't matter wheather this is PIC, 8051, ...... it always works.