intermitent resets

dave.t · Guest

Hi all!

I have a project which uses an 16f877 running at 4mhz using a ceramic resonator. When I power up - the system does not always reset properly (although the prototype does!). I have a 10k resistor from mclr to +5v and a 10uf tant from mclr to ground and have set the fuses correctly. Could it be the resonator? What make of resonator is recommended for this pic?
I have read microchips AN607 application note but it does not state which make of resonator to use.
Any help would greatly appreciated!

Dave
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Original Post ID: 11873

Steve H · Guest

A couple of ideas:

1) You need to put a diode across the 10 k resistor to MCLR. This will discharge the capacitor every time the power rail drops. Otherwise the power has to off for a long period of time before you get a real reset.

2) You need to put 100 ohms in series with the MCLR line to prevent the capacitor charge from latching up the PIC when the power rail goes away.

3) Did you use a ceramic resonator with built in caps? You need loading capacitors to get the proper phase shift on the oscillator. I have used many types of resonators from several manufacturers and I have not had problems with any. Panasonic makes some fine parts and you can get them from DigiKey.

Steve H.
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Original Post ID: 11885

Steve H · Guest

One more thing....

Make sure you use the PUT (power on timer) and BOR (brown out reset) fuses.

Steve H.
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This message was ported from CCS's old forum
Original Post ID: 11886

Tomi · Guest

Do you have any external circuit connected to your PIC board what has a separated power supply? If so then turning BOR on as Steve suggested could be a solution but check the PIC's power line: if an external circuit feeds one (or more) of your PIC pins by 5V logic level then you can measure 4 - 4.4V what is pretty enough for the PIC.

:=Hi all!
:=
:=I have a project which uses an 16f877 running at 4mhz using a ceramic resonator. When I power up - the system does not always reset properly (although the prototype does!). I have a 10k resistor from mclr to +5v and a 10uf tant from mclr to ground and have set the fuses correctly. Could it be the resonator? What make of resonator is recommended for this pic?
:=I have read microchips AN607 application note but it does not state which make of resonator to use.
:=Any help would greatly appreciated!
:=
:=Dave
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This message was ported from CCS's old forum
Original Post ID: 11891

Hans Wedemeyer · Guest

All advice given by others is good.

However you can simply connect the MCLR line to +5V (no 10K needed) and use the PUT.

That will guarantee a clean reset everytime your system is powered ON, and save a resistor and Cap.
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This message was ported from CCS's old forum
Original Post ID: 11936