[PIC]: PIC16F871 newbie oscillator issues

The Situation

I have a set of questions all of which relate to the general topic of the
oscillator on the PIC16F871.  I developed the usual first PIC program, the
blinking led.  After several go arounds it worked, the led blinked.  Since
then I have tried other programs and gone back to the original program and
cannot get anything to run.  For some reason ( probably because it is one of
the things that I am least clear on ) I have decided that it might be the
oscillator.  The hardware I am using is a 20 MHz crystal ( and other
attempts with a 16 MHz crystal ) with 17 pf capacitors to ground.  I think
the correct oscillator setting is HS.  I developed the program with MPLab
and included a line:

 __CONFIG _HS_OSC & _PWRTE_ON & _BODEN_OFF & _CP_OFF & _WDT_ON

I programmed with PICALL & AVR programmer - P16PRO mode, Windows version.
This has configuration settings for the oscillator configuration where I
also used HS.  The hex file used was the output from MPLab.  Simulation both
in MPLab and another simulator seem to indicate the program might work.  I
had until recently thought of the fuses as pretty much the same as any other
part of the flash memory.

I did not always use HS, sometime by mistake sometimes by intention to see
if  I could get the sucker to run.  I also have a 30 MHz scope, and have
been unable to observe a clock signal. I used a 10 x probe to reduce loading
and tested on osc2 which I think can stand more loading.

When I ask the programmer to verify the chip all seems ok, including
shifting from one program to another.  I also tried using a second PIC chip
with about the same results.

The circuit is built on a proto board.  I have checked the wiring several
time, think it is ok.

The Questions.

Once the clock fuses are set can they be changed ( not the code protection
one, just the oscillator ones )?

If you reprogram the fuses over and over do you get: the last setting, a
tendency to or them together to all 1's, a tendency to or them together to
all 0's?

If they get ored together can I still use the chip somehow perhaps with an
external oscillator?

How will the configuration setting in the hex file interact with the
configuration settings in the PICALL programmer?

Can I expect the Scope to work if I use a 10x 10 meg shielded cable?  I
assume the OSC2 pin is a better place to probe than OSC1.

Built an external clock using a couple of SN74ls04n gates and the crystal,
plan to try it out soon, it does show a clock signal.

Also thought I would try setting up for an RC clock .  Worth a try?

Finally thought it might be easier to run as 4 MHz.  Worth a try?

Perhaps I am forcing the chip into sleep mode?

Other suggestions?


Russ Hensel

A client server programmer in Boston with some PIC hobby interests






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