I am building one of these http://plazma.kapsi.fi/diy/lanc_controller/
Its a LANC Controller (ie. remote control) for a Sony camcoder.

The project uses a PIC16F84A chip, which is a programmable
microcontroller.
I am very comfortable working with assembler, but I am not so
profficient with electronics. I'm having a problem with this project.

I've assembled everything on a breadboard (the chip is programmed and
verified), but it doesn't seem to work. I have a striking suspicion
that I bought a wrong crystal oscillator, and I can't find any
information on how to choose the correct device.

Do crystal oscilators come in different flavors when it comes to
waveform shape? Since this oscillator is used as a clock in the
circuit, does that mean that I need one that generates a square wave?

The oscillator I bought and am currently using is this one: http://
www.newark.com/jsp/Passives/Crystals/VISHAY+DALE/XT49S-204M/
displayProduct.jsp?sku=18C1488

Also I bought 22pF capacitors instead of the 18pF suggested in the
project. Would that affect anything?

As a related question, I've noticed that the clock signal circuit for
many chips is the same as the one in this project (crystal oscilator
and two caps), how do you know what capacitor values to use? I've read
in several places that it depends on the characteristics of the
specific oscillator, but I could not find any more detail.

Thanks in advance,
Nick.

This is a standard type of crystal clock. Note that IMHO you bought a
crystal, not a crystal oscillator (the latter implies some internal IC
logic to run the clock), but you did buy the right part. Based on the
crystal's part number, it wants 20pF caps (each crystal wants a
specific cap value, read the data sheets) but 22pF is probably close
enough, at most the frequency might be off a little.

Now, do you have a way of checking to see if the crystal *is*
oscillating? Note that only one side of the crystal is safe to probe,
if you probe the other side you'll likely stop the clock (it won't
hurt anything, though) so check both pins. On some MCUs, the crystal
always runs, on others it has to be started in software.

After reading the specs for the crystal and the PIC, I make the
following observation: The crystal is an AT strip cut crystal. The
PIC spec says that a series resistor may be needed for AT strip cut
crystals; the PIC spec has a schematic. Also, make sure the FOSC
configuration bits are correctly chosen.

The way you choose which mode you need is to look at
how the crystal will be driven. If it'll be driven with an
inverting amplifier, like the one in the PIC16F845A, then you go for
parallel resonance. If it'll be driven by a non-inverting
amplifier, then you specify series resonance.

That oscillator is classic, is called a "Pierce" oscillator, and
Google will give you lots of hits on how it works.

You pointed out that the documentation mentions that a series
resistor may be needed for an AT strip cut crystal (something I
missed alltogether). What would be the purpose of this resistor and
why is it specific to AT strip cut crystals? Also, what value should
that resistor be? I find it odd that the documentation states
nothign of the value that should be used.

I am building one of thesehttp://plazma.kapsi.fi/diy/lanc_controller/
Its a LANC Controller (ie. remote control) for a Sony camcoder.

The project uses a PIC16F84A chip, which is a programmable
microcontroller.
I am very comfortable working with assembler, but I am not so
profficient with electronics. I'm having a problem with this project.

I've assembled everything on a breadboard (the chip is programmed and
verified), but it doesn't seem to work. I have a striking suspicion
that I bought a wrong crystal oscillator, and I can't find any
information on how to choose the correct device.

Do crystal oscilators come in different flavors when it comes to
waveform shape? Since this oscillator is used as a clock in the
circuit, does that mean that I need one that generates a square wave?

The oscillator I bought and am currently using is this one: http://www.newark.com/jsp/Passives/Crystals/VISHAY+DALE/XT49S-204M/
displayProduct.jsp?sku=18C1488

Also I bought 22pF capacitors instead of the 18pF suggested in the
project. Would that affect anything?
As a related question, I've noticed that the clock signal circuit for
many chips is the same as the one in this project (crystal oscilator
and two caps), how do you know what capacitor values to use? I've read
in several places that it depends on the characteristics of the
specific oscillator, but I could not find any more detail.

Thanks in advance,
Nick.

On Jan 29, 3:07 pm, John Fields <jfie...@austininstruments.com> wrote:
The way you choose which mode you need is to look at
how the crystal will be driven. If it'll be driven with an
inverting amplifier, like the one in the PIC16F845A, then you go for
parallel resonance. If it'll be driven by a non-inverting
amplifier, then you specify series resonance.

I thought about this and still can't figure out why a non-invertin amp
configuration differs from an inverting one. I'm assuming it has
something to do with what kind of impedance the amp (buffer in this
case?) likes to "see", is that so? But the type of an amp used does
not affect this property, or am I wrong?

That oscillator is classic, is called a "Pierce" oscillator, and
Google will give you lots of hits on how it works.

Thanks, now that I know how this configuration is called, it'll be
much easier to find information on it.


Thanks again for all your input. I appreciate the help.
This group looks like its full of knowledgeable people.

-Nick

You need to select the "XT" crystal setting.
This can be set in your programmer software.

Dave

Yes. The reason for the inversion is that there's a 180° phase
shift through the crystal when it's operating in parallel resonance
and that, combined with the 180°shift through the amplifier makes
the circuit inherently unstable, causing it to oscillate.
---
JF