Starting Crystals

[camerart]

Hi all,

This is my first time on the site.

I sometimes get trouble with Crystals not starting up. I am making a new circuit, modifying a circuit where the Crystals always start up, but on the new circuit it doesn't. The Crystal (Watch type 32766 Hz) is connected to the input of a 4060 divider circuit, as the suggestion on the PDF. I have used the same components as the working cicuit. I found that when checking if the Crystal was working using an Oscilloscope on pin 10, it would start, then even just the Oscilloscope lead would start it. I made a 60mm length of screened lead, connected it to pin 10 and the screen to Gnd, it now works. Is it likely to be reliable?

Cheers, Camerart.

 

[Mitchekj]

Sounds like the capacitance from the probe is what was letting you start. Try a 10pF cap instead? Or whatever your probe's capacitance is. I don't know anything about your specific IC there, so I don't know if there are any other issues.

 

[(*steve*)]

When I've gone looking for crystal oscillators (especially for these watch crystals) I note that this is not an uncommon problem.

Be aware that it can take a surprisingly long time for the oscillator to start up and stabilise, even in the best of conditions.

Your long thin capacitor could be reliable, but I would go with Mitchekj's suggestion of a more conventional capacitor

 

[camerart]

Thank you.

I did leave it for a long time, while getting it to start.

I will try adding a Capacitor in place of my wire.

How do I find the Capacitance of the Oscilloscope? Is it written on it?

Cheers, C.

 

[(*steve*)]

They're typically 10pF to 20pF. Coax on its own may be around 1pF per cm (it depends a LOT on the type of coax).

6cm of coax -- try about 10pF

 

[camerart]

Ok, Steve,

I'll try that tomorrow.

Next week I will have access to a capacitance meter, I'll check the wire.

Thanks, back Friday.

 

[camerart]

Hi,

I substituted the a 10pF cap for the wire and the XTL started, then I moved the Cap to be parallel with the Cap on that track. It also worked, so I changed the cap, at first ot be 33pF didn't work, then 47pF did.

Having also read up a little I decided on 22pf and 100pF Caps 10mH Resistor across Pins 10 and 11, and 100K Resistor on the 100pF track. This worked on 3 X circuits. and seems stable.

Thanks for your help.