Generation problem

Hello everybody!

I have problem with oscillator. When I use PIC16F84 AP/4 and with “canned” 4MHz oscillator controller works great but with 4 MHz crystal and two 22pF ceramic capacitors generator doesn’t work. I played with value of capacitors, I tried to change position of the crystal and capacitors, but there is still no generation. What other parameters might affect on generation? What typical amplitude of generation if VDD 5V ? I use Hameg 20Mhz oscilloscope. Should I use small capacitor to connect probe to make measurements of oscillator?
Thank you in advance
Vladimir

Just thinking out loud . . have you tried to load and run a simple program?
What I am wondering is if your scope inputs impeadance is so low that the act of checking it loads the oscillator enough to stop it. The "canned" oscillators have a much more robust output than on chip unit. try putting an 100 kilo to 1 meg ohm resistor in series with your input, it will reduce the pattern height but if you geet anything on screen then you know you are on track. . .
JS

Have you got your oscillator mode set right? I think it is HS for crystal..don't have my documaentation in front of me but I have had trouble with forgetting this.

Hello, thank you everybody for replies.
Yes, I downloaded program into PIC controller, and I‘m 100% sure there is not generation , because if I use external source of oscillation controller wakes up and works properly. I think my prototype PCB isn’t good and there are some parasitic influence between components. What are there general rules exist for placing components around PIC controller?
Vladimir

Hi Vladimir,
Running at 4 MHZ you should not have too many RF issues as the frequency is so low. When you program the chip - make sure the programmer is set to " XT " for 4 mhz if it is set to " HS "or " RC" it will not oscillate with a resonator or crystal. Since 4 MHZ is the default speed for this chip I do not believe you need to DEFINE it in the code, I never have for 4 mhz and they always work. Also make sure no other loads are attached to pins 15 or 16, just the crystal and 22pf caps.
I Hope this helps.
JS