Generation problem

[vladimir059@hot]

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

[Archangel]

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

[dmairspotter]

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.

[vladimir059@hot]

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

[Archangel]

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