Crystal oscillator start-up resistor still required

[trastikata]

Just a reminder that things we take for given now, are not always as we expect.

Please note that even in some relative modern dsPIC's, an external, parallel to the crystal, high value resistor is still required.

[rick.curl]

Thanks for the reminder!
If you look at the schematic of the chip you can see there is supposed to be another resistor inside the chip- so the external one is in parallel with it. My guess is that this is actually a silicon anomaly because they forgot to add the internal resistor - so the external resistor is just a way of covering their ass.
The internal oscillators have gotten so good that it's rare for me to use an external crystal. I'm curious to know if anyone has had an oscillator fail to start until they added the extra resistor.
I will go ahead and add it just out of caution in the event I use an external crystal, though.

-Rick   

[okmn]

i  used  the rezistor when i saw it in that orginal microchip dev. board schematic.

[david]

That Dev board circuit is showing a series resistor for the RTC tuning fork type xtal (very high impedance and Q) whereas the first example is a parallel resistor for a high frequency xtal.
1 Meg is usually fine for lower frequency xtals like 4MHz but as you get up to 25-30MHz you would want to drop the value down to 100k.  The most critical components are the xtal loading capacitors which are dictated by the xtal used, not the circuit as they set the correct xtal current and frequency.  I know many oscillators will work without them but your xtal current will be very low, you'll be high in frequency and it may not start up on limit parameters like voltage, gm, temperature.
220k in series with a 20MHz xtal is not going to work.

David

[okmn]

it is 32.768khz for secondary osc.

[david]

Yes - no problem having such a high series resistance for that type of xtal at that frequency but the opening post was about parallel resistance for the main oscillator.  Sometimes a small amount of series resistant can be used in high frequency oscillators but usually only a few hundred ohms.

Cheers,
David