Crystal series resistor value when using PLL

[newguy]

A question to those of you who have used the PLL on the 18F PICs. I've never used the PLL before, but I'm pretty much being forced to because of a very "tempermental" peripheral. The PIC communicates with this peripheral via standard serial link @ 115.2 kpbs. The peripheral can't handle any baud error at all from the nominal 115.2 kbps. It looks like I'm going to use a 7.3728 MHz ECS crystal with a 18F2680, and use the x4 PLL to bump the frequency up to 29.4912 MHz. This frequency is exactly 256 x 115.2 kbps, which means that it's as accurate as I can get it.

The PIC's datasheet suggests that I'll need a resistor in series with the crystal, and that a good starting point is 330 ohms. For those of you that have used the PLL, is this resistance value okay? I'd like something a little more concrete than "a good starting point." The issue for me is that I won't be able to breadboard this - it's straight to a PCB, and I won't be able to easily swap out surface mount resistors. I'd like to have something that works from the outset.

[Ttelmah]

The 'answer', is totally dependant on your crystal. The PLL, is not as fussy as it originally was. On early chips, it was very prone to problems if overdriven, while latter chips have tended to be much better in this regard. You need to get an specfication for the crystal you intend to use. Microchip, have a Spice model for the internal oscillator, which they will send you if asked, and you can then model the oscillator system, and have a good chance of getting it right.

Best Wishes