In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Beau Schwabe wrote:

<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">[url=http://forums.parallax.com/forums/profile.aspx?f=7&amp;m=291334&amp;p=6050]Zoot[/url],<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>

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<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">A resistor placed between XO amd XI, in combination with the internal pin capacitance, forms a low-pass filter. This filter slows down the clock edges, which significantly reduces voltage-droop and ground bounce on the power supply rail and helps to reduce jitter.  At 50MHz (10nS pulses) there may be enough attenuation from the low-pass filter that is formed that the crystal does not have enough energy to continue oscillating.  It is possible for a crystal oscillator to operate without an external resistor or with the IFBD turned off, but the oscillator will not be as stable.<o:p></o:p>

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<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">My guess is that parasitic board capacitance is your culprit more than the IFBD resistance (approximately 1 Meg) ... A 10k resistor seems a little low to me.

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