Simon:

I have used the Cygnal 'F020 evaluaton board for numerous projects and prototyping activities. I write in Keil C51 language and here is the code that I use to switch over to the external 22.1184 MHz oscillator.

//----------------------------------------------------------------
// Oscillator Configuration
//----------------------------------------------------------------

	OSCXCN = 0x67;	// EXTERNAL Oscillator Control Register	
    for (;n < 255; n++) ;           // XTLVLD blanking interval (>1ms)
    while ( (OSCXCN & 0x80) == 0 );	// wait for xtal osc to start up

	OSCICN = 0x0C;	// Internal Oscillator Control Register

Michael Karas