Can't find where XTAL 12M is selected in the Arduino Core

Official Hardware Mega Due
1

Hi folks, this might be an academic question, but it's bothering me. We know that there is a 12M external oscillator on the Due, but I don't see where in the code it is selected. I have heard elsewhere that this oscillator is selected, but the 32KHz chip isn't. Just trying to confirm these simple facts, but searching has not turned up anything obvious.

Thanks
Doug

2

Never fails. Post a question and suddenly the answer is obvious. This appears to be happening in system_sam3xa.c within function SystemInit() which is called by variant.cpp at startup.

Specifically

	/* Switch to 3-20MHz Xtal oscillator */
	PMC->CKGR_MOR = SYS_CKGR_MOR_KEY_VALUE | SYS_BOARD_OSCOUNT | 
	                           CKGR_MOR_MOSCRCEN | CKGR_MOR_MOSCXTEN | CKGR_MOR_MOSCSEL;

I guess I just didn't expect the Atmel libraries to assume the presence of the XTAL.

Not super sure what CKGR_MOR_MOSCRCEN is doing in there.