[board] Use external crystal on SAMV71 Xplained Ultra board

Author: chris-durand

src/modm/board/samv71_xplained_ultra/board.hpp

@@ -25,7 +25,7 @@ using namespace modm::platform;
 
 struct SystemClock
 {
-	// 300MHz system clock generated by PLLA from internal Rc 12MHz clock
+	// 300 MHz system clock generated by PLLA from external 12 MHz crystal
 	static constexpr uint32_t PllAMult = 25;
 	static constexpr uint32_t Frequency = 300_MHz;
 	static constexpr uint32_t Mck = Frequency / 2;  // 150 MHz max.
@@ -35,10 +35,11 @@ struct SystemClock
 	static bool inline
 	enable()
 	{
-		ClockGen::setFlashLatency<Frequency / 2>();  // Flash runs off MCK
+		ClockGen::setFlashLatency<Mck>();
+
+		ClockGen::enableMainExternalCrystal<12_MHz>(std::chrono::microseconds{1000});
+		ClockGen::selectMainClockSource(MainClockSource::External);
 
-		ClockGen::enableMainInternal(MainInternalFreq::Rc12Mhz);
-		ClockGen::selectMainClockSource(MainClockSource::Internal);
 		ClockGen::enablePllA<PllAMult>();
 		ClockGen::selectMasterClk<MasterClkSource::PLLA_CLK, MasterClkPrescaler::CLK_1, MasterClkDivider::Div2>();
 		ClockGen::updateCoreFrequency<Frequency>();