quickly noted the concept in disco_f411ve/board.hpp

src/modm/board/disco_f411ve/board.hpp

@@ -22,14 +22,34 @@ namespace Board
 /// @{
 using namespace modm::literals;
 
-/// STM32F411 running at 84MHz generated from the external 8MHz crystal
 struct SystemClock
 {
-	static constexpr uint32_t Frequency = 84_MHz;
+	static constexpr uint32_t ExternalCrystalClock = 8_MHz;
+	static constexpr Rcc::PllFactors pllFactors{
+		.pllM = 8,
+		.pllN = 336,
+		.pllP = 4,
+		.pllQ = 7,
+	};
+	static constexpr uint32_t MainPllClock = ExternalCrystalClock / pllFactors.pllM * pllFactors.pllN;
+
+	static constexpr Rcc::AhbPrescaler Ahb_prescaler = Rcc::AhbPrescaler::Div1;
+	static constexpr Rcc::Apb1Prescaler Apb1_prescaler = Rcc::Apb1Prescaler::Div2;
+	static constexpr Rcc::Apb2Prescaler Apb2_prescaler = Rcc::Apb2Prescaler::Div1;
+
+	// ------------------------------------------
+
+	static constexpr uint32_t Frequency = MainPllClock / pllFactors.pllP; // 84_Mhz
+	static constexpr uint32_t Usb = MainPllClock / pllFactors.pllQ; // 48_Mhz
+
 	static constexpr uint32_t Ahb = Frequency;
 	static constexpr uint32_t Apb1 = Frequency / 2;
 	static constexpr uint32_t Apb2 = Frequency;
 
+	// @todo find the right place
+	static_assert(Apb1 <= 50_MHz, "Apb1 has max. 50MHz!");
+	static_assert(Apb2 <= 100_MHz, "Apb2 has max. 100MHz!");
+
 	static constexpr uint32_t Adc = Apb2;
 
 	static constexpr uint32_t Spi1 = Apb2;
@@ -62,27 +82,18 @@ struct SystemClock
 	static constexpr uint32_t Timer10 = Apb2Timer;
 	static constexpr uint32_t Timer11 = Apb2Timer;
 
-	static constexpr uint32_t Usb = 48_MHz;
-
 	static bool inline enable()
 	{
-		Rcc::enableExternalCrystal();  // 8MHz
-		const Rcc::PllFactors pllFactors{
-			.pllM = 8,    // 8MHz / M=8 -> 1MHz
-			.pllN = 336,  // 1MHz * N=336 -> 84MHz
-			.pllP = 4,    // 336MHz / P=4 -> 100MHz = F_cpu
-			.pllQ = 7,    // 336MHz / P=7 -> 100MHz = F_cpu
-		};
+		/// STM32F411 running at 84MHz generated from the external 8MHz crystal
+		Rcc::enableExternalCrystal();
 		Rcc::enablePll(Rcc::PllSource::ExternalCrystal, pllFactors);
 		// set flash latency for 100MHz
 		Rcc::setFlashLatency<Frequency>();
 		// switch system clock to PLL output
 		Rcc::enableSystemClock(Rcc::SystemClockSource::Pll);
-		Rcc::setAhbPrescaler(Rcc::AhbPrescaler::Div1);
-		// APB1 has max. 50MHz
-		// APB2 has max. 100MHz
-		Rcc::setApb1Prescaler(Rcc::Apb1Prescaler::Div2);
-		Rcc::setApb2Prescaler(Rcc::Apb2Prescaler::Div1);
+		Rcc::setAhbPrescaler(Ahb_prescaler);
+		Rcc::setApb1Prescaler(Apb1_prescaler);
+		Rcc::setApb2Prescaler(Apb2_prescaler);
 		// update frequencies for busy-wait delay functions
 		Rcc::updateCoreFrequency<Frequency>();