add 80MHz clock example

00-80mhz/Makefile

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+COMMON_DIR := ../common
+
+CFILES += main.c
+
+include $(COMMON_DIR)/rules.mk

00-80mhz/README.md

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+# 0. 80MHz system clock
+
+This example shows how to set up the STM32 microcontroller to execute at its faster clock speed of 80MHz.
+
+Achieving this speed requires two things:
+
+1. Setting up the Phase-Locked Loop (PLL) to multiply a source clock up to 80MHz.
+2. Configuring flash wait-states to ensure that the processor does not execute faster than instructions can be fetched.
+
+Once the system clock is set to 80MHz, the Microcontroller Clock Output (MCO) is configured to output the system clock
+over pin PA8. This output can be viewed with an oscilloscope to confirm the clock's speed.
+
+See `main.c` for comments describing the configuration steps in more detail.
+

00-80mhz/main.c

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+#include <stm32l476xx.h>
+
+int main(void)
+{
+        // Disable interrupts
+	asm("cpsid i");
+
+	// Running at 80MHz requires configuring flash wait-states:
+	FLASH->ACR = (FLASH->ACR & ~(FLASH_ACR_LATENCY)) | FLASH_ACR_LATENCY_4WS;
+
+        // Configure the PLL to output 80MHz:
+        //  1. MSI clock is 4MHz at reset, use MSI as source for PLL
+        //  2. Set PLLN multiplier to 40: 4MHz * 40 = 160MHz
+        //  3. PLLM divider is set to zero, no division there
+        //  4. Set PLLR system clock divider to zero = division by two
+        //     160MHz / 2 = 80MHz
+        RCC->PLLCFGR = RCC_PLLCFGR_PLLSRC_MSI |
+                       (40 << RCC_PLLCFGR_PLLN_Pos) |
+                       (0 << RCC_PLLCFGR_PLLR_Pos) |
+                       RCC_PLLCFGR_PLLREN;
+
+        // Enable the PLL
+	RCC->CR |= RCC_CR_PLLON;
+	while ((RCC->CR & RCC_CR_PLLRDY) != RCC_CR_PLLRDY);
+
+        // Set the system clock to use PLL
+        // Also clear the HPRE divider to be safe 
+	RCC->CFGR = (RCC->CFGR & ~(RCC_CFGR_HPRE_Msk | RCC_CFGR_SW_Msk)) |
+	            RCC_CFGR_SW_PLL;
+	while ((RCC->CFGR & RCC_CFGR_SWS_PLL) != RCC_CFGR_SWS_PLL);
+
+        // The processor is now running at 80MHz!
+
+        // Enable MCO to output the system clock over PA8
+        RCC->CFGR &= ~(RCC_CFGR_MCOPRE_Msk);
+        RCC->CFGR |= 2 << RCC_CFGR_MCOSEL_Pos;
+
+        // Configure PA8 for alternate function zero (MCO)
+        RCC->AHB2ENR |= RCC_AHB2ENR_GPIOAEN;
+        GPIOA->MODER &= ~(GPIO_MODER_MODE8_Msk);
+        GPIOA->MODER |= 2 << GPIO_MODER_MODE8_Pos;
+        GPIOA->AFR[1] &= ~(GPIO_AFRH_AFSEL8_Msk);
+
+        // Using a volatile assembly instruction ensures that this loop is not
+        // optimized away by the compiler.
+	while (1)
+            asm volatile("nop");
+}
+