Implement RCC's clock configuration

For now, flash configuration is hard-coded. This will fixed in future
commits

drivers/rcc.c

@@ -0,0 +1,126 @@
+/** @file rcc.c
+ * Module handling Reset and Clocks Control (RCC).
+ *
+ * The module provides functions to configure clocks according to presets as
+ * well as to enable/disable/reset peripherals.
+ */
+
+//--includes--------------------------------------------------------------------
+
+#include "rcc.h"
+#include "rcc_regs.h"
+
+
+//--local definitions-----------------------------------------------------------
+
+static void apply_default_preset(void);
+static void apply_speed_preset(void);
+
+
+//--local variables-------------------------------------------------------------
+
+static volatile struct RCC* regs = (struct RCC*)RCC_BASE_ADDRESS;
+
+
+//--public functions------------------------------------------------------------
+
+void rcc_configure(enum RccPreset preset)
+{
+	//disable all peripherals during clock configuration
+	union RCC_APB1ENR apb1_enr = regs->APB1ENR;
+	regs->APB1ENR.word = 0x0;
+	union RCC_APB2ENR apb2_enr = regs->APB2ENR;
+	regs->APB2ENR.word = 0x0;
+
+	switch (preset) {
+		case RCC_PRESET_DEFAULT:
+			apply_default_preset();
+			break;
+		case RCC_PRESET_SPEED:
+			apply_speed_preset();
+			break;
+		default:
+			break;
+	}
+
+	//re-enable peripherals
+	regs->APB1ENR = apb1_enr;
+	regs->APB2ENR = apb2_enr;
+}
+
+
+//--local functions-------------------------------------------------------------
+
+/**
+ * Apply the default clock preset, using HSI whithout PLL or prescalers. This is
+ * similar to the reset configuration. Most buses run at 8Mhz, except ADCs wich
+ * run at 4Mhz, and Systick wich only reaches 1Mhz.
+ */
+static void apply_default_preset(void)
+{
+	//ensures HSI is enabled
+	regs->CR.HSION = 0x1;
+	while (regs->CR.HSIRDY != 0x1);
+
+	//set HSI as main clock source and disable prescalers
+	regs->CFGR.word &= !0x077fff3;
+
+	//disable all options
+	regs->CR.HSITRIM = 0x10;
+	regs->CR.HSEON = 0x0;
+	regs->CR.HSEBYP = 0x0;
+	regs->CR.CCSON = 0x0;
+	regs->CR.PLLON = 0x0;
+
+	//disable all interrupts
+	regs->CIR.LSIRDYIE = 0x0;
+	regs->CIR.LSERDYIE = 0x0;
+	regs->CIR.HSIRDYIE = 0x0;
+	regs->CIR.HSERDYIE = 0x0;
+	regs->CIR.PLLRDYIE = 0x0;
+}
+
+/**
+ * Apply the speed preset, using HSE with PLL to achieve 72MHz on AHB, APB2 and
+ * timer buses. APB1 is limited to its maximum clock of 36Mhz and the ADCs run
+ * at 12MHz. Systick runs at 9Mhz.
+ */
+static void apply_speed_preset(void)
+{
+	//restore sane values
+	apply_default_preset();
+
+	//try enabling HSE, fallback to HSI if HSE fails
+	regs->CR.HSEON = 0x1;
+	for (uint32_t i=0; i<1000; ++i) {
+		__asm__("nop");
+	}
+	if (regs->CR.HSERDY == 0x1) {
+		regs->CFGR.PLLSCR = 0x1;
+	} else {
+		regs->CR.HSEON = 0;
+	}
+
+	//configure PLL, fallback to HSI if PLL fails
+	regs->CFGR.PLLMUL = 0x7; //PLL x9
+	regs->CR.PLLON = 0x1;
+	for (uint32_t i=0; i<1000; ++i) {
+		__asm__("nop");
+	}
+	if (regs->CR.PLLRDY != 0x1) {
+		regs->CR.PLLON = 0x0;
+		return; //clock low enough, no need for prescalers
+	}
+
+	//configure prescalers
+	regs->CFGR.PPRE1 = 0x4;  // /2
+	regs->CFGR.ADCPRE = 0x2; // /6
+
+	//configure flash
+	volatile uint32_t* ACR = (uint32_t*)0x40022000;
+	*ACR |= 0x12;
+
+	//switch to PLL output
+	regs->CFGR.SW = 0x2;
+}
+

drivers/rcc.h

@@ -0,0 +1,38 @@
+/** @file rcc.h
+ * Module handling Reset and Clocks Control (RCC).
+ *
+ * The module provides functions to configure clocks according to presets as
+ * well as to enable/disable/reset peripherals.
+ */
+
+#ifndef _RCC_H_
+#define _RCC_H_
+
+//--includes--------------------------------------------------------------------
+
+#include "stdint.h"
+
+
+//--type definitions------------------------------------------------------------
+
+/**
+ * Available clock configuration presets
+ */
+enum RccPreset {
+	RCC_PRESET_DEFAULT, //sane values, identical to reset config
+	RCC_PRESET_SPEED,   //highest clocks, uses 8MHz HSE if available
+};
+
+
+//--functions-------------------------------------------------------------------
+
+/**
+ * Configures the clocks and buses according to the given preset.
+ *
+ * @param preset the preset to use for configuration
+ */
+void rcc_configure(enum RccPreset preset);
+
+
+#endif //_RCC_H_
+

drivers/rcc_regs.h

@@ -1,3 +1,10 @@
+/** @file rcc_regs.h
+ * Module defining Reset and Clocks Control (RCC) registers.
+ *
+ * Mainly made to be used by the rcc module. It is recommanded to go through
+ * the functions provided by that module instead of directly using the registers
+ * defined here.
+ */
 
 #ifndef _RCC_REGS_H_
 #define _RCC_REGS_H_