global: add common function to start DFU and systeme memory

global.c

@@ -16,6 +16,8 @@
 #include <libopencm3/stm32/rcc.h> // real-time control clock library
 #include <libopencm3/stm32/gpio.h> // general purpose input output library
 #include <libopencm3/stm32/exti.h> // external interrupt defines
+#include <libopencm3/stm32/syscfg.h> // system definitions
+#include <libopencm3/usb/dwc/otg_fs.h> // USB OTG utilities
 
 #include "global.h" // common methods
 
@@ -201,6 +203,66 @@ void board_setup(void)
 	user_input_used = 0;
 }
 
+/** disconnect USB by sending a reset condition */
+static void usb_disconnect(void)
+{
+	if (OTG_FS_GUSBCFG & OTG_GUSBCFG_FDMOD) { // USB configured as device
+		// pull USB D+ low for a short while
+		OTG_FS_DCTL |= OTG_DCTL_SDIS; // disconnect DP pull-up to simulate a disconnect
+		// in case there is an external pull-up resistor, pull DP low
+		// I have no idea why, but once USB is configured, I can't use PA12/DP back as GPIO
+		gpio_mode_setup(GPIOA, GPIO_MODE_OUTPUT, GPIO_PUPD_NONE, GPIO12); // be sure the D+ pin can be used as GPIO output
+		gpio_set_output_options(GPIOA, GPIO_OTYPE_PP, GPIO_OSPEED_2MHZ, GPIO12); // use push-pull output
+		gpio_clear(GPIOA, GPIO12); // pull D+ low
+		for (volatile uint32_t i = 0; i < 0x2000; i++); // USB disconnected must be at least 10 ms long, at most 100 ms
+	}
+}
+
+void system_memory(void)
+{
+	usb_disconnect(); // disconnect from USB (if necessary)
+
+	// for more details, see https://stm32f4-discovery.net/2017/04/tutorial-jump-system-memory-software-stm32/
+
+	// deinit RCC (according to STM32CubeF4 source code)
+	RCC_CR |= RCC_CR_HSION; // enable high speed internal clock
+	while (!(RCC_CR & RCC_CR_HSIRDY)); // wait until clock is ready
+	RCC_CR |= (0x10U << RCC_CR_HSITRIM_SHIFT); // set HSITRIM[4:0] bits to the reset value
+	RCC_CFGR = 0;// reset CFGR register
+	while (((RCC_CFGR >> RCC_CFGR_SWS_SHIFT) & RCC_CFGR_SWS_MASK) != RCC_CFGR_SWS_HSI); // wait it till clock switch is ready
+	RCC_CR &= ~(RCC_CR_HSEON | RCC_CR_HSEBYP | RCC_CR_CSSON); // clear HSEON, HSEBYP and CSSON bits
+	while (RCC_CR & RCC_CR_HSERDY); // wait till HSE is disabled
+	RCC_CR &= ~RCC_CR_PLLON; // Clear PLLON bit
+	while (RCC_CR & RCC_CR_PLLRDY); // wait till PLL is disabled
+	//RCC_PLLCFGR = 0x24003010; // reset PLLCFGR register to default value (value for STM32F401)
+	RCC_CIR &= ~(RCC_CIR_LSIRDYIE | RCC_CIR_LSERDYIE | RCC_CIR_HSIRDYIE | RCC_CIR_HSERDYIE | RCC_CIR_PLLRDYIE); // disable all interrupts
+	RCC_CIR |= (RCC_CIR_LSIRDYC | RCC_CIR_LSERDYC | RCC_CIR_HSIRDYC | RCC_CIR_HSERDYC | RCC_CIR_PLLRDYC | RCC_CIR_CSSC); // clear all interrupt flags
+	RCC_CR &= ~RCC_CSR_LSION; // clear LSION bit
+	RCC_CSR |= RCC_CSR_RMVF; // reset all CSR flags
+
+	// switch to system memory
+	RCC_APB2ENR = RCC_APB2ENR_SYSCFGEN; // enable system configure clock (all others are not required)
+	cm_disable_interrupts(); // disable all interrupts
+	SYSCFG_MEMRM = 1; // map system memory to 0x0000 0000 (this bypasses the BOOT0 pin)
+	const uint32_t address = 0x1FFF0000; // system memory address
+	__asm__ volatile ("MSR msp,%0" : :"r"(*(uint32_t*)address)); // set stack pointer to address provided in the beginning of the bootloader (loaded into a register first)
+	(*(void(**)(void))((uint32_t)address + 4))(); // start system memory (by jumping to the reset function which address is stored as second entry of the vector table)
+	// we should not reach this point
+}
+
+void dfu_bootloader(void)
+{
+	usb_disconnect(); // disconnect from USB (if necessary)
+
+	// set DFU magic to specific RAM location
+	__dfu_magic[0] = 'D';
+	__dfu_magic[1] = 'F';
+	__dfu_magic[2] = 'U';
+	__dfu_magic[3] = '!';
+	scb_reset_system(); // reset system (core and peripherals)
+	while (true); // wait for the reset to happen
+}
+
 #if defined(BUTTON_PIN)
 /** interrupt service routine called when button is pressed */
 void GPIO_EXTI_ISR(BUTTON_PIN)(void)

global.h

@@ -495,4 +495,11 @@ char user_input_get(void);
 void user_input_store(char c);
 /** setup board peripherals */
 void board_setup(void);
-
+/** start embedded bootloader (e.g. system memory)
+ *  @warning the USB DFU bootloader does not start
+ *  @note after jumping to the bootloader, the USB device is detected, but does not enumerate. I have no idea about the cause. Not configuring the USB did not solve the issue. Re-initializing RCC did not help. Even the STM32CodeMX generated code was not able to have a working USB DFU reboot. Calibrating HSI according to https://community.st.com/s/question/0D50X0000BG0TfH/stm32f4-usb-dfu-bootloader-not-working-reliably did not solve the issue. Maybe there is something with the board. Booting the embedded USB DFU bootloader by hand by pressing the BOOT0 button works.
+ *  @note the USART bootloader works fine though
+ */
+void system_memory(void);
+/** start DFU bootloader */
+void dfu_bootloader(void);