The STM32 App Note 2606 discusses this, but there is no simple code example.
This answer has been tested on the STM32F072 Nucleo board using IAR EWARM. This answer uses the "STM32 Standard Peripheral Library" and nothing else.
Note that the best/easiest way to verify you are successfully in bootloader mode (DFU mode) is to hookup a USB-2-UART converter (get one here from Sparkfun: http://sfe.io/p9873 for $15) on lines PA_9 (USART1_TX) and PA_10 (USART1_RX) (don't forget to connect ground as well). I was not able to use the Nucleo USART2 default connection (/dev/ttyACM0), hence the external USB-2-USART connection. Then create a simple C program to write 0x7F on the USART connection. If you are in DFU mode, it will reply with one byte: 0x79. I use Ubuntu, so my test program compiles and runs on Linux.
Also, the easiest way to test bootloader mode (aka DFU mode) is to jumper the BOOT0 line to +3.3V. These are right next to each other on the Nucleo.
Add to main.c main() routine:
// Our STM32 F072 has:
// 16k SRAM in address 0x2000 0000 - 0x2000 3FFF
*((unsigned long *)0x20003FF0) = 0xDEADBEEF;
// Reset the processor
NVIC_SystemReset();
Add some code to Libraries/sysconfig/system_stm32f0xx.c at the beginning of the SystemInit() function:
// Define our function pointer
void (*SysMemBootJump)(void);
void SystemInit (void)
{
// Check if we should go into bootloader mode.
//
// Set the main stack pointer __set_MSP() to its default value. The default
// value of the main stack pointer is found by looking at the default value
// in the System Memory start address. Do this in IAR View -> Memory. I
// tried this and it showed address: 0x200014A8 which I then tried here.
// The IAR compiler complained that it was out of range. After some
// research, I found the following from "The STM32 Cortex-M0 Programming
// Manual":
// Main Stack Pointer (MSP)(reset value). On reset, the processor
// loads the MSP with the value from address 0x00000000.
//
// So I then looked at the default value at address 0x0 and it was 0x20002250
//
// Note that 0x1fffC800 is "System Memory" start address for STM32 F0xx
//
if ( *((unsigned long *)0x20003FF0) == 0xDEADBEEF ) {
*((unsigned long *)0x20003FF0) = 0xCAFEFEED; // Reset our trigger
__set_MSP(0x20002250);
// 0x1fffC800 is "System Memory" start address for STM32 F0xx
SysMemBootJump = (void (*)(void)) (*((uint32_t *) 0x1fffC804)); // Point the PC to the System Memory reset vector (+4)
SysMemBootJump();
while (1);
}
... // The rest of the vanilla SystemInit() function
Create a simple utility to see if you are in bootloader mode (aka DFU mode). This compiles and runs on Linux. Make sure you get your serial port right. It will likely be /dev/ttyUSB0 as shown below.
//
// A bare-bones utility: Test if the STM32 is in DFU mode
// (aka bootloader mode, aka firmware update mode).
//
// If it is in DFU mode, you can send it 0x7F over a UART port and it
// will send 0x79 back.
//
// For details, see the STM32 DFU USART spec.
//
#include <termios.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <string.h>
#include <errno.h> // errno
#define DEFAULT_SERDEVICE "/dev/ttyUSB0"
//#define DEFAULT_SERDEVICE "/dev/ttyACM0"
int main(int argc, char **argv)
{
int fd, cooked_baud = B9600;
char *sername = DEFAULT_SERDEVICE;
struct termios oldsertio, newsertio;
unsigned char mydata[2] = {0};
mydata[0] = 0x7F;
mydata[1] = 0;
/* Not a controlling tty: CTRL-C shouldn't kill us. */
fd = open(sername, O_RDWR | O_NOCTTY);
if ( fd < 0 )
{
perror(sername);
exit(-1);
}
tcgetattr(fd, &oldsertio); /* save current modem settings */
/*
* 8 data, EVEN PARITY, 1 stop bit. Ignore modem control lines. Enable
* receive. Set appropriate baud rate. NO HARDWARE FLOW CONTROL!
*/
newsertio.c_cflag = cooked_baud | CS8 | CLOCAL | CREAD | PARENB;
/* Raw input. Ignore errors and breaks. */
newsertio.c_iflag = IGNBRK | IGNPAR;
/* Raw output. */
newsertio.c_oflag = OPOST;
/* No echo and no signals. */
newsertio.c_lflag = 0;
/* blocking read until 1 char arrives */
newsertio.c_cc[VMIN]=1;
newsertio.c_cc[VTIME]=0;
/* now clean the modem line and activate the settings for modem */
tcflush(fd, TCIFLUSH);
tcsetattr(fd,TCSANOW,&newsertio);
// Here is where the magic happens
write(fd,&mydata[0],1);
int red = read(fd,&mydata[1],1);
if (red < 0) {
fprintf(stderr, "Error: read() failed, errno [%d], strerrer [%s]\n",
errno, strerror(errno));
}
tcsetattr(fd,TCSANOW,&oldsertio);
close(fd);
printf("Read [%d] bytes: [0x%x]\n", red, mydata[1]);
return 0;
}