blink.c

struct rcc {
  volatile unsigned long rc;
  volatile unsigned long cfg;
  volatile unsigned long cir;
  volatile unsigned long apb2;
  volatile unsigned long apb1;
  volatile unsigned long ape3;
  volatile unsigned long ape2;
  volatile unsigned long ape1;
  volatile unsigned long bdcr;
  volatile unsigned long csr;
};

#define GPIOA_ENABLE  0x04
#define GPIOB_ENABLE  0x08
#define GPIOC_ENABLE  0x10

#define RCC_BASE      (struct rcc *) 0x40021000


struct gpio {
  volatile unsigned long cr[2];
  volatile unsigned long idr;
  volatile unsigned long odr;
  volatile unsigned long bsrr;
  volatile unsigned long brr;
  volatile unsigned long lock;
};

#define GPIOA_BASE    (struct gpio *) 0x40010800
#define GPIOB_BASE    (struct gpio *) 0x40010C00
#define GPIOC_BASE    (struct gpio *) 0x40011000

#define MODE_OUT_2    0x02

#define CONF_GP_UD    0x0
#define CONF_GP_OD    0x4


#define FAST 200
#define SLOW 400

#define SPEED SLOW


void
delay(void)
{
  int i;

  for (i = 0; i < (1000 * SPEED); i++)
    __asm__("nop");
}


struct gpio *gp;
unsigned long on_mask;
unsigned long off_mask;

void
led_init( int bit )
{
  int conf;
  int shift;
  struct rcc *rp = RCC_BASE;

  /* Turn on GPIO C */
  rp->ape2 |= GPIOC_ENABLE;
  gp = GPIOC_BASE;

  shift = (bit - 8) * 4;
  conf = gp->cr[1] & ~(0xf << shift);
  conf |= (MODE_OUT_2|CONF_GP_OD) << shift;
  gp->cr[1] = conf;

  on_mask = 1 << bit;
  off_mask = 1 << (bit+16);
}

void
led_on( void )
{
  gp->bsrr = on_mask;
}

void led_off( void )
{
  gp->bsrr = off_mask;
}

#define PC13 13

void
startup( void )
{
  led_init( PC13 );

  while(1) {
    led_on();
    delay();
    led_off();
    delay();
  }
}

blink.lds

/* blink.lds
 * linker script for blink demo.
 *
 * Memory layout on my STM32F103 is pretty much like this:
 *
 * 0x00000000 - 0x07ffffff - aliased to flash or sys memory depending on BOOT jumpers
 * 0x08000000 - 0x0800ffff - Flash (64K ... or 128K)
 * 0x1ffff000 - 0x1ffff7ff - Boot firmware in system memory
 * 0x1ffff800 - 0x1fffffff - option bytes
 * 0x20000000 - 0x20004fff - SRAM (20k)
 * 0x40000000 - 0x40023400 - peripherals
 */
MEMORY
{
   flash(RX)  : ORIGIN = 0x00000000, LENGTH = 128K
   sram(WAIL) : ORIGIN = 0x20000000, LENGTH = 20K
}

SECTIONS
{
   .text : { *(.text*) } > flash
   .bss  : { *(.bss*) } > sram
}

locore.s

/* locore.s
 * Assembler startup file for the STM32F103
 * Tom Trebisky  9-24-2016
 */

# The Cortex M3 is a thumb only processor
.cpu cortex-m3
.thumb

.word   0x20005000  /* stack top address */
.word   _reset      /* 1 Reset */
.word   spin        /* 2 NMI */
.word   spin        /* 3 Hard Fault */
.word   spin        /* 4 MM Fault */
.word   spin        /* 5 Bus Fault */
.word   spin        /* 6 Usage Fault */
.word   spin        /* 7 RESERVED */
.word   spin        /* 8 RESERVED */
.word   spin        /* 9 RESERVED*/
.word   spin        /* 10 RESERVED */
.word   spin        /* 11 SV call */
.word   spin        /* 12 Debug reserved */
.word   spin        /* 13 RESERVED */
.word   spin        /* 14 PendSV */
.word   spin        /* 15 SysTick */
.word   spin        /* 16 IRQ0 */
.word   spin        /* 17 IRQ1 */
.word   spin        /* 18 IRQ2 */
.word   spin        /* 19 ...   */
                    /* On to IRQ67 */

spin:   b spin

.thumb_func
_reset:
    bl startup
    b .

/* THE END */

Makefile

# Makefile for blink demo

#TOOLS = arm-linux-gnu
TOOLS = arm-none-eabi

# Assembling with gcc makes it want crt0 at link time.
#AS = $(TOOLS)-gcc
AS = $(TOOLS)-as
# Use the -g flag if you intend to use gdb
#CC = $(TOOLS)-gcc -mcpu=cortex-m3 -mthumb
CC = $(TOOLS)-gcc -mcpu=cortex-m3 -mthumb -g

#LD = $(TOOLS)-gcc
LD = $(TOOLS)-ld.bfd
OBJCOPY = $(TOOLS)-objcopy
DUMP = $(TOOLS)-objdump -d
GDB = $(TOOLS)-gdb

OBJS = locore.o blink.o

all: blink.bin blink.dump

blink.dump:     blink.elf
        $(DUMP) blink.elf >blink.dump

blink.bin:      blink.elf
        $(OBJCOPY) blink.elf blink.bin -O binary

blink.elf:      $(OBJS)
        $(LD) -T blink.lds -o blink.elf $(OBJS)

locore.o:       locore.s
        $(AS) locore.s -o locore.o

blink.o:        blink.c
        $(CC) -c blink.c

# To burn the image:
#  Connect the STLINK gadget to the target.
#  Be sure and start openOCD in this directory.
#    (or else it won't be able to find the bin file).
#  Then use "make flash" to start an openocd console session.
#  Type: reset halt
#  Type: flash write_image erase blink.bin 0x08000000
flash:
        @echo "type: flash write_image erase blink.bin 0x08000000"
        telnet localhost 4444
gdb:
        $(GDB) --eval-command="target remote localhost:3333" blink.elf

gdbtui:
        $(GDB) -tui --eval-command="target remote localhost:3333" blink.elf

clean:
        rm -f *.o blink.elf blink.bin blink.dump

Very nice minimal example, wish they'd release examples like this.