1.8 Three-stage Rocket

As I merge the cstartup with the ledtick code, I split the functionalities between three files according to the three phases: boot, initialization and main execution.

startup.c is mainly concerned with the early set up of the board: initializing the memory, calling system initialization and then executing the main C function if initialization is successful.

init.c holds the middleware implementation and will abstract the peripherals into higher level interfaces. Its entry point is the init() function.

the main() C function will be the focus of the last file and it should be written in a subset of standard C. As an example I will use success.c.

/* success.c -- success does nothing, successfully */
#include <stdlib.h>

int main( void) {
    return EXIT_SUCCESS ;
}

startup.c

Beside the interrupt vector and the Reset_Handler() that calls init() and main(), I have created stubs for all the System Exceptions listed in the Programming Manual. For those, if there is no implementation avalable in init.c, the linker will use the Default_Handler() provided with the __attribute__() Gnu C extension.

/* Memory locations defined by linker script */
extern long __StackTop ;        /* &__StackTop points after end of stack */
void Reset_Handler( void) ;     /* Entry point for execution */
extern const long __etext[] ;   /* start of initialized data copy in flash */
extern long __data_start__[] ;
extern long __bss_start__[] ;
extern long __bss_end__ ;       /* &__bss_end__ points after end of bss */

/* Stubs for System Exception Handler */
void Default_Handler( void) ;
#define dflt_hndlr( fun) void fun##_Handler( void) \
                                __attribute__((weak,alias("Default_Handler")))
dflt_hndlr( NMI) ;
dflt_hndlr( HardFault) ;
dflt_hndlr( SVCall) ;
dflt_hndlr( PendSV) ;
dflt_hndlr( SysTick) ;

/* Interrupt vector table:
 * 1  Stack Pointer reset value
 * 15 System Exceptions
 * NN Device specific Interrupts
 */
typedef void (*isr_p)( void) ;
isr_p const isr_vector[ 16] __attribute__((section(".isr_vector"))) = {
    (isr_p) &__StackTop,
/* System Exceptions */
    Reset_Handler,
    NMI_Handler,
    HardFault_Handler,
    0,  0,  0,  0,  0,  0,  0,
    SVCall_Handler,
    0,  0,
    PendSV_Handler,
    SysTick_Handler
} ;

extern int init( void) ;
extern int main( void) ;

void Reset_Handler( void) {
    const long  *f ;    /* from, source constant data from FLASH */
    long    *t ;        /* to, destination in RAM */

/* Assume:
**  __bss_start__ == __data_end__
**  All sections are 4 bytes aligned
*/
    f = __etext ;
    for( t = __data_start__ ; t < __bss_start__ ; t += 1)
        *t = *f++ ;

    while( t < &__bss_end__)
        *t++ = 0 ;

    if( init() == 0)
        main() ;

    for( ;;)
        __asm( "WFI") ; /* Wait for interrupt */
}

void Default_Handler( void) {
    for( ;;) ;
}

Except for the future addition of stubs to handle the device specific interrupts, this file will not grow much anymore.

init.c

This is the embryo of an hardware abstraction layer where most of the device specific code will be added. The current code is the peripherals part of ledtick.c.

#define SYSTICK             ((volatile long *) 0xE000E010)
#define SYSTICK_CSR         SYSTICK[ 0]
#define SYSTICK_RVR         SYSTICK[ 1]
#define SYSTICK_CVR         SYSTICK[ 2]

#define RCC                 ((volatile long *) 0x40021000)
#define RCC_AHBENR          RCC[ 5]
#define RCC_AHBENR_IOPBEN   0x00040000  /*  18: I/O port B clock enable */

#define GPIOB               ((volatile long *) 0x48000400)
#define GPIOB_MODER         GPIOB[ 0]
#define GPIOB_ODR           GPIOB[ 5]

int init( void) {
/* By default SYSCLK == HSI [8MHZ] */

/* SYSTICK */
    SYSTICK_RVR = 1000000 - 1 ;     /* HBA / 8 */
    SYSTICK_CVR = 0 ;
    SYSTICK_CSR = 3 ;               /* HBA / 8, Interrupt ON, Enable */
    /* SysTick_Handler will execute every 1s from now on */

/* User LED ON */
    RCC_AHBENR |= RCC_AHBENR_IOPBEN ;   /* Enable IOPB periph */
    GPIOB_MODER |= 1 << (1 * 2) ;       /* PB1 Output [01], over default 00 */
    /* OTYPER Push-Pull by default */
    /* PB1 output default LOW at reset */

    return 0 ;
}

void SysTick_Handler( void) {
    GPIOB_ODR ^= 1 << 1 ;   /* Toggle PB1 (User LED) */
}

Makefile

As I now build from multiple source files, I have modified the Makefile to list the sources that combine together. All steps I have done so far can be found in the commented SRCS lines. Single file steps can be build explicitly (make ledon.hex) or implicitly (make) after removing the comment on the corresponding SRCS line. Multiple file steps can only be build implicitly when their SRCS line is uncommented.

### Build environment selection

ifeq (linux, $(findstring linux, $(MAKE_HOST)))
 GCCDIR = $(HOME)/Packages/arm-gnu-toolchain-13.3.rel1-x86_64-arm-none-eabi
else
 GCCDIR = "D:/Program Files (x86)/GNU Arm Embedded Toolchain/arm-gnu-toolchain-13.3.rel1-mingw-w64-i686-arm-none-eabi"
endif

BINPFX  = @$(GCCDIR)/bin/arm-none-eabi-
CC      = $(BINPFX)gcc
LD      = $(BINPFX)ld
OBJCOPY = $(BINPFX)objcopy
OBJDUMP = $(BINPFX)objdump
SIZE    = $(BINPFX)size

### STM32F030F4P6 based board

PROJECT = f030f4
#SRCS = boot.c
#SRCS = ledon.c
#SRCS = blink.c
#SRCS = ledtick.c
#SRCS = cstartup.c
SRCS = startup.c init.c success.c
OBJS = $(SRCS:.c=.o)
CPU = -mthumb -mcpu=cortex-m0
CFLAGS = $(CPU) -g -Wall -Wextra -Os
LD_SCRIPT = $(PROJECT).ld

### Build rules

.PHONY: clean all

all: $(PROJECT).hex $(PROJECT).bin

clean:
    @echo CLEAN
    @rm -f *.o *.elf *.map *.lst *.bin *.hex

$(PROJECT).elf: $(OBJS)
    @echo $@
    $(LD) -T$(LD_SCRIPT) -Map=$(PROJECT).map -cref -o $@ $(OBJS)
    $(SIZE) $@
    $(OBJDUMP) -hS $@ > $(PROJECT).lst

%.elf: %.o
    @echo $@
    $(LD) -T$(LD_SCRIPT) -Map=$*.map -cref -o $@ $<
    $(SIZE) $@
    $(OBJDUMP) -hS $@ > $*.lst

%.bin: %.elf
    @echo $@
    $(OBJCOPY) -O binary $< $@

%.hex: %.elf
    @echo $@
    $(OBJCOPY) -O ihex $< $@

A successful build will generate the files f030f4.hex, f030f4.bin, f030f4.map, f030f4.lst.

Build and Test

Even if stdlib.h is included in success.c, there is no C libraries needed to complete the build as only the constant EXIT_SUCCESS from that header is used. Furthermore, default location of the header files is derived by the compiler from the location of gcc.

$ make
f030f4.elf
   text    data     bss     dec     hex filename
    216       0       0     216      d8 f030f4.elf
f030f4.hex
f030f4.bin

Building shows an increase in code, still no data.

Once f030f4.hex is loaded into the board, the behavior is the same as ledtick.hex. The new file structure and data initialization didn’t introduce any ~~bugs~~ changes, just code overhead.

Checkpoint

This step was mainly to achieve a better structure for future evolution.

Next, I will make the code available in a public git repository.