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A bare metal programming guide on STM32 microcontroller

License: MIT License

C++ 0.83% C 99.06% Makefile 0.11%

bare-metal-programming-guide's Introduction

A bare metal programming guide

This guide is written for developers who wish to start programming microcontrollers using GCC compiler and bare metal approach. We are going to use a Nucleo-F429ZI development board with STM32F429 microcontroller (buy on Mouser). But basic principles would be applicable to any other microcontroller.

In this guide, I'll show how to program a microcontroller using just a compiler and a datasheet, nothing else. Later I'll explain what are the vendor's CMSIS headers, how and why they should be used. We'll learn how to blink LEDs, how to redirect printf() to UART, how to set up the system clock, how to use interrupts, and even how to run a web server with device dashboard.

To proceed, the following tools are required:

Tools setup instructions for Mac. Start a terminal, and execute:

$ /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
$ brew install --cask gcc-arm-embedded make stlink

Tools setup instructions for Linux (Ubuntu). Start a terminal, and execute:

$ sudo apt -y install gcc-arm-none-eabi make stlink-tools

Tools setup instructions for Windows:

Also, download two datasheets:

Introduction

A microcontroller (uC, or MCU) is a small computer. Typically it has CPU, RAM, flash to store firmware code, and a bunch of pins that stick out. Some pins are used to power the MCU, usually marked as GND (ground) and VCC pins. Other pins are used to communicate with the MCU, by means of high/low voltage applied to those pins. One of the simplest ways of communication is an LED attached to a pin: one LED contact is attached to the ground pin (GND), and another contact is attached to a signal pin via a current-limiting resistor. A firmware code can set high or low voltage on a signal pin, making LED blink:

Memory and registers

The 32-bit address space of the MCU is divided by regions. For example, some region of memory is mapped to the internal MCU flash at a specific address. Firmware code instructions are read and executed by reading from that memory region. Another region is RAM, which is also mapped to a specific address. We can read and write any values to the RAM region.

From STM32F429 datasheet, we can take a look at section 2.3.1 and learn that RAM region starts at address 0x20000000 and has size of 192KB. From section 2.4 we can learn that flash is mapped at address 0x08000000. Our MCU has 2MB flash, so flash and RAM regions are located like this:

From the datasheet we can also learn that there are many more memory regions. Their address ranges are given in the section 2.3 "Memory Map". For example, there is a "GPIOA" region that starts at 0x40020000 and has length of 1KB.

These memory regions correspond to a different "peripherals" inside the MCU - a piece of silicon circuitry that make certain pins behave in a special way. A peripheral memory region is a collection of 32-bit registers. Each register is a 4-byte memory range at a certain address, that maps to a certain function of the given peripheral. By writing values into a register - in other words, by writing a 32-bit value at a certain memory address, we can control how given peripheral should behave. By reading registers, we can read back peripheral's data or configuration.

There are many different peripherals. One of the simpler ones are GPIO (General Purpose Input Output), which allow user to set MCU pins into "output mode" and set high or low voltage on them. Or, set pins into an "input mode" and read voltage values from them. There is a UART peripheral which can transmit and receive serial data over two pins using RS232 protocol. There are many other peripherals.

Often, there are multiple "instances" of the same peripheral, for example GPIOA, GPIOB, ... which control different set of MCU pins. Likewise, there could be UART1, UART2, ... which allow to implement multiple UART channels. On Nucleo-F429, there are several GPIO and UART peripherals.

For example, GPIOA peripheral starts at 0x40020000, and we can find GPIO register description in section 8.4. The datasheet says that GPIOA_MODER register has offset 0, that means that it's address is 0x40020000 + 0, and this is the format of the register:

The datasheet shows that the 32-bit MODER register is a collection of 2-bit values, 16 in total. Therefore, one MODER register controls 16 physical pins, Bits 0-1 control pin 0, bits 2-3 control pin 1, and so on. The 2-bit value encodes pin mode: 0 means input, 1 means output, 2 means "alternate function" - some specific behavior described elsewhere, and 3 means analog. Since the peripheral name is "GPIOA", then pins are named "A0", "A1", etc. For peripheral "GPIOB", pin naming would be "B0", "B1", ...

If we write 32-bit value 0 to the register MODER, we'll set all 16 pins, from A0 to A15, to input mode:

  * (volatile uint32_t *) (0x40020000 + 0) = 0;  // Set A0-A15 to input mode

By setting individual bits, we can selectively set specific pins to a desired mode. For example, this snippet sets pin A3 to output mode:

  * (volatile uint32_t *) (0x40020000 + 0) &= ~(3 << 6);  // CLear bits 6-7
  * (volatile uint32_t *) (0x40020000 + 0) |= 1 << 6;     // Set bits 6-7 to 1

Some registers are not mapped to the MCU peripherals, but they are mapped to the ARM CPU configuration and control. For example, there is a "Reset at clock control" unit (RCC), described in section 6 of the datasheet. It describes registers that allow to set systems clock and other things.

Human-readable peripherals programming

In the previous section we have learned that we can read and write peripheral register by direct accessing certain memory addresses. Let's look at the snippet that sets pin A3 to output mode:

  * (volatile uint32_t *) (0x40020000 + 0) &= ~(3 << 6);  // CLear bits 6-7
  * (volatile uint32_t *) (0x40020000 + 0) |= 1 << 6;     // Set bits 6-7 to 1

That is pretty cryptic. Without extensive comments, such code would be quite hard to understand. We can rewrite this code to a much more readable form. The idea is to represent the whole peripheral as a structure that contains 32-bit fields. Let's see what registers exist for the GPIO peripheral in the section 8.4 of the datasheet. They are MODER, OTYPER, OSPEEDR, PUPDR, IDR, ODR, BSRR, LCKR, AFR. Their offsets are with offsets 0, 4, 8, etc... . That means we can represent them as a structure with 32-bit fields, and make a define for GPIOA:

struct gpio {
  volatile uint32_t MODER, OTYPER, OSPEEDR, PUPDR, IDR, ODR, BSRR, LCKR, AFR[2];
};

#define GPIOA ((struct gpio *) 0x40020000)

Then, for setting GPIO pin mode, we can define a function:

// Enum values are per datasheet: 0, 1, 2, 3
enum {GPIO_MODE_INPUT, GPIO_MODE_OUTPUT, GPIO_MODE_AF, GPIO_MODE_ANALOG};

static inline void gpio_set_mode(struct gpio *gpio, uint8_t pin, uint8_t mode) {
  gpio->MODER &= ~(3U << (pin * 2));        // Clear existing setting
  gpio->MODER |= (mode & 3) << (pin * 2);   // Set new mode
}

Now, we can rewrite the snippet for A3 like this:

gpio_set_mode(GPIOA, 3 /* pin */, GPIO_MODE_OUTPUT);  // Set A3 to output

Our MCU has several GPIO peripherals (also called "banks"): A, B, C, ... K. From section 2.3 we can see that they are 1KB away from each other: GPIOA is at address 0x40020000, GPIOB is at 0x40020400, and so on:

#define GPIO(bank) ((struct gpio *) (0x40020000 + 0x400 * (bank)))

We can create pin numbering that includes the bank and the pin number. To do that, we use 2-byte uint16_t value, where upper byte indicates GPIO bank, and lower byte indicates pin number:

#define PIN(bank, num) ((((bank) - 'A') << 8) | (num))
#define PINNO(pin) (pin & 255)
#define PINBANK(pin) (pin >> 8)

This way, we can specify pins for any GPIO bank:

  uint16_t pin1 = PIN('A', 3);    // A3   - GPIOA pin 3
  uint16_t pin2 = PIN('G', 11);   // G11  - GPIOG pin 11

Let's rewrite the gpio_set_mode() function to take our pin specification:

static inline void gpio_set_mode(uint16_t pin, uint8_t mode) {
  struct gpio *gpio = GPIO(PINBANK(pin)); // GPIO bank
  uint8_t n = PINNO(pin);                 // Pin number
  gpio->MODER &= ~(3U << (n * 2));        // Clear existing setting
  gpio->MODER |= (mode & 3) << (n * 2);   // Set new mode
}

Now the code for A3 is self-explanatory:

  uint16_t pin = PIN('A', 3);            // Pin A3
  gpio_set_mode(pin, GPIO_MODE_OUTPUT);  // Set to output

Note that we have created a useful initial API for the GPIO peripheral. Other peripherals, like UART (serial communication) and others - can be implemented in a similar way. This is a good programming practice that makes code self-explanatory and human readable.

MCU boot and vector table

When STM32F429 MCU boots, it reads a so-called "vector table" from the beginning of flash memory. A vector table is a concept common to all ARM MCUs. That is a array of 32-bit addresses of interrupt handlers. First 16 entries are reserved by ARM and are common to all ARM MCUs. The rest of interrupt handlers are specific to the given MCU - these are interrupt handlers for peripherals. Simpler MCUs with few peripherals have few interrupt handlers, and more complex MCUs have many.

Vector table for STM32F429 is documented in Table 62. From there we can learn that there are 91 peripheral handlers, in addition to the standard 16.

At this point, we are interested in the first two entries of the vector table, because they play a key role in the MCU boot process. Those two first values are: initial stack pointer, and an address of the boot function to execute (a firmware entry point).

So now we know, that we must make sure that our firmware should be composed in a way that the 2nd 32-bit value in the flash should contain an address of out boot function. When MCU boots, it'll read that address from flash, and jump to our boot function.

Minimal firmware

Let's create a file main.c, and specify our boot function that initially does nothing (falls into infinite loop), and specify a vector table that contains 16 standard entries and 91 STM32 entries. In your editor of choice, create main.c file and copy/paste the following into main.c file:

// Startup code
__attribute__((naked, noreturn)) void _reset(void) {
  for (;;) (void) 0;  // Infinite loop
}

// 16 standard and 91 STM32-specific handlers
__attribute__((section(".vectors"))) void (*tab[16 + 91])(void) = {
  0, _reset
};

For function _reset(), we have used GCC-specific attributes naked and noreturn - they mean, standard function's prologue and epilogue should not be created by the compiler, and that function does not return.

The void (*tab[16 + 91])(void) expression means: define an array of 16 + 91 pointers to functions, that return nothing (void) and take to arguments. Each such function is an IRQ handler (Interrupt ReQuest handler). An array of those handlers is called a vector table.

The vector table tab we put in a separate section called .vectors - that we need later to tell the linker to put that section right at the beginning of the generated firmware - and consecutively, at the beginning of flash memory. We leave the rest of vector table filled with zeroes.

Note that we do not set the first entry in the vector table, which is an initial value for the stack pointer. Why? Because we don't know the correct value for it. We'll handle it later.

Compilation

Let's compile our code:

$ arm-none-eabi-gcc -mcpu=cortex-m4 main.c -c

That works! The compilation produced a file main.o which contains our minimal firmware that does nothing. The main.o file is in ELF binary format, which contains several sections. Let's see them:

$ arm-none-eabi-objdump -h main.o
...
Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         00000002  00000000  00000000  00000034  2**1
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  1 .data         00000000  00000000  00000000  00000036  2**0
                  CONTENTS, ALLOC, LOAD, DATA
  2 .bss          00000000  00000000  00000000  00000036  2**0
                  ALLOC
  3 .vectors      000001ac  00000000  00000000  00000038  2**2
                  CONTENTS, ALLOC, LOAD, RELOC, DATA
  4 .comment      0000004a  00000000  00000000  000001e4  2**0
                  CONTENTS, READONLY
  5 .ARM.attributes 0000002e  00000000  00000000  0000022e  2**0
                  CONTENTS, READONLY

Note that VMA/LMA addresses for sections are set to 0 - meaning, main.o is not yet a complete firmware, because it does not contain the information where those section should be loaded in the address space. We need to use a linker to produce a full firmware firmware.elf from main.o.

The section .text contains firmware code, in our case it is just a _reset() function, 2-bytes long - a jump instruction to its own address. There is an empty .data section and an empty .bss section (data that is initialized to zero) . Our firmware will be copied to the flash region at offset 0x8000000, but our data section should reside in RAM - therefore our _reset() function should copy the contents of the .data section to RAM. Also it has to write zeroes to the whole .bss section. Our .data and .bss sections are empty, but let's modify our _reset() function anyway to handle them properly.

Also, our _reset() function should set the initial stack pointer, cause our vector table has zero in the corresponding entry at index 0.

In order to do all that, we must know where stack starts, and where data and bss section start. This we can specify in the "linker script", which is a file with the instructions to the linker, where to put various sections in the address space, and which symbols to create.

Linker script

Create a minimal linker script link.ld, and copy-paste contents from step-0-minimal/link.ld. Below is the explanation:

ENTRY(_reset);

This line tells the linker the value of the "entry point" attribute in the generated ELF header - so this is a duplicate to what a vector table has. This is an aid for a debugger (like Ozone, described below) that helps to set a breakpoint at the beginning of the firmware. A debugger does not know about a vector table, so it relies on the ELF header.

MEMORY {
  flash(rx)  : ORIGIN = 0x08000000, LENGTH = 2048k
  sram(rwx) : ORIGIN = 0x20000000, LENGTH = 192k  /* remaining 64k in a separate address space */
}

This tells the linker that we have two memory regions in the address space, their addresses and sizes.

_estack     = ORIGIN(sram) + LENGTH(sram);    /* stack points to end of SRAM */

This tell a linker to create a symbol estack with value at the very end of the RAM memory region. That will be our initial stack value!

  .vectors  : { KEEP(*(.vectors)) }   > flash
  .text     : { *(.text*) }           > flash
  .rodata   : { *(.rodata*) }         > flash

These lines tell the linker to put vectors table on flash first, followed by .text section (firmware code), followed by the read only data .rodata.

The next goes .data section:

  .data : {
    _sdata = .;   /* .data section start */
    *(.first_data)
    *(.data SORT(.data.*))
    _edata = .;  /* .data section end */
  } > sram AT > flash
  _sidata = LOADADDR(.data);

Note that we tell linker to create _sdata and _edata symbols. We'll use them to copy data section to RAM in the _reset() function.

Same for .bss section:

  .bss : {
    _sbss = .;              /* .bss section start */
    *(.bss SORT(.bss.*) COMMON)
    _ebss = .;              /* .bss section end */
  } > sram

Startup code

Now we can update our _reset() function. We initialize stack pointer, copy data section to RAM, and initialise bss section to zeroes. Then, we call main() function - and fall into infinite loop in case if main() returns:

int main(void) {
  return 0; // Do nothing so far
}

// Startup code
__attribute__((naked, noreturn)) void _reset(void) {
  asm("ldr sp, = _estack");  // Set initial stack pointer

  // memset .bss to zero, and copy .data section to RAM region
  extern long _sbss, _ebss, _sdata, _edata, _sidata;
  for (long *src = &_sbss; src < &_ebss; src++) *src = 0;
  for (long *src = &_sdata, *dst = &_sidata; src < &_edata;) *src++ = *dst++;

  main();             // Call main()
  for (;;) (void) 0;  // Infinite loop in the case if main() returns
}

The following diagram visualises how _reset() initialises .data and .bss:

Now we are ready to produce a full firmware file firmware.elf:

$ arm-none-eabi-gcc -T link.ld -nostdlib main.o -o firmware.elf

Let's examine sections in firmware.elf:

$ arm-none-eabi-objdump -h firmware.elf
...
Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .vectors      000001ac  08000000  08000000  00010000  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  1 .text         00000058  080001ac  080001ac  000101ac  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
...

Now we can see that the .vectors section will reside at the very beginning of flash memory at address 0x8000000, then the .text section right after it, at 0x80001ac. Our code does not create any variables, so there is no data section.

Flash firmware

We're ready to flash this firmware! First, extract sections from the firmware.elf into a single contiguous binary blob:

$ arm-none-eabi-objcopy -O binary firmware.elf firmware.bin

And use st-link utility to flash the firmware.bin. Plug your board to the USB, and execute:

$ st-flash --reset write firmware.bin 0x8000000

Done! We've flashed a firmware that does nothing.

Makefile: build automation

Instead of typing those compilation, linking and flashing commands, we can use make command line tool to automate the whole process. make utility uses a configuration file named Makefile where it reads instructions how to execute actions. This automation is great because it also documents the process of building firmware, used compilation flags, etc.

The Makefile format is simple:

action1:
	command ...     # Comments can go after hash symbol
	command ....    # IMPORTANT: command must be preceded with the TAB character

action2:
	command ...     # Don't forget about TAB. Spaces won't work!

Now, we can invoke make with the action name (also called target) to execute a corresponding action:

$ make action1

It is possible to define variables and use them in commands. Also, actions can be file names that needs to be created:

firmware.elf:
	COMPILATION COMMAND .....

And, any action can have a list of dependencies. For example, firmware.elf depends on our source file main.c. Whenever main.c file changes, the make build command rebuilds firmware.elf:

build: firmware.elf

firmware.elf: main.c
	COMPILATION COMMAND

Now we are ready to write a Makefile for our firmware. We define a build action / target:

CFLAGS  ?=  -W -Wall -Wextra -Werror -Wundef -Wshadow -Wdouble-promotion \
            -Wformat-truncation -fno-common -Wconversion \
            -g3 -Os -ffunction-sections -fdata-sections -I. \
            -mcpu=cortex-m4 -mthumb -mfloat-abi=hard -mfpu=fpv4-sp-d16 $(EXTRA_CFLAGS)
LDFLAGS ?= -Tlink.ld -nostartfiles -nostdlib --specs nano.specs -lc -lgcc -Wl,--gc-sections -Wl,-Map=$@.map
SOURCES = main.c 

build: firmware.elf

firmware.elf: $(SOURCES)
	arm-none-eabi-gcc $(SOURCES) $(CFLAGS) $(LDFLAGS) -o $@

There, we define compilation flags. The ?= means that's a default value; we could override them from the command line like this:

$ make build CFLAGS="-O2 ...."

We specify CFLAGS, LDFLAGS and SOURCES variables. Then we tell make: if you're told to build, then create a firmware.elf file. It depends on the main.c file, and to create it, start arm-none-eabi-gcc compiler with a given flags. $@ special variable expands to a target name - in our case, firmware.elf.

Let's call make:

$ make build
arm-none-eabi-gcc main.c  -W -Wall -Wextra -Werror -Wundef -Wshadow -Wdouble-promotion -Wformat-truncation -fno-common -Wconversion -g3 -Os -ffunction-sections -fdata-sections -I. -mcpu=cortex-m4 -mthumb -mfloat-abi=hard -mfpu=fpv4-sp-d16  -Tlink.ld -nostartfiles -nostdlib --specs nano.specs -lc -lgcc -Wl,--gc-sections -Wl,-Map=firmware.elf.map -o firmware.elf

If we run it again:

$ make build
make: Nothing to be done for `build'.

The make utility examines modification times for main.c dependency and firmware.elf - and does not do anything if firmware.elf is up to date. But if we change main.c, then next make build will recompile:

$ touch main.c # Simulate changes in main.c
$ make build

Now, what is left - is the flash target:

firmware.bin: firmware.elf
	$(DOCKER) $(CROSS)-objcopy -O binary $< $@

flash: firmware.bin
	st-flash --reset write $(TARGET).bin 0x8000000

That's it! Now, make flash terminal command creates a firmware.bin file, and flashes it to the board. It'll recompile the firmware if main.c changes, because firmware.bin depends on firmware.elf, and it in turn depends on main.c. So, now the development cycle would be these two actions in a loop:

# Develop code in main.c
$ make flash

It is a good idea to add a clean target to remove build artifacts:

clean:
	rm -rf firmware.*

A complete project source code you can find in step-0-minimal folder.

Blinky LED

Now as we have the whole build / flash infrastructure set up, it is time to teach our firmware to do something useful. Something useful is of course blinking an LED. A Nucleo-F429ZI board has three built-in LEDs. In a Nucleo board datasheet section 6.5 we can see which pins built-in LEDs are attached to:

  • PB0: green LED
  • PB7: blue LED
  • PB14: red LED

Let's modify main.c file and add our definitions for PIN, gpio_set_mode(). In the main() function, we set the blue LED to output mode, and start an infinite loop. First, let's copy the definitions for pins and GPIO we have discussed earlier. Note we also add a convenience macro BIT(position):

#include <inttypes.h>
#include <stdbool.h>

#define BIT(x) (1UL << (x))
#define PIN(bank, num) ((((bank) - 'A') << 8) | (num))
#define PINNO(pin) (pin & 255)
#define PINBANK(pin) (pin >> 8)

struct gpio {
  volatile uint32_t MODER, OTYPER, OSPEEDR, PUPDR, IDR, ODR, BSRR, LCKR, AFR[2];
};
#define GPIO(bank) ((struct gpio *) (0x40020000 + 0x400 * (bank)))

// Enum values are per datasheet: 0, 1, 2, 3
enum { GPIO_MODE_INPUT, GPIO_MODE_OUTPUT, GPIO_MODE_AF, GPIO_MODE_ANALOG };

static inline void gpio_set_mode(uint16_t pin, uint8_t mode) {
  struct gpio *gpio = GPIO(PINBANK(pin));  // GPIO bank
  int n = PINNO(pin);                      // Pin number
  gpio->MODER &= ~(3U << (n * 2));         // Clear existing setting
  gpio->MODER |= (mode & 3) << (n * 2);    // Set new mode
}

Some microcontrollers, when they are powered, have all their peripherals powered and enabled, automatically. STM32 MCUs, however, by default have their peripherals disabled in order to save power. In order to enable a GPIO peripheral, it should be enabled (clocked) via the RCC (Reset and Clock Control) unit. In the datasheet section 7.3.10 we find that the AHB1ENR (AHB1 peripheral clock enable register) is responsible to turn GPIO banks on or off. First we add a definition for the whole RCC unit:

struct rcc {
  volatile uint32_t CR, PLLCFGR, CFGR, CIR, AHB1RSTR, AHB2RSTR, AHB3RSTR,
      RESERVED0, APB1RSTR, APB2RSTR, RESERVED1[2], AHB1ENR, AHB2ENR, AHB3ENR,
      RESERVED2, APB1ENR, APB2ENR, RESERVED3[2], AHB1LPENR, AHB2LPENR,
      AHB3LPENR, RESERVED4, APB1LPENR, APB2LPENR, RESERVED5[2], BDCR, CSR,
      RESERVED6[2], SSCGR, PLLI2SCFGR;
};
#define RCC ((struct rcc *) 0x40023800)

In the AHB1ENR register documentation we see that bits from 0 to 8 inclusive set the clock for GPIO banks GPIOA - GPIOE:

int main(void) {
  uint16_t led = PIN('B', 7);            // Blue LED
  RCC->AHB1ENR |= BIT(PINBANK(led));     // Enable GPIO clock for LED
  gpio_set_mode(led, GPIO_MODE_OUTPUT);  // Set blue LED to output mode
  for (;;) asm volatile("nop");          // Infinite loop
  return 0;
}

Now, what is left to do, is to find out how to set a GPIO pin on and off, and then modify the main loop to set an LED pin on, delay, off, delay. Looking at the datasheet section 8.4.7, wee see that the register BSRR is responsible for setting voltage high or low. The low 16 bit are used to set the ODR register (i.e. set pin high), and high 16 bit are used to reset the ODR register (i.e. set pin low). Let's define an API function for that:

static inline void gpio_write(uint16_t pin, bool val) {
  struct gpio *gpio = GPIO(PINBANK(pin));
  gpio->BSRR |= (1U << PINNO(pin)) << (val ? 0 : 16);
}

Next we need to implement a delay function. We do not require an accurate delay at this moment, so let's define a function spin() that just executes a NOP instruction a given number of times:

static inline void spin(volatile uint32_t count) {
  while (count--) asm("nop");
}

Finally, we're ready to modify our main loop to implement LED blinking:

  for (;;) {
    gpio_write(pin, true);
    spin(999999);
    gpio_write(pin, false);
    spin(999999);
  }

Run make flash and enjoy blue LED flashing. A complete project source code you can find in step-1-blinky.

Blinky with SysTick interrupt

In order to implement an accurate time keeping, we should enable ARM's SysTick interrupt. SysTick a 24-bit hardware counter, and is part of ARM core, therefore it is documented by the ARM datasheet. Looking at the datasheet, we see that SysTick has four registers:

  • CTRL - used to enable/disable systick
  • LOAD - an initial counter value
  • VAL - a current counter value, decremented on each clock cycle
  • CALIB - calibration register

Every time VAL drops to zero, a SysTick interrupt is generated. The SysTick interrupt index in the vector table is 15, so we need to set it. Upon boot, our board Nucleo-F429ZI runs at 16Mhz. We can configure the SysTick counter to trigger interrupt each millisecond.

First, let's define a SysTick peripheral. We know 4 registers, and from the datasheet we can learn that the SysTick address is 0xe000e010. So:

struct systick {
  volatile uint32_t CTRL, LOAD, VAL, CALIB;
};
#define SYSTICK ((struct systick *) 0xe000e010)

Next, add an API function that configures it. We need to enable SysTick in the SYSTICK->CTRL register, and also we must clock it via the RCC->APB2ENR, described in the section 7.4.14:

#define BIT(x) (1UL << (x))
static inline void systick_init(uint32_t ticks) {
  if ((ticks - 1) > 0xffffff) return;  // Systick timer is 24 bit
  SYSTICK->LOAD = ticks - 1;
  SYSTICK->VAL = 0;
  SYSTICK->CTRL = BIT(0) | BIT(1) | BIT(2);  // Enable systick
  RCC->APB2ENR |= BIT(14);                   // Enable SYSCFG
}

By default, Nucleo-F429ZI board runs at 16Mhz. That means, if we call systick_init(16000000 / 1000);, then SysTick interrupt will be generated every millisecond. We should have interrupt handler function defined - here it is, we simply increment a 32-bit millisecond counter:

static volatile uint32_t s_ticks;
void SysTick_Handler(void) {
  s_ticks++;
}

And we should add this handler to the vector table:

__attribute__((section(".vectors"))) void (*tab[16 + 91])(void) = {
    0, _reset, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, SysTick_Handler};

Now we have a precise millisecond clock! Let's create a helper function for arbitrary periodic timers:

// t: expiration time, prd: period, now: current time. Return true if expired
bool timer_expired(uint32_t *t, uint64_t prd, uint64_t now) {
  if (now + prd < *t) *t = 0;                    // Time wrapped? Reset timer
  if (*t == 0) *t = now + prd;                   // First poll? Set expiration
  if (*t > now) return false;                    // Not expired yet, return
  *t = (now - *t) > prd ? now + prd : *t + prd;  // Next expiration time
  return true;                                   // Expired, return true
}

Now we are ready to update our main loop and use a precise timer for LED blink. For example, let's use 250 milliseconds blinking interval:

  uint32_t timer, period = 250;          // Declare timer and 250ms period
  for (;;) {
    if (timer_expired(&timer, period, s_ticks)) {
      static bool on;       // This block is executed
      gpio_write(led, on);  // Every `period` milliseconds
      on = !on;             // Toggle LED state
    }
    // Here we could perform other activities!
  }

Note that using SysTick, and a helper timer_expired() function, we made our main loop (also called superloop) non-blocking. That means that inside that loop we can perform many actions - for example, have different timers with different periods, and they all will be triggered in time.

A complete project source code you can find in step-2-systick folder.

Add UART debug output

Now it's time to add a human-readable diagnostics to our firmware. One of the MCU peripherals is a serial UART interface. Looking at the datasheet section 2.3, we see that there are several UART/USART controllers - i.e. pieces of circuitry inside MCU that, properly configured, can exchange data via certain pins. A mimimal UART setup uses two pins, RX (receive) and TX (transmit).

In a Nucleo board datasheet section 6.9 we see that one of the controllers, USART3, is using pins PD8 (TX) and PD9 (RX) and is connected to the on-board ST-LINK debugger. That means that if we configure USART3 and output data via the PD9 pin, we can see it on our workstation via the ST-LINK USB connection.

Thus, let us create a handy API for the UART, the way we did it for GPIO. Datasheet section 30.6 summarises UART registers - so here is our UART struct:

struct uart {
  volatile uint32_t SR, DR, BRR, CR1, CR2, CR3, GTPR;
};
#define UART1 ((struct uart *) 0x40011000)
#define UART2 ((struct uart *) 0x40004400)
#define UART3 ((struct uart *) 0x40004800)

To configure UART, we need to:

  • Enable UART clock by setting appropriate bit in RCC->APB2ENR register
  • Set "alternate function" pin mode for RX and TX pins. There can be several alternate functions (AF) for any given pin, depending on the peripheral that is used. The AF list can be found in the STM32F429ZI table 12
  • Set baud rate (receive/transmit clock frequency) via the BRR register
  • Enable the peripheral, receive and transmit via the CR1 register

We already know how to set a GPIO pin into a specific mode. If a pin is in the AF mode, we also need to specify the "function number", i.e. which exactly peripheral takes control. This can be done via the "alternate function register", AFR, of the GPIO peripheral. Reading the AFR register description in the datasheet, we can see that the AF number occupies 4 bits, thus the whole setup for 16 pins occupies 2 registers. If a p

static inline void gpio_set_af(uint16_t pin, uint8_t af_num) {
  struct gpio *gpio = GPIO(PINBANK(pin));  // GPIO bank
  int n = PINNO(pin);                      // Pin number
  gpio->AFR[n >> 3] &= ~(15UL << ((n & 7) * 4));
  gpio->AFR[n >> 3] |= ((uint32_t) af_num) << ((n & 7) * 4);
}

In order to completely hide register-specific code from the GPIO API, let's move the GPIO clock init to the gpio_set_mode() function:

static inline void gpio_set_mode(uint16_t pin, uint8_t mode) {
  struct gpio *gpio = GPIO(PINBANK(pin));  // GPIO bank
  int n = PINNO(pin);                      // Pin number
  RCC->AHB1ENR |= BIT(PINBANK(pin));       // Enable GPIO clock
  ...

Now we're ready to create a UART initialization API function:

#define FREQ 16000000  // CPU frequency, 16 Mhz
static inline void uart_init(struct uart *uart, unsigned long baud) {
  // https://www.st.com/resource/en/datasheet/stm32f429zi.pdf
  uint8_t af = 0;           // Alternate function
  uint16_t rx = 0, tx = 0;  // pins

  if (uart == UART1) RCC->APB2ENR |= BIT(4);
  if (uart == UART2) RCC->APB1ENR |= BIT(17);
  if (uart == UART3) RCC->APB1ENR |= BIT(18);

  if (uart == UART1) af = 4, tx = PIN('A', 9), rx = PIN('A', 10);
  if (uart == UART2) af = 4, tx = PIN('A', 2), rx = PIN('A', 3);
  if (uart == UART3) af = 7, tx = PIN('D', 8), rx = PIN('D', 9);

  gpio_set_mode(tx, GPIO_MODE_AF);
  gpio_set_af(tx, af);
  gpio_set_mode(rx, GPIO_MODE_AF);
  gpio_set_af(rx, af);
  uart->CR1 = 0;                           // Disable this UART
  uart->BRR = FREQ / baud;                 // FREQ is a CPU frequency 
  uart->CR1 |= BIT(13) | BIT(2) | BIT(3);  // Set UE, RE, TE
}

And, finally, functions for reading and writing to the UART. The datasheet section 30.6.1 tells us that the status register SR tells us whether data is ready:

static inline int uart_read_ready(struct uart *uart) {
  return uart->SR & BIT(5);  // If RXNE bit is set, data is ready
}

The data byte itself can be fetched from the data register DR:

static inline uint8_t uart_read_byte(struct uart *uart) {
  return (uint8_t) (uart->DR & 255);
}

Transmitting a single byte can be done via the data register too. After setting a byte to write, we need to wait for the transmission to end, indicated via bit 7 in the status register:

static inline void uart_write_byte(struct uart *uart, uint8_t byte) {
  uart->DR = byte;
  while ((uart->SR & BIT(7)) == 0) spin(1);
}

And writing a buffer:

static inline void uart_write_buf(struct uart *uart, char *buf, size_t len) {
  while (len-- > 0) uart_write_byte(uart, *(uint8_t *) buf++);
}

Now, initialise UART in our main() function:

  ...
  uart_init(UART3, 115200);              // Initialise UART

Now, we're ready to print a message "hi\r\n" every time LED blinks!

    if (timer_expired(&timer, period, s_ticks)) {
      ...
      uart_write_buf(UART3, "hi\r\n", 4);  // Write message
    }

Rebuild, reflash, and attach a terminal program to the ST-LINK port. On my Mac workstation, I use cu. It also can be used on Linux. On Windows, using putty utility can be a good idea. Run a terminal and see the messages:

$ cu -l /dev/cu.YOUR_SERIAL_PORT -s 115200
hi
hi

A complete project source code you can find in step-3-uart folder.

Redirect printf() to UART

In this section, we replace uart_write_buf() call by printf() call, which gives us an ability to do formatted output - and increase our abilities to print diagnostic information, implemeting so called "printf-style debugging".

A GNU ARM toolchain that we're using comes not only with a GCC compiler and other tools, but with a C library called newlib, https://sourceware.org/newlib. A newlib library was developed by RedHat for embedded systems.

If our firmware calls a standard C library function, for example strcmp(), then a newlib code will be added to our firmware by the GCC linker.

Some of the standard C functions that newlib implements, specifically, file input/output (IO) operations, implemented by the newlib is a special fashion: those functions eventually call a set of low-level IO functions called "sycalls".

For example:

  • fopen() eventually calls _open()
  • fread() eventually calls a low level _read()
  • fwrite(), fprintf(), printf() eventually call a low level _write()
  • malloc() eventually calls _sbrk(), and so on.

Thus, by modifying a _write() syscall, we can redirect printf() to whatever we want. That mechanism is called "IO retargeting".

Note: STM32 Cube also uses ARM GCC with newlib, that's why Cube projects typically include syscalls.c file. Other toolchains, like TI's CCS, Keil's CC, might use a different C library with a bit different retargeting mechanism. We use newlib, so let's modify _write() syscall to print to the UART3.

Before that, let's organise our source code in the following way:

  • move all API definitions to the file mcu.h
  • move startup code to startup.c
  • create an empty file syscalls.c for newlib "syscalls"
  • modify Makefile to addd syscalls.c and startup.c to the build

After moving all API definitions to the mcu.h, our main.c file becomes quite compact. Note that it does not have any mention of the low-level registers, just a high level API functions that are easy to understand:

#include "mcu.h"

static volatile uint32_t s_ticks;
void SysTick_Handler(void) {
  s_ticks++;
}

int main(void) {
  uint16_t led = PIN('B', 7);            // Blue LED
  systick_init(16000000 / 1000);         // Tick every 1 ms
  gpio_set_mode(led, GPIO_MODE_OUTPUT);  // Set blue LED to output mode
  uart_init(UART3, 115200);              // Initialise UART
  uint32_t timer = 0, period = 250;      // Declare timer and 250ms period
  for (;;) {
    if (timer_expired(&timer, period, s_ticks)) {
      static bool on;                      // This block is executed
      gpio_write(led, on);                 // Every `period` milliseconds
      on = !on;                            // Toggle LED state
      uart_write_buf(UART3, "hi\r\n", 4);  // Write message
    }
    // Here we could perform other activities!
  }
  return 0;
}

Great, now let's retarget printf to the UART3. In the empty syscalls.c, copy/paste the following code:

#include "mcu.h"

int _write(int fd, char *ptr, int len) {
  (void) fd, (void) ptr, (void) len;
  if (fd == 1) uart_write_buf(UART3, ptr, (size_t) len);
  return -1;
}

Here we say: if the file descriptor we're writing to is 1 (which is a standard output descriptor), then write the buffer to the UART3. Otherwise, ignore. This is the essence of retargeting!

Rebuilding this firmware results in a bunch of linker errors:

../../arm-none-eabi/lib/thumb/v7e-m+fp/hard/libc_nano.a(lib_a-sbrkr.o): in function `_sbrk_r':
sbrkr.c:(.text._sbrk_r+0xc): undefined reference to `_sbrk'
closer.c:(.text._close_r+0xc): undefined reference to `_close'
lseekr.c:(.text._lseek_r+0x10): undefined reference to `_lseek'
readr.c:(.text._read_r+0x10): undefined reference to `_read'
fstatr.c:(.text._fstat_r+0xe): undefined reference to `_fstat'
isattyr.c:(.text._isatty_r+0xc): undefined reference to `_isatty'

Since we've used a newlib stdio function, we need to supply newlib with the rest of syscalls. Let's add just a simple stubs that do nothing:

int _fstat(int fd, struct stat *st) {
  (void) fd, (void) st;
  return -1;
}

void *_sbrk(int incr) {
  (void) incr;
  return NULL;
}

int _close(int fd) {
  (void) fd;
  return -1;
}

int _isatty(int fd) {
  (void) fd;
  return 1;
}

int _read(int fd, char *ptr, int len) {
  (void) fd, (void) ptr, (void) len;
  return -1;
}

int _lseek(int fd, int ptr, int dir) {
  (void) fd, (void) ptr, (void) dir;
  return 0;
}

Now, rebuild gives no errors. Last step: replace the uart_write_buf() call in the main() function with printf() call that prints something useful, e.g. a LED status and a current value of systick:

printf("LED: %d, tick: %lu\r\n", on, s_ticks);  // Write message

The serial output looks like this:

LED: 1, tick: 250
LED: 0, tick: 500
LED: 1, tick: 750
LED: 0, tick: 1000

Congratulations! We learned how IO retargeting works, and can now printf-debug our firmware. A complete project source code you can find in step-4-printf folder.

Debug with Segger Ozone

What if our firmware is stuck somewhere and printf debug does not work? What if even a startup code does not work? We would need a debugger. There are many options, but I'd recommend using an Ozone debugger from Segger. Why? Because it is stand-alone. It does not need any IDE set up. We can feed our firmware.elf directly to Ozone, and it'll pick up our source files.

So, download Ozone from the Segger website. Before we can use it with our Nucleo board, we need to convert ST-LINK firmware on the onboard debugger to the jlink firmware that Ozone understands. Follow the instructions on the Segger site.

Now, run Ozone. Choose our device in the wizard:

Select a debugger we're going to use - that should be a ST-LINK:

Choose our firmware.elf file:

Leave the defaults on the next screen, click Finish, and we've got our debugger loaded (note the mcu.h source code is picked up):

Click the green button to download, run the firmware, and we're stopped here:

Now we can single-step through code, set breakpoints, and do the usual debugging stuff. One thing that could be noted, is a handy Ozone peripheral view:

Using it, we can directly examine or set the state of the peripherals. For example, let's turn on a green on-board LED (PB0):

  1. We need to clock GPIOB first. Find Peripherals -> RCC -> AHB1ENR, and enable GPIOBEN bit - set it to 1:

2. Find Peripherals -> GPIO -> GPIOB -> MODER, set MODER0 to 1 (output):

3. Find Peripherals -> GPIO -> GPIOB -> ODR, set ODR0 to 1 (on):

Now, a green LED should be on! Happy debugging.

Vendor CMSIS headers

In the previous sections, we have developed the firmware using only datasheets, editor, and GCC compiler. We have created peripheral structure definitions manually, using datasheets.

Now as you know how it all works, it is time to introduce CMSIS headers. What is it ? These are header files with all definitions, created and supplied by the MCU vendor. They contain definitions for everything that MCU contains, therefore they rather big.

CMSIS stands for Common Microcontroller Software Interface Standard, thus it is a common ground for the MCU manufacturers to specify peripheral API. Since CMSIS is an ARM standard, and since CMSIS headers are supplied by the MCU vendor, they are the source of authority. Therefore, using vendor headers is is a preferred way, rather than writing definitions manually.

In this section, we will replace our API functions in the mcu.h by the CMSIS vendor header, and leave the rest of the firmware intact.

STM32 CMSIS headers for F4 family can be found at https://github.com/STMicroelectronics/cmsis_device_f4 repo. From there, copy the following files into our firmware directory, step-5-cmsis:

Those two files depend on a standard ARM CMSIS includes, download them too:

From the mcu.h, remove all peripheral API and definitions, and leave only standard C inludes, vendor CMSIS include, defines to PIN, BIT, FREQ, and timer_expired() helper function:

#pragma once

#include <inttypes.h>
#include <stdbool.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/stat.h>

#include "stm32f429xx.h"

#define FREQ 16000000  // CPU frequency, 16 Mhz
#define BIT(x) (1UL << (x))
#define PIN(bank, num) ((((bank) - 'A') << 8) | (num))
#define PINNO(pin) (pin & 255)
#define PINBANK(pin) (pin >> 8)

static inline void spin(volatile uint32_t count) {
  while (count--) asm("nop");
}

static inline bool timer_expired(uint32_t *t, uint32_t prd, uint32_t now) {
  ...
}

If we try to rebuild the firmware - make clean build, then GCC will fail complaining about missing systick_init(), GPIO_MODE_OUTPUT, uart_init(), and UART3. Let's add those using STM32 CMSIS files.

Let's start from systick_init(). The core_cm4.h header defines SysTick_Type structure which is identical to our struct systick, and has an appropriate #define for SysTick peripheral. Also, stm32f429xx.h has a RCC_TypeDef structure and appropriate #define for the RCC. Therefore our systick_init() function remains almost unchanged: we only have to replace SYSTICK with SysTick:

static inline void systick_init(uint32_t ticks) {
  if ((ticks - 1) > 0xffffff) return;  // Systick timer is 24 bit
  SysTick->LOAD = ticks - 1;
  SysTick->VAL = 0;
  SysTick->CTRL = BIT(0) | BIT(1) | BIT(2);  // Enable systick
  RCC->APB2ENR |= BIT(14);                   // Enable SYSCFG
}

Next goes gpio_set_mode() function. The stm32f429xx.h header has GPIO_TypeDef structure, identical to our struct gpio. Let's use it:

#define GPIO(bank) ((GPIO_TypeDef *) (GPIOA_BASE + 0x400 * (bank)))
enum { GPIO_MODE_INPUT, GPIO_MODE_OUTPUT, GPIO_MODE_AF, GPIO_MODE_ANALOG };

static inline void gpio_set_mode(uint16_t pin, uint8_t mode) {
  GPIO_TypeDef *gpio = GPIO(PINBANK(pin));  // GPIO bank
  int n = PINNO(pin);                      // Pin number
  RCC->AHB1ENR |= BIT(PINBANK(pin));       // Enable GPIO clock
  gpio->MODER &= ~(3U << (n * 2));         // Clear existing setting
  gpio->MODER |= (mode & 3) << (n * 2);    // Set new mode
}

The gpio_set_af() and gpio_write() functions is also trivial - simply replace struct gpio with GPIO_TypeDef, and that's all.

Next goes UART. There is a USART_TypeDef, and defines for USART1, USART2, USART3. Let's use them:

#define UART1 USART1
#define UART2 USART2
#define UART3 USART3

In the uart_init() and the rest of UART functions, change struct uart to USART_TypeDef. The rest stays the same!

And we are done. Rebuild, reflash the firmware. The LED blinks, the UART shows the output. Congratulations, we have adopted our firmware code to use vendor CMSIS header files. Now let's reorganise the repository a bit by moving all standard files into include directory and updating Makefile to let GCC know about it:

...
  -g3 -Os -ffunction-sections -fdata-sections -I. -Iinclude \

We have left with a project template that can be reused for the future projects. A complete project source code you can find in step-5-cmsis

Setting up clocks

After boot, Nucleo-F429ZI CPU runs at 16MHz. The maximum frequency is 180MHz. Note that system clock frequency is not the only factor we need to care about. Peripherals are attached to different buses, APB1 and APB2 which are clocked differently. Their clock speeds are configured by the frequency prescaler values, set in the RCC. The main CPU clock source can also be different - we can use either an external crystal oscillator (HSE) or an internal oscillator (HSI). In our case, we'll use HSI.

When CPU executes instructions from flash, a flash read speed (which is around 25MHz) becomes a bottleneck if CPU clock gets higher. There are several tricks that can help. Instruction prefetch is one. Also, we can give a clue to the flash controller, how faster the system clock is: that value is called flash latency. For 180MHz system clock, the FLASH_LATENCY value is 5. Bits 8 and 9 in the flash controller enable instruction and data caches:

  FLASH->ACR |= FLASH_LATENCY | BIT(8) | BIT(9);      // Flash latency, caches

The clock source (HSI or HSE) goes through a piece of hardware called PLL, which multiplies source frequency by a certain value. Then, a set of frequency dividers are used to set the system clock and APB1, APB2 clocks. In order to obtain the maximum system clock of 180MHz, multiple values of PLL dividers and APB prescalers are possible. Section 6.3.3 of the datasheet tells us the maximum values for APB1 clock: <= 45MHz, and the APB2 clock: <= 90MHz. That narrows down the list of possible combinations. Here we chose the values manually. Note that tools like CubeMX can automate the process and make it easy and visual.

enum { APB1_PRE = 5 /* AHB clock / 4 */, APB2_PRE = 4 /* AHB clock / 2 */ };
enum { PLL_HSI = 16, PLL_M = 8, PLL_N = 180, PLL_P = 2 };  // Run at 180 Mhz
#define PLL_FREQ (PLL_HSI * PLL_N / PLL_M / PLL_P)
#define FREQ (PLL_FREQ * 1000000)

Now we're ready for a simple algorithm to set up the clock for CPU and peripheral buses may look like this:

  • Optionally, enable FPU
  • Set flash latency
  • Decide on a clock source, and PLL, APB1 and APB2 prescalers
  • Configure RCC by setting respective values:
static inline void clock_init(void) {                 // Set clock frequency
  SCB->CPACR |= ((3UL << 10 * 2) | (3UL << 11 * 2));  // Enable FPU
  FLASH->ACR |= FLASH_LATENCY | BIT(8) | BIT(9);      // Flash latency, caches
  RCC->PLLCFGR &= ~((BIT(17) - 1));                   // Clear PLL multipliers
  RCC->PLLCFGR |= (((PLL_P - 2) / 2) & 3) << 16;      // Set PLL_P
  RCC->PLLCFGR |= PLL_M | (PLL_N << 6);               // Set PLL_M and PLL_N
  RCC->CR |= BIT(24);                                 // Enable PLL
  while ((RCC->CR & BIT(25)) == 0) spin(1);           // Wait until done
  RCC->CFGR = (APB1_PRE << 10) | (APB2_PRE << 13);    // Set prescalers
  RCC->CFGR |= 2;                                     // Set clock source to PLL
  while ((RCC->CFGR & 12) == 0) spin(1);              // Wait until done
}

What is left, is to call clock_init() from main, then rebuild and reflash. And our board runs at its maximum speed, 180MHz! A complete project source code you can find in step-6-clock

Web server with device dashboard

The Nucleo-F429ZI comes with Ethernet on-board. Ethernet hardware needs two components: a PHY (which transmits/receives electrical signals to the media like copper, optical cable, etc) and MAC (which drives PHY controller). On our Nucleo, the MAC controller is built-in, and the PHY is external (specifically, is is Microchip's LAN8720a).

MAC and PHY can talk several interfaces, we'll use RMII. For that, a bunch of pins must be configured to use their Alternative Function (AF). To implement a web server, we need 3 software components:

  • a network driver, which sends/receives Ethernet frames to/from MAC controller
  • a network stack, that parses frames and understands TCP/IP
  • a network library that understands HTTP

We will use Mongoose Network Library which implements all of that in a single file. It is a dual-licensed library (GPLv2/commercial) that was designed to make network embedded development fast and easy.

So, copy mongoose.c and mongoose.h to our project. Now we have a driver, a network stack, and a library at hand. Mongoose also provides a large set of examples, and one of them is a device dashboard example. It implements lots of things - like dashboard login, real-time data exchange over WebSocket, embedded file system, MQTT communication, etcetera. So let's use that example. Copy two extra files:

  • net.c - implements dashboard functionality
  • packed_fs.c - contains HTML/CSS/JS GUI files

What we need is to tell Mongoose which functionality to enable. That can be done via compilation flags, by setting preprocessor constants. Alternatively, the same constants can be set in the mongoose_custom.h file. Let's go the second way. Create mongoose_custom.h file with the following contents:

#pragma once
#define MG_ARCH MG_ARCH_NEWLIB
#define MG_ENABLE_MIP 1
#define MG_ENABLE_PACKED_FS 1
#define MG_IO_SIZE 512
#define MG_ENABLE_CUSTOM_MILLIS 1

Now it's time to add some networking code to main.c. We #include "mongoose.c", initialise Ethernet RMII pins and enable Ethernet in the RCC:

  uint16_t pins[] = {PIN('A', 1),  PIN('A', 2),  PIN('A', 7),
                     PIN('B', 13), PIN('C', 1),  PIN('C', 4),
                     PIN('C', 5),  PIN('G', 11), PIN('G', 13)};
  for (size_t i = 0; i < sizeof(pins) / sizeof(pins[0]); i++) {
    gpio_init(pins[i], GPIO_MODE_AF, GPIO_OTYPE_PUSH_PULL, GPIO_SPEED_INSANE,
              GPIO_PULL_NONE, 11);
  }
  nvic_enable_irq(61);                          // Setup Ethernet IRQ handler
  RCC->APB2ENR |= BIT(14);                      // Enable SYSCFG
  SYSCFG->PMC |= BIT(23);                       // Use RMII. Goes first!
  RCC->AHB1ENR |= BIT(25) | BIT(26) | BIT(27);  // Enable Ethernet clocks
  RCC->AHB1RSTR |= BIT(25);                     // ETHMAC force reset
  RCC->AHB1RSTR &= ~BIT(25);                    // ETHMAC release reset

Mongoose's driver uses Ethernet interrupt, thus we need to update startup.c and add ETH_IRQHandler to the vector table. Let's reorganise vector table definition in startup.c in a way that does not require any modification to add an interrupt handler function. The idea is to use a "weak symbol" concept:

void __attribute__((weak)) DefaultIRQHandler(void) {
  for (;;) (void) 0;
}
#define WEAK_ALIAS __attribute__((weak, alias("DefaultIRQHandler")))

WEAK_ALIAS void NMI_Handler(void);
WEAK_ALIAS void HardFault_Handler(void);
WEAK_ALIAS void MemManage_Handler(void);
...
__attribute__((section(".vectors"))) void (*tab[16 + 91])(void) = {
    0, _reset, NMI_Handler, HardFault_Handler, MemManage_Handler,
    ...

Notice that the vector table now has entries for every possible IRQ handler, like NMI_Handler(), HardFault_Handler(), etcetera; but all of them are "aliased" to the function DefaultIRQHandler() which is marked weak. This mechanism allows to provide a default handler function for all interrupts. If, however, user code defines a function with the same name as it appears in the vector table, then the linker will choose user function, not the default weak one. In our case, there is a ETH_IRQHandler() weak handler defined in the vector table, and Mongoose's STM32 driver code defines its own ETH_IRQHandler() which replaces it.

The next step is to initialise Mongoose library: create an event manager, setup network driver, and start a listening HTTP connection:

  struct mg_mgr mgr;        // Initialise Mongoose event manager
  mg_mgr_init(&mgr);        // and attach it to the MIP interface
  mg_log_set(MG_LL_DEBUG);  // Set log level

  struct mip_driver_stm32 driver_data = {.mdc_cr = 4};  // See driver_stm32.h
  struct mip_if mif = {
      .mac = {2, 0, 1, 2, 3, 5},
      .use_dhcp = true,
      .driver = &mip_driver_stm32,
      .driver_data = &driver_data,
  };
  mip_init(&mgr, &mif);
  extern void device_dashboard_fn(struct mg_connection *, int, void *, void *);
  mg_http_listen(&mgr, "http://0.0.0.0", device_dashboard_fn, &mgr);
  MG_INFO(("Init done, starting main loop"));

What is left, is to add a mg_mgr_poll() call into the main loop.

Now, add mongoose.c, net.c and packed_fs.c files to the Makefile. Rebuild, reflash the board. Attach a serial console to the debug output, observe that the board obtains an IP address over DHCP:

847 3 mongoose.c:6784:arp_cache_add     ARP cache: added 0xc0a80001 @ 90:5c:44:55:19:8b
84e 2 mongoose.c:6817:onstatechange     READY, IP: 192.168.0.24
854 2 mongoose.c:6818:onstatechange            GW: 192.168.0.1
859 2 mongoose.c:6819:onstatechange            Lease: 86363 sec
LED: 1, tick: 2262
LED: 0, tick: 2512

Fire up a browser at that IP address, and get a working dashboard, with real-time graph over WebSocket, with MQTT, authentication, and other things! See full description for more details.

Device dashboard

A complete project source code you can find in step-7-webserver directory.

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