An bare metal, make-based SDK for the ESP32, ESP32C3 chips. It is written from scratch using datasheets ( ESP32 C3 TRM, ESP32 TRM ). It is completely independent from the ESP-IDF and does not use any ESP-IDF tools or files. The only tool required is a GCC crosscompiler.

A screenshot below demonstrates a examples/c3ws2812 RGB LED firmware flashed on a ESP32-C3-DevKitM-1 board. It takes < 2 seconds for a full firmware rebuild and flash:

Install a GCC RISCV compiler and export environment variables:

• Using Docker on Linux or Mac. Slower builds, but works off-the-shelf:

  $ export MDK=/path/to/mdk                 # Points to MDK directory
  $ export PATH=$PATH:$MDK/tools            # Add $MDK/tools to $PATH
  $ export PORT=/dev/cu.usb*                # Serial port for flashing

• Native MacOS (installation takes time):

  $ brew tap riscv/riscv
  $ brew install riscv-gnu-toolchain --with-multilib
  $ export MDK=/path/to/mdk                 # Points to MDK directory
  $ export PATH=$PATH:$MDK/tools            # Add $MDK/tools to $PATH
  $ export TOOLCHAIN=riscv64-unknown-elf    # $TOOLCHAIN-gcc must run GCC
  $ export ARCH=ESP32C3                     # Choices: ESP32C3, ESP32
  $ export PORT=/dev/cu.usb*                # Serial port for flashing

• Native Linux: install GCC, e.g. from https://github.com/espressif/crosstool-NG, then

  $ export MDK=/path/to/mdk                 # Points to MDK directory
  $ export PATH=$PATH:$MDK/tools            # Add $MDK/tools to $PATH
  $ export TOOLCHAIN=riscv32-esp-elf        # $TOOLCHAIN-gcc must run GCC
  $ export ARCH=ESP32C3                     # Choices: ESP32C3, ESP32
  $ export PORT=/dev/ttyUSB0                # Serial port for flashing

Verify setup by building and flashing a blinky example firmware. From repository root, execute:

$ make -C examples/blinky clean build flash monitor

Firmware Makefile should look like this:

SOURCES = main.c another_file.c

EXTRA_CFLAGS ?=
EXTRA_LINKFLAGS ?=

include $(MDK)/make/build.mk

Environment / Makefile variables:

Makefile targets:

Preprocessor definitions

API support matrix:

• GPIO src/gpio.h

  void gpio_output(int pin);              // Set pin mode to OUTPUT
  void gpio_input(int pin);               // Set pin mode to INPUT
  void gpio_write(int pin, bool value);   // Set pin to low (false) or high
  void gpio_toggle(int pin);              // Toggle pin value
  bool gpio_read(int pin);                // Read pin value

• SPI src/spi.h, src/spi.c

  // SPI descriptor. Specifies pins for MISO, MOSI, CLK and chip select
  struct spi { int miso, mosi, clk, cs[3]; };
  
  bool spi_init(struct spi *spi);           // Init SPI
  void spi_begin(struct spi *spi, int cs);  // Start SPI transaction
  void spi_end(struct spi *spi, int cs);    // End SPI transaction
  unsigned char spi_txn(struct spi *spi, unsigned char);   // Do SPI transaction

• UART src/uart.h, src/uart.c

  void uart_init(int no, int tx, int rx, int baud);   // Initialise UART
  bool uart_read(int no, uint8_t *c);   // Read byte. Return true on success
  void uart_write(int no, uint8_t c);   // Write byte. Block if FIFO is full

• LEDC
• WDT src/wdt.h

  void wdt_disable(void);   // Disable watchdog

• Timer src/timer.h

  struct timer {
    uint64_t period;       // Timer period in micros
    uint64_t expire;       // Expiration timestamp in micros
    void (*fn)(void *);    // Function to call
    void *arg;             // Function argument
    struct timer *next;    // Linkage
  };
  
  #define TIMER_ADD(head_, p_, fn_, arg_)
  void timers_poll(struct timer *head, uint64_t now);

• System src/sys.h

  int sdk_ram_used(void);           // Return used RAM in bytes
  int sdk_ram_free(void);           // Return free RAM in bytes
  unsigned long time_us(void);      // Return uptime in microseconds
  void delay_us(unsigned long us);  // Block for "us" microseconds
  void delay_ms(unsigned long ms);  // Block for "ms" milliseconds
  void spin(unsigned long count);   // Execute "count" no-op instructions

• Log src/log.h, src/log.c

  void sdk_log(const char *fmt, ...);   // Log message to UART 0
                                        // Supported specifiers:
                                        // %d, %x, %s, %p
  void sdk_hexdump(const void *buf, size_t len);  // Hexdump buffer

• TCP/IP

Firmware examples could be built on Mac/Linux as normal UNIX binaries. In the firmware directory, type

make unix

That builds a build/firmware executable. To support that, all hardware API are mocked out. The typical API implementation looks like:

#if defined(ESP32C3)
...
#elif defined(ESP32)
...
#elif defined(__unix) || defined(__unix__) || defined(__APPLE__)
...  <-- Here goes a mocked-out hardware API implementation
#endif

Flashing ESP32 chips is done via UART. In order to do so, ESP32 should be rebooted in the flashing mode, by pulling IO0 low during boot. Then, a ROM bootloader uses SLIP framing for a simple serial protocol, which is described at https://github.com/espressif/esptool/wiki/Serial-Protocol.

Using that SLIP protocol, it is possible to write images to flash at any offset. That is what tools/esputil.c implements. The image should be of the following format:

• COMMON HEADER - 4 bytes, contains number of segments in the image and flash params
• ENTRY POINT ADDRESS - 4 bytes, the beginning of the image code
• EXTENDED HEADER - 16 bytes, contains chip ID and extra flash params
• One or more SEGMENTS, which are padded to 16 bytes

 | COMMON HEADER |  ENTRY  |           EXTENDED HEADER          | SEGM1 | ... | 
 | 0xe9 N F1 F2  | X X X X | 0xee 0 0 0 C 0 V 0 0 0 0 0 0 0 0 1 |       | ... | 

   0xe9 - Espressif image magic number. All images must start with 0xe9
   N    - a number of segments in the image
   F1  - flash mode. 0: QIO, 1: QOUT, 2: DIO, 3: DOUT
   F2  - flash size (high 4 bits) and flash frequency (low 4 bits):
            size: 0: 1MB, 0x10: 2MB, 0x20: 4MB, 0x30: 8MB, 0x40: 16MB
            freq: 0: 40m, 1: 26m, 2: 20m, 0xf: 80m
   ENTRY - 4-byte entry point address in little endian
   C     - Chip ID. 0: ESP32, 5: ESP32C3
   V     - Chip revision

Image header format includes two bytes, F1 and F2, which desribe SPI flash parameters that ROM bootloader uses to load the rest of the firmware. Those two bytes encode three parameters:

• Flash mode (F1 byte - can be 0, 1, 2, 3)
• FLash size (hight 4 bits of F2 byte - can be 0, 1, 2, 3, 4)
• Flash frequency (low 4 bits of F2 byte - can be 0, 1, 2, f)

By default, esputil fetches flash params F1 and F2 from the existing bootloader by reading first 4 bytes of the bootloader from flash. It is possible to manually set flash params via the -fp flag, which is an integer value that represent 3 hex nimbles. For example fp 0x22f sets flash to DIO, 4MB, 80MHz:

$ esputil -fp 0x22f flash 0 build/firmware.bin

Some boards fail to talk to flash: when you attempt to esputil flash them, they'll time out with the flash_begin/erase failed, for example trying to flash a bootloader on a ESP32-PICO-D4-Kit:

$ esputil flash 4096 build/bootloader/bootloader.bin 
Error: can't read bootloader @ addr 0x1000
Erasing 24736 bytes @ 0x1000
flash_begin/erase failed

This is because ROM bootloader on such boards have wrong SPI pins settings. Espressif's esptool.py alleviates that by uploading its own piece of software into ESP32 RAM, which does the right thing. esputil uses ROM bootloader, and in order to fix an issue, a -fspi FLASH_PARAMS parameter can be set which manually sets flash SPI pins. The format of the FLASH_PARAMS is five comma-separated integers for CLK,Q,D,HD,CS pins.

A previously failed ESP32-PICO-D4-Kit example can be fixed by passing a correct SPI pin settings:

$ esputil -fspi 6,17,8,11,16 flash 4096 build/bootloader/bootloader.bin 
Written build/bootloader/bootloader.bin, 24736 bytes @ 0x1000