MDK (Minimal Development Kit) - a baremetal ESP32/ESP32C3 SDK
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:
Environment setup
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
Firmware Makefile should look like this:
SOURCES = main.c another_file.c
EXTRA_CFLAGS ?=
EXTRA_LINKFLAGS ?=
include $(MDK)/make/build.mk
Environment reference
Environment / Makefile variables:
| Name | Description |
|---|---|
| ARCH | Architecture. Possible values: ESP32C3, ESP32. Default: ESP32C3 |
| TOOLCHAIN | GCC binary prefix. Default: riscv64-unknown-elf |
| PORT | Serial port. Default: /dev/ttyUSB0 |
| FPARAMS | Flash parameters, see below. Default: 0x21f |
| EXTRA_CFLAGS | Extra compiler flags. Default: empty |
| EXTRA_LINKFLAGS | Extra linker flags. Default: empty |
Makefile targets:
| Name | Description |
|---|---|
| clean | Clean up build artifacts |
| build | Build firmware in a project's build/ directory |
| flash | Flash firmware. Needs PORT variable set |
| monitor | Run serial monitor. Needs PORT variable set |
| unix | Build Mac/Linux executable firmware, see "UNIX mode" section below |
Preprocessor definitions
| Name | Description |
|---|---|
| LED1 | User LED pin. Default: 2 |
| BTN1 | User button pin. Default: 9 |
API reference
API support matrix:
| Name | GPIO | SPI | I2C | UART | WiFi | Timer | System | RTOS |
|---|---|---|---|---|---|---|---|---|
| ESP32C3 | yes | yes | - | yes | - | yes | yes | - |
| ESP32 | yes | yes | - | - | - | yes | yes | - |
- 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
UNIX mode
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
esputil
esputil is a command line tool for managing Espressif devices. It is a replacement of esptool.py. esputil is written in C, its source code is in tools/esputil.c. Below is a quick reference:
$ esputil -h
Defaults: BAUD=115200, PORT=/dev/ttyUSB0
Usage:
esputil [-v] [-b BAUD] [-p PORT] monitor
esputil [-v] [-b BAUD] [-p PORT] info
esputil [-v] [-b BAUD] [-p PORT] [-fp FLASH_PARAMS] [-fspi FLASH_SPI] flash OFFSET BINFILE ...
esputil mkbin OUTPUT.BIN ENTRYADDR SECTION_ADDR SECTION.BIN ...
esputil mkhex ADDRESS1 BINFILE1 ADDRESS2 BINFILE2 ...
esputil [-tmp TMP_DIR] unhex HEXFILE
Example: flash MDK-built ESP32C3 firmware:
$ esputil flash 0 ./build/firmware.bin
Example: flash ESP-IDF built firmware on ESP32-PICO-Kit board:
$ esputil -fspi 6,17,8,11,16 flash
0x1000 build/bootloader/bootloader.bin \
0x8000 build/partitions.bin \
0xe000 build/ota_data_initial.bin \
0x10000 build/firmware.bin
Build esputil
Linux, macOS:
$ make -C tools esputil
Windows:
Choose one of the below compilers, and use the provided command from the mdk root folder. Check to make sure the path points to where you installed the compiler.
Clang: (From PowerShell)
& 'C:\Program Files\LLVM\bin\clang.exe' -v -o esputil.exe tools\esputil.c
TCC: (From PowerShell)
& 'C:\Program Files\tcc\tcc.exe' -v -o esputil.exe tools\esputil.c
MSVC: (From Developer Command Prompt)
cl tools\esputil.c
ESP32 flashing
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
Flash parameters
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
FLash SPI pin settings
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
1.4k Aug 2, 2022
90 Aug 4, 2022
600 Aug 9, 2022
5 Feb 4, 2022
1.6k Aug 3, 2022
104 Aug 3, 2022
251 Aug 5, 2022
278 Aug 10, 2022
22 Jun 18, 2022
29 Aug 4, 2022
11 Jun 23, 2022
20 Jul 18, 2022
89 Jul 4, 2022
414 Aug 8, 2022
161 Aug 2, 2022
18 Jan 9, 2022
8 Feb 26, 2022
57 Jul 26, 2022
23 Aug 2, 2022