jolt — STM32L0 UART programmer

A small, focused Rust CLI that programs STM32L0 microcontrollers (STM32L0x1 / L0x2 / L0x3) over a serial port using the STM32 embedded UART bootloader (ST AN3155 protocol). It flashes, erases, reads device info, and includes a serial monitor.

Any board exposing the STM32L0 bootloader UART works. jolt can also enter the bootloader automatically — with no buttons — on boards whose USB-UART bridge drives the MCU's NRST and BOOT0 pins, which is how it's been tested on the HARDWARIO TOWER Radio Dongle and TOWER Core Module. On other boards you may need to enter the bootloader manually (see How it works).

Flash size is not assumed: a full-chip erase adapts to the connected part's density automatically, so the whole STM32L0 range (16–192 KiB) is supported.

Build

cargo build --release
# binary at target/release/jolt

macOS builds with a plain cargo build (the serialport crate pulls only IOKit deps, no libudev).

Usage

jolt <COMMAND> [OPTIONS]

Commands:
  list     List all serial ports
  info     Read bootloader info (chip id, version, commands) — read-only
  flash    Flash a raw firmware .bin file
  erase    Erase the entire flash (adapts to the device's density)
  reset    Reset the device into the application or the bootloader
  monitor  Open the serial port and print incoming data (serial monitor)

Global options:
  -p, --port <PATH>   Serial port (default: the only port present, else required)
  -v, --verbose       Verbose output (e.g. per-attempt bootloader-entry errors)

The bootloader runs at 115200 baud, 8-E-1 — 115200 is the maximum baud rate ST specifies for the USART bootloader.

Examples

jolt list
jolt --port /dev/cu.usbserial-112140 info
jolt -p /dev/cu.usbserial-112140 flash firmware.bin
jolt -p /dev/cu.usbserial-112140 flash firmware.bin --no-verify   # skip read-back verify
jolt -p /dev/cu.usbserial-112140 flash firmware.bin --no-run      # leave in bootloader
jolt -p /dev/cu.usbserial-112140 flash firmware.bin --go          # start via Go, not reset
jolt -p /dev/cu.usbserial-112140 erase
jolt -p /dev/cu.usbserial-112140 reset --app                      # or --bootloader
jolt -p /dev/cu.usbserial-112140 monitor                          # 115200 8N1, Ctrl-C to exit
jolt -p /dev/cu.usbserial-112140 monitor -b 9600 --parity even    # 9600 8E1
jolt -p /dev/cu.usbserial-112140 monitor --reset                  # reset into app first, catch boot logs
jolt -p /dev/cu.usbserial-112140 monitor > log.txt                # banner on stderr, device bytes only

monitor is read-only: it streams whatever the device sends to stdout until you press Ctrl-C. The frame format defaults to 115200 8N1 (the application console; note this differs from the bootloader's 8-E-1) and is configurable via -b/--baudrate, -d/--databits (5–8), --parity (none/even/odd), and -s/--stopbits (1–2). Opening the port drives the bridge to the run state so the application keeps running; --reset additionally pulses NRST into the application so you catch its boot output.

flash accepts a raw .bin only (written at 0x08000000). Convert ELF/HEX first, e.g. arm-none-eabi-objcopy -O binary firmware.elf firmware.bin.

If exactly one serial port is present, --port may be omitted; otherwise pass it explicitly (use jolt list to find it).

How it works

To enter the bootloader automatically, jolt pulses NRST while raising BOOT0, in raw modem-line terms (true = line asserted):

(rts=T,dtr=T) → (rts=T,dtr=F) → (rts=F,dtr=T) → (rts=F,dtr=F) → send 0x7F

(T,F) asserts RESET; switching to (F,T) raises BOOT0 while the 1 µF cap on the TOWER boards lets NRST ramp up slowly, so BOOT0 is high when the chip latches it and boots system memory. This sequence is tuned for that auto-reset circuit (FT231X driving NRST/BOOT0 through transistors). On a board without it, pull BOOT0 high and reset the MCU yourself, then run jolt — the protocol itself is hardware-independent.

Once in the bootloader, jolt speaks the standard ST protocol (init 0x7F, ACK 0x79). Get ID reports the product id (e.g. 0x447 for STM32L0x3); any recognized STM32L0 id is accepted, and an unknown id only prints a warning. Erase uses an explicit page list (the STM32L0 bootloader rejects the 0xFFFF mass-erase code); because the bootloader won't report the flash size, a full-chip erase walks the page list up to the family maximum and stops as soon as the device NACKs an out-of-range page — i.e. once the part's flash is wiped.

Tests

cargo test covers the protocol logic with no hardware: command/address/data checksums and framing, the Get-ID reply parse, the Extended-Erase page list, write padding, and chunk/address bookkeeping.

License

Licensed under the MIT License — © 2026 HARDWARIO a.s.