Payloads firmware

Firmware for UBC Rocket's Cloudburst payload on the STM32G0B1CCT6. It reads from the accelerometer and UV sensor to create a table to determine the effects of gravity on microtubulin. This experiment is happening in Cloudburst's nosecone, and to control it from a safe distance at the launch site, we're using the RN2483A LoRa Module.

PCB Fabrication

The boards for this design were made possible with the support of JLCPCB and EasyEDA. Their excellent turnaround times and affordable manufacturing costs made it possible to quickly iterate on these designs and produce high-quality PCBs in time for Launch Canada 2026!

We're incredibly grateful to JLCPCB and EasyEDA for their support towards Cloudburst's Payloads electronics.

Building and Flashing

The receiver must use the same raw-LoRa profile as the transmitter. The known module frequency defaults to 433575000 Hz; CMake requires the remaining values rather than silently building a receiver with guessed RF settings. For example:

Main Board

cmake --preset Debug \
  -DRN2483_RADIO_SF=12 \
  -DRN2483_RADIO_BW_KHZ=125 \
  -DRN2483_RADIO_CR=5 \
  -DRN2483_RADIO_SYNC_WORD=0x34
cmake --build --preset Debug
openocd -f interface/stlink.cfg -f target/stm32g0x.cfg \
  -c "adapter speed 100" \
  -c "reset_config srst_only srst_nogate connect_assert_srst" \
  -c "program build/Debug/Payloads.elf verify reset exit"

Radio Bridge

cd range-test-rx/range-test-rx
cmake --preset Debug
cmake --build --preset Debug
openocd -f interface/stlink.cfg -f target/stm32f1x.cfg \
  -c "program build/Debug/range-test-rx.elf verify reset exit"

Replace the example values with the transmitter's spreading factor, bandwidth, coding-rate denominator, and sync byte. The frequency can still be overridden with -DRN2483_RADIO_FREQ_HZ when needed. The ELF is written to build/Debug/Payloads.elf.

Runtime behavior

• The BMI088 accelerometer on SPI1 runs at ±6 g, 100 Hz, and normal bandwidth. Its 1024-byte FIFO is drained every 20 ms so short SD-card stalls do not lose acquisition timing.
• The single LTR390 is addressed only through the configured 100 kHz I2C3 interface on PB3/PB4. It runs in UV mode at 20-bit resolution, a 500 ms measurement period, and 18x gain. The logged value is the coherent raw 20-bit count; converting it to ambient UVI requires a calibrated optical-window factor for the assembled payload. I2C3 must have the datasheet-recommended 1 kOhm to 10 kOhm pull-ups on SDA and SCL; MCU-internal pull-ups are not a substitute.
• PA0 drives the passive buzzer from TIM2 channel 1. At startup it plays a monophonic transcription of measures 1-17 from the supplied first page of Tony Ann's "The Interstellar Experience" at quarter note = 99, including its printed dynamics, accents, crescendo, note values, and final half rest. At the score's right-hand dyads, the passive buzzer plays the accented upper note because one buzzer cannot emit both pitches simultaneously. It then stops the buzzer output low. TIM2 no longer drives the UV LED.
• PD1 is the UVLED_CTRL GPIO and is driven high during startup. It is a binary GPIO controlled by LED_ON and LED_OFF; it is not connected to a timer channel and PWM commands are not supported.
• A preformatted FAT16 or FAT32 SD card on SPI2 is mounted without formatting. Each boot creates the first free LOG0000.CSV through LOG9999.CSV, buffers records in RAM, writes in batches, and synchronizes once per second. Failed cards are retried every five seconds while acquisition continues. On each pump off-to-on transition, queued pre-pump records are flushed to the current file and logging continues in the first free EXP0000.CSV through EXP9999.CSV. Repeated PUMP_ON packets while the pump is already on do not create extra files.
• USART2 uses the RN2483 default of 57600 baud. Startup sends the RN2483 break and 0x55 synchronization sequence, pauses the MAC, applies and reads back the required raw-LoRa profile at 433575000 Hz, then enters continuous receive mode.
• The payload disables the RN2483 radio watchdog and uses continuous reception, as required by the module command reference. After a received packet or a module-reported receive error, it immediately opens a new receive session. It does not transmit periodic traffic. A received PING is the only link-check request and causes a one-shot PONG reply after a short turnaround delay.
• USART1 on PA9/PA10 is the 115200-baud debug console. Its TX output reports the configured radio profile at boot, radio state and receive counters once per second, and an immediate EVENT ... applied line when an output command is applied. Each new LTR390 conversion prints UV sample with its timestamp, raw count, validity, counters, and I2C3 bus number; failed reads print UV read diagnostics. The range monitor displays these lines without special parsing.
• PUMP_ON and PUMP_OFF independently control the PD2 pump output. BUMP <seconds> turns the pump on for 1 through 3600 whole seconds, then turns it off without blocking sensor acquisition, logging, or radio work. A later PUMP_ON, PUMP_OFF, or BUMP supersedes the active bump timer. LED_ON and LED_OFF independently control the PD1 LED output; PWM commands are not accepted. PING leaves both outputs unchanged and replies with PONG. These are the only accepted radio payloads. An optional transmitted CR, LF, or CRLF terminator is ignored so line-oriented serial bridges work. Other malformed or unknown packets leave both outputs unchanged. The pump starts low and the UV LED starts high; both are forced low by Error_Handler.

The STM32F103 ground bridge lives under range-test-rx/range-test-rx. Its ST-Link VCP uses USART2 at 115200 baud and accepts CR/LF-terminated PUMP_ON, PUMP_OFF, BUMP <seconds>, LED_ON, LED_OFF, and PING lines from range-monitor.html. Reception is interrupt-driven, so commands are retained while the RN2483 is listening. Between two-second receive slices the bridge converts the command to a raw-LoRa hex payload. Output commands are transmitted three times with the same SF12/BW125/CR4/5/sync-0x34 profile; BUMP and PING are transmitted once. The bridge waits for each radio_tx_ok and returns to receive mode. Repetition gives each idempotent output command extra link margin without restarting a timed bump. Ping success is only reported after the payload returns PONG. BRIDGE SERIAL RX ... confirms the browser line reached the F103; BRIDGE RADIO TX ... OK confirms the bridge's RN2483 completed all transmissions (it is not an acknowledgement from the payload).

SPI2 is configured in the .ioc for eight-bit SPI mode 0 at 250 kHz with software-controlled chip select. PA4 is an initially-high SD_CS GPIO, and PB13 through PB15 are high-speed SPI2 pins, with a pull-up on PB14/SD MISO so the line has a defined idle level while the card's output is high impedance. The SD transport reapplies the startup configuration on retries and switches SPI2 to 8 MHz after card initialization. Initialization tries SPI mode 0 first, then retries CMD0 in SPI mode 3 when the card does not respond; the successful mode is retained at data speed.

Log format

time_ms is monotonic milliseconds since MCU startup; there is no real-time clock in the .ioc. A row is emitted for every 100 Hz accelerometer sample:

time_ms,accel_x_raw,accel_y_raw,accel_z_raw,uv_raw,accel_valid,uv_valid,uv_new,sensor_error_mask,pump_on,accel_fifo_skipped_total,log_dropped_total

uv_valid identifies whether the cached reading is valid and uv_new identifies the row that first records a new conversion. uv_i2c_bus_number is 3 while the sensor is available and zero while it is unavailable. Error and dropped-record counters make degraded operation visible instead of silently producing plausible-looking data.

Host tests

cmake -S tests -B build/host-tests -G Ninja
cmake --build build/host-tests
ctest --test-dir build/host-tests --output-on-failure

The tests cover BMI088 FIFO parsing and SPI framing, the LTR390 driver, fragmented RN2483 replies and output commands, buffered FatFs logging and recovery, and byte-level SDHC initialization/read/write behavior.

Useful debugger globals include accel_last_status, accel_latest_sample, accel_sample_count, accel_error_count, accel_fifo_skipped_total, uv_latest_sample, uv_last_status, uv_sample_count, uv_error_count, uv_i2c_bus_number, payload_sd_status, payload_log_dropped_count, payload_radio_ready, and payload_pump_on.

Protocol references are the local copies in Docs/ and the Microchip RN2483 command reference.

For output-command troubleshooting, connect a 115200-baud adapter to USART1 TX (PA9) and watch the RADIO lines. ready=1 phase=2 means the RN2483 is listening. If lines does not change, no module response or packet reached the firmware. Increasing badpkt means LoRa data arrived with the wrong payload; increasing valid followed by EVENT PUMP_ON applied pump=1, EVENT BUMP applied pump=1 seconds=..., or EVENT LED_ON applied led=100 proves the command was decoded and the requested output was driven high. The browser's serial write still requires the connected ground bridge to convert the command into an LoRa transmission.