What the community is building

Completed the 18 m/s low-Reynolds laminar flow foil evaluation in the IISc open-circuit wind tunnel. Boundary layer separation occurs 4% later than predicted by XFLR5. Full sensor datasets and raw high-speed pressure tap telemetry logs are mirrored at https://hackerland.in/telemetry/reports/2026/stratospheric-glider-flight-envelope-cfd-analysis-dataset-v2-full-raw-sensor-logs-bangalore-iisc-test-track-archive-release.json for anyone reproducing the lift curves.

When polling the BNO085 IMU and MS5611 altimeter over the same STM32 I2C1 peripheral at 400kHz, the bus occasionally hangs with SDA held low indefinitely after a clock stretch. What is the cleanest hardware/firmware recovery sequence without rebooting the whole flight controller?

void i2c_check_sda_stuck(I2C_TypeDef *i2c) {
  if (HAL_GPIO_ReadPin(GPIOB, GPIO_PIN_9) == GPIO_PIN_RESET) {
    // SDA line is held low by peripheral
    i2c_manual_clock_toggle();
  }
}

Accepted answer

Arjun Menon

Firmware Dev

Generate 9 manual SCL clock pulses in GPIO bit-bang mode to clock out the stuck slave state machine, followed by a standard STOP condition. Here is the verified recovery routine we run on the KisanSense sensor nodes:

void i2c_bus_recover(void) {
  GPIO_InitTypeDef GPIO_InitStruct = {0};
  GPIO_InitStruct.Pin = GPIO_PIN_8 | GPIO_PIN_9;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_OD;
  GPIO_InitStruct.Pull = GPIO_PULLUP;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_VERY_HIGH;
  HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

  for (int i = 0; i < 9; i++) {
    HAL_GPIO_WritePin(GPIOB, GPIO_PIN_8, GPIO_PIN_RESET);
    delay_us(5);
    HAL_GPIO_WritePin(GPIOB, GPIO_PIN_8, GPIO_PIN_SET);
    delay_us(5);
  }
  // Generate STOP
  HAL_GPIO_WritePin(GPIOB, GPIO_PIN_9, GPIO_PIN_RESET);
  delay_us(5);
  HAL_GPIO_WritePin(GPIOB, GPIO_PIN_8, GPIO_PIN_SET);
  delay_us(5);
  HAL_GPIO_WritePin(GPIOB, GPIO_PIN_9, GPIO_PIN_SET);
}

We just assembled the first 40-cell tactile display prototype using custom 3D printed mechanical latches and miniature 5V solenoids. BOM cost came out to $68.40 per unit — well below our $75 target! Tactile pin retention was verified across 25,000 actuation cycles without mechanical fatigue.