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9-DOF IMU Chip Layout

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About This MicroSim

The IMU board on the swarm robot is called a 9-DOF IMU. "9-DOF" means nine degrees of freedom: three axes from a gyroscope, three from an accelerometer, and three from a magnetometer. It looks like one small board, but it is really two separate sensor chips that share one I2C bus:

  • The L3GD20 gyroscope answers at I2C address 0x6B. It gives 3 axes of rotation rate in degrees per second.
  • The LSM303DLHC holds two sensors in one chip, and each one has its own address: a 3-axis accelerometer (in g) at 0x19 and a 3-axis magnetometer (in gauss) at 0x1E.

The board also carries a bonus BMP180 temperature and air-pressure chip at 0x77. That is why the board's label says "10 DOF". We don't use it, but it still answers a scan.

Both chips connect to the same two wires: GPIO16 (SDA) and GPIO17 (SCL), which make I2C bus 0 on the Pico W. I2C lets many devices share one pair of wires, as long as each device has a different address (a number that picks which chip should answer).

The What i2c.scan() prints card shows the output of the chapter's scan code. With a working module it lists four addresses, not one. The scenario buttons show what the scan prints when one chip's wiring has failed, which is the first row of the chapter's troubleshooting table.

Clone boards can use different addresses

The gyroscope has an address-select pin, so on some boards it shows up at 0x6A instead of 0x6B. Always run i2c.scan() and use the addresses your own board reports.

This MicroSim goes with Chapter 13: Swarm Robotics and Advanced Engineering Patterns, in the section "Meet the 9-DOF IMU: L3GD20 + LSM303DLHC."

How to Use

  1. Hover over (or tap) each box, from left to right. Read the explanation in the Details card.
  2. Hover over the addr 0x6B and addr 0x19 + 0x1E labels. How does the Pico W pick which chip answers?
  3. Count the gray boxes on the right. How many axes does each one have? Add them up. Do you get 9?
  4. Press Gyro not answering, then Accel/Mag not answering. Look at the scan output each time. Which addresses are missing?
  5. Explain to a partner why reading this module needs two small drivers, not one.

Lesson Plan

Learning Objective

Students will explain (Bloom's Taxonomy: Understand) that a "9-DOF IMU module" is two separate I2C sensor chips sharing one bus, each answering at its own address or addresses, rather than one combined chip, and will interpret an i2c.scan() result in those terms.

Grade Level

Grades 8–12

Duration

10–15 minutes

Prerequisites

Activities

  1. Prediction (3 min): Before showing the diagram, ask: "The board says 9-DOF. How many addresses will i2c.scan() print?" Most students predict one; record the split. (A working board prints four.)
  2. Guided walk-through (5 min): Project the diagram and hover each node from left to right. Pause at the LSM303DLHC and ask why one chip can produce six of the nine values, and why it answers at two addresses. Point out the BMP180 as a chip the scan finds even though the code never uses it.
  3. Fault reasoning (4 min): Use the two failure scenarios. Students state which addresses disappear and which readings the robot would lose (rotation rate versus acceleration and heading).
  4. Hardware check (3 min): If modules are available, students run the chapter's scan and WHO_AM_I code and compare the printed addresses with the diagram, noting any clone-board address differences.

Discussion Questions

  • Why can the time-of-flight sensor, the OLED display, and both IMU chips all share GPIO16 and GPIO17?
  • What would happen if two chips on the bus had the same address?
  • Why does the chapter warn that motors near the LSM303DLHC distort one of its readings but not the other?

Assessment

  • Formative: Students label a blank copy of the diagram with the three sensor addresses and the units of each data type.
  • Exit ticket: "i2c.scan() prints ['0x6b', '0x77']. Which chip is missing, and which robot features stop working?" (Expected: the LSM303DLHC; tilt sensing and the compass heading.)
  • Rubric (4-point): Exemplary — explains the two-chip design, all three sensor addresses, and how the bus selects a chip; Proficient — names both chips and their addresses; Developing — knows there are two chips but confuses which sensor is on which; Beginning — treats the module as a single device.

References

  1. Inertial measurement unit (Wikipedia) — what an IMU measures and why several sensor types are combined.
  2. I²C (Wikipedia) — how devices share SDA and SCL and how 7-bit addresses select one device.
  3. MicroPython machine.I2C documentation — scan() and readfrom_mem() used in the chapter code.
  4. LSM303DLHC accelerometer/magnetometer datasheet (STMicroelectronics) — the chip's two I2C addresses and its register map.
  5. Magnetometer (Wikipedia) — how a magnetometer works as an electronic compass.
  6. 9-DOF IMU Kit — bench test of this exact module, with the shared l3gd20.py and lsm303dlhc.py drivers and a probe that confirms every address.