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I2C Bus Explorer

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About This MicroSim

The I2C bus lets one controller talk to many devices over just two wires: SDA (Serial Data, GPIO 16 on the Maker Pi RP2040) and SCL (Serial Clock, GPIO 17). Two pull-up resistors hold both wires at 3.3 V when nobody is talking.

The top of this MicroSim shows the robot's bus: the RP2040 controller, the VL53L0X distance sensor at address 0x29, and the SSD1306 OLED display at address 0x3C. Both devices hear every message. They tell messages apart by address, a 7-bit number that each device owns.

The middle shows a timing diagram: the SCL clock in green and the SDA data in blue. One write transaction has seven parts:

  1. START: SDA falls while SCL is high.
  2. ADDRESS: 7 address bits, most significant bit first.
  3. R/W: one bit, 0 for write.
  4. ACK: the device with that address pulls SDA low to say "I'm here."
  5. DATA: 8 data bits.
  6. ACK: the device confirms it got the byte.
  7. STOP: SDA rises while SCL is high.

Only the addressed device answers. It lights green, and the other device turns gray and shows "ignoring." If no device has the address, nobody pulls SDA low on the ACK bit. That is a NACK, and MicroPython reports OSError: [Errno 5] EIO.

Scan bus does what i2c.scan() does on your robot. It probes every usable address from 0x08 to 0x77 and lists the ones that answer.

How to Use

  1. Choose who to Talk to, the Clock speed, and the Data byte.
  2. Press Step to send one bit at a time, or Send to watch the whole transaction. The orange marker shows the bit being sent.
  3. Read the 0 and 1 above each SDA bit and compare them with the binary address in the header line.
  4. Choose Nobody home (0x50) and send again. Watch the ACK bit.
  5. Press Scan bus and read the list of addresses that answered.
  6. Compare the transaction time at 100 kHz and 400 kHz.

This MicroSim goes with the I2C section of Chapter 6: Electronics, DC Motors, and Communication Protocols.

Lesson Plan

Grade Level

Grades 8–12 (introductory electronics with a physical robot)

Duration

20 minutes

Prerequisites

Learning Objective

Students will be able to trace (Bloom's Taxonomy: Apply) an I2C write transaction (START, address, ACK, data, STOP) and explain how addresses let two devices share the same two wires.

Activities

  1. Decode the address (4 min). Students convert 0x29 to 7-bit binary by hand (0101001), then step through the address bits and check each one.
  2. Trace a full write (5 min). Students step through a transaction to the OLED and label each part of the timing diagram on a worksheet: START, address, R/W, ACK, data, ACK, STOP.
  3. Break it (4 min). Students send to 0x50, describe what happens on the ACK bit, and connect the EIO error to a real wiring mistake.
  4. Scan and compare (4 min). Students run Scan bus and compare the result with the chapter's i2c.scan() example.
  5. Speed check (3 min). Students compute the transaction time at 100 kHz and 400 kHz and verify it with the header line.

Assessment

  • Challenge: Send to 0x50 and find out why nothing answers. Answer: no device uses that address, so nobody pulls SDA low for the ACK bit. Then run Scan bus and confirm that the two addresses that answer are 0x29 and 0x3c.
  • Exit ticket: "Two sensors both use address 0x29. What problem will you see on the bus, and how could you fix it?"
  • Rubric (4-point): Exemplary traces every part of a transaction, decodes the address bits, and explains ACK vs. NACK in terms of who drives SDA. Proficient names the parts in order and explains why only one device answers. Developing identifies START and STOP but confuses the ACK bit. Beginning cannot explain how two devices share the wires.

References

  1. Chapter 6: Electronics, DC Motors, and Communication Protocols - the I2C bus, addresses, and the robot's SDA and SCL pins.
  2. MicroPython machine.I2C - the I2C class and i2c.scan().
  3. I²C (Wikipedia) - how START, addresses, ACK, and STOP work on the bus.
  4. NXP UM10204: I2C-bus specification and user manual - the official I2C specification.
  5. Protocol Comparison (Learning MicroPython) - the earlier MicroSim whose layout this one follows.