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Week 06 – Embedded Programming

Group Assignment – I2C Signal Analysis Using Oscilloscope

1. Oscilloscope Description

For this assignment, I used the Rigol DHO804 Digital Oscilloscope (70 MHz). Oscilloscope

Main Specifications:

  • Bandwidth: 70 MHz
  • 4 Analog Channels
  • Sample rate: up to 1.25 GSa/s
  • 12-bit vertical resolution
  • UltraVision architecture
  • Automatic frequency and voltage measurements.

The oscilloscope was used to analyze I2C communication between the ESP8266 and the bus lines SCL and SDA.

2. Test Setup

  • Microcontroller: ESP8266
  • Protocol: I2C
  • Logic level: 3.3V
  • Channel 1 (Yellow) → SCL
  • Channel 2 (Blue) → SDA
  • Common ground between ESP8266 and oscilloscope.

Important:

I performed two different tests:

  1. I2C communication without external pull-up resistors
  2. I2C communication with pull-up resistors added

3. Test 1 – Without Pull-Up Resistors

I first tested the I2C bus without external pull-up resistors.

I2C uses open-drain (open-collector) outputs, meaning:

  • Devices can only pull the line LOW
  • The line needs a pull-up resistor to go HIGH

Waveform Without Pull-Ups

No Pullup Test

Observations:

  • Rising edges were slower
  • HIGH level was not perfectly stable
  • Signal shape was not ideal
  • Possible noise and weak logic HIGH

This happens because: - Without pull-up resistors, the line cannot properly return to 3.3V

4. Test 2 – With Pull-Up Resistors

After that, I added external pull-up resistors on:

  • SDA → 3.3V
  • SCL → 3.3V

(typically 4.7kΩ or 10kΩ resistors)

Waveform With Pull-Ups

With Pullup Test

Observations:

  • Clean rising edges
  • Stable 3.3V HIGH level
  • Sharp transitions
  • Much cleaner digital signal

Measured voltage levels:

  • HIGH ≈ 3.07V – 3.08V
  • LOW ≈ 70mV – 80mV

This confirms proper 3.3V logic behavior.

5. I2C Signal Analysis

5.1 Clock Signal (SCL)

Clock Signal

Measured frequency:

  • ~47.6 kHz
  • ~51.7 kHz

Average clock speed:

≈ 50 kHz

5.2 Data Line (SDA)

Data Line

Observations:

  • SDA changes only when SCL is LOW
  • SDA remains stable while SCL is HIGH

This confirms correct I2C protocol timing.

Each clock pulse corresponds to one data bit.

6. Physical Setup

Full Test Environment

Full Setup

  • Rigol DHO804 oscilloscope
  • ESP8266 development board
  • Breadboard wiring
  • Probes connected to SCL and SDA
  • Laptop for programming

Oscilloscope Display

Oscilloscope Front

The display shows:

  • Yellow: SCL
  • Blue: SDA
  • Microsecond time base

Week 06 — Electronics Design (Individual Assignment)

Software Used

For this assignment, I used KiCad for schematic and PCB design.

I chose KiCad because:

  • It has a simpler and more intuitive interface.
  • Library management is easier for small and medium projects.
  • It is open-source and well suited for academic use.
  • It integrates schematic, PCB, 3D viewer and Gerber export in one environment.

Design Rules Reference (PCB)

For general PCB layout rules and best practices, I used the following reference:

David L. Jones
PCB Design Tutorial
Revision A — June 29, 2004

📎 PCB Design Tutorial — David L. Jones (PDF)

Key Design Principles Applied

From this document, I followed these simple rules:

  • Finish and verify schematic before starting PCB layout.
  • Use grid and snap for clean placement and routing.
  • Prefer 45° trace routing.
  • Keep traces short and direct.
  • Avoid unnecessary vias.
  • Run ERC and DRC before fabrication.
  • Use ground plane for better stability.

Schematic Design

For this assignment, I designed a simple development board based on:

STM32F030K6T6

The goal was to create a minimal but functional STM32 dev board that includes:

  • 3.3V power regulation
  • Decoupling capacitors
  • NRST reset circuit
  • SWD programming header
  • Basic IO breakout
  • Power input (USB / external input if applicable)

Full Schematic Overview

Full Schematic

1️⃣ USB-C Power Input

The board is powered using a USB-C receptacle (Power Only configuration).

Design details:

  • VBUS provides 5V
  • CC1 and CC2 use 5.1kΩ resistors to GND (R1, R2) to indicate sink device
  • Shield connected through RC network (R3 1MΩ + C1 1nF) to GND
  • VBUS labeled as VBUS_5V

This provides safe and standard USB-C 5V power input.

USB-C Section

USB-C Section

2️⃣ Voltage Regulation (5V → 3.3V)

The regulator used is:

AMS1117-3.3

Configuration:

  • Input: VBUS_5V
  • Output: +3.3V
  • C2: 10µF input capacitor
  • C3: 10µF output capacitor

This generates stable 3.3V for the STM32 and peripherals.

Regulator Section

Regulator Section

6️⃣ SWD Programming Header

A 10-pin SWD header (J2) is used.

Connected signals:

  • SWDIO
  • SWCLK
  • NRST
  • +3.3V
  • GND

This allows programming with ST-Link or compatible debugger.

SWD Section

SWD Section

7️⃣ Status LED

A simple LED circuit was added:

  • D1 (Green LED)
  • R5 (470Ω current limiting resistor)

Connected to MCU pin (LED_STATUS).

Used for:

  • Status indication
  • Debug output
  • Simple firmware testing