# Protocol Quick Reference

> **类别：reference** | **主题：通信协议快速参考** | **信源：** 多平台汇总

## Table of Contents

- [I2C (Inter-Integrated Circuit)](#i2c-inter-integrated-circuit)
  - [Overview](#overview)
  - [Signal Lines](#signal-lines)
  - [Speed Modes](#speed-modes)
  - [Pull-up Requirements](#pull-up-requirements)
  - [Common I2C Addresses (Collision Detection)](#common-i2c-addresses-collision-detection)
  - [Platform Notes](#platform-notes)
- [SPI (Serial Peripheral Interface)](#spi-serial-peripheral-interface)
  - [Overview](#overview)
  - [Signal Lines](#signal-lines)
  - [Speed](#speed)
  - [Clock Modes](#clock-modes)
  - [Pull-up Requirements](#pull-up-requirements)
  - [Platform Notes](#platform-notes)
- [UART (Universal Asynchronous Receiver/Transmitter)](#uart-universal-asynchronous-receivertransmitter)
  - [Overview](#overview)
  - [Signal Lines](#signal-lines)
  - [Common Baud Rates](#common-baud-rates)
  - [Frame Format](#frame-format)
  - [Voltage Levels](#voltage-levels)
  - [Level Shifting](#level-shifting)
  - [Platform Notes](#platform-notes)
- [PWM (Pulse Width Modulation)](#pwm-pulse-width-modulation)
  - [Overview](#overview)
  - [Key Parameters](#key-parameters)
  - [Frequency by Application](#frequency-by-application)
  - [Servo Control Specifics](#servo-control-specifics)
  - [Platform Notes](#platform-notes)
- [1-Wire](#1-wire)
  - [Overview](#overview)
  - [Signal Line](#signal-line)
  - [Pull-up Requirement](#pull-up-requirement)
  - [Common 1-Wire Devices](#common-1-wire-devices)
  - [Platform Notes](#platform-notes)
- [CAN (Controller Area Network)](#can-controller-area-network)
  - [Overview](#overview)
  - [Signal Lines](#signal-lines)
  - [Common Transceivers](#common-transceivers)
  - [Speed and Termination](#speed-and-termination)
  - [Platform Notes](#platform-notes)
- [ADC (Analog-to-Digital Converter)](#adc-analog-to-digital-converter)
  - [Overview](#overview)
  - [Key Parameters](#key-parameters)
  - [Platform Comparison](#platform-comparison)
  - [ESP32 ADC Attenuation](#esp32-adc-attenuation)
  - [External ADC Options](#external-adc-options)
  - [Input Protection](#input-protection)
  - [Platform Notes](#platform-notes)

---

## I2C (Inter-Integrated Circuit)

### Overview

- Two-wire synchronous serial bus
- Multi-device: multiple slaves on same bus
- Addressable: 7-bit (128) or 10-bit (1024) addresses
- Open-drain: requires external pull-up resistors
- Half-duplex: bidirectional on single data line

### Signal Lines

- SDA：Direction=Bidirectional, Type=Open-drain, Description=Serial data
- SCL：Direction=Master→Slave, Type=Open-drain, Description=Serial clock

### Speed Modes

- Standard：Speed=100 kHz, Notes=Universal compatibility
- Fast：Speed=400 kHz, Notes=Most common for sensors
- Fast Plus：Speed=1 MHz, Notes=Requires stronger pull-ups
- High Speed：Speed=3.4 MHz, Notes=Rarely used in hobby projects

### Pull-up Requirements

**REQUIRED** on both SDA and SCL lines.

- Standard/Fast：Recommended Value=4.7kΩ, Notes=Most common choice
- Fast Plus：Recommended Value=2.2kΩ, Notes=Stronger pull needed
- High Speed / Long wires：Recommended Value=1kΩ, Notes=Compensates for capacitance

**Calculation Formula:**
```
R = (VCC - VOL) / IOL
R = (3.3V - 0.4V) / 3mA = 967Ω minimum
```

**Consequences of Wrong Value:**
- **Too high (>10kΩ):** Slow rise times, communication errors, fails at higher speeds
- **Too low (<1kΩ):** Excessive current draw, devices cannot pull line LOW, bus contention

### Common I2C Addresses (Collision Detection)

- 0x20-0x27：Device(s)=MCP23017 GPIO expander, PCF8574
- 0x27, 0x3F：Device(s)=PCF8574 LCD backpack
- 0x29：Device(s)=VL53L0X ToF distance sensor
- 0x39：Device(s)=APDS9960 gesture/color sensor
- 0x3C, 0x3D：Device(s)=SSD1306 OLED display
- 0x40：Device(s)=INA219 current sensor, PCA9685 PWM
- 0x48-0x4B：Device(s)=ADS1115/ADS1015 ADC
- 0x50-0x57：Device(s)=AT24C EEPROM
- 0x5A：Device(s)=MLX90614 IR thermometer
- 0x60：Device(s)=Si5351 clock generator
- 0x68：Device(s)=DS3231 RTC, MPU6050 IMU
- 0x76, 0x77：Device(s)=BME280/BMP280 sensor

**Note:** Many devices have address pins (A0, A1, A2) to resolve conflicts.

### Platform Notes

**Raspberry Pi:**
- I2C1 (GPIO2/3) is the primary user-accessible bus
- I2C0 (GPIO0/1) is **reserved** for HAT EEPROM detection — do not use
- Enable with `dtparam=i2c_arm=on` in config.txt
- Pi 4/5 have additional I2C buses via dtoverlay

**ESP32:**
- Any GPIO pair works via GPIO matrix — no fixed pins
- Convention: GPIO21 (SDA), GPIO22 (SCL)
- Do NOT use input-only pins (GPIO34-39) — they cannot drive SDA

---

## SPI (Serial Peripheral Interface)

### Overview

- Four-wire synchronous serial bus
- Full-duplex: simultaneous send and receive
- One chip select (CS) per slave device
- No addressing: CS line selects device
- Push-pull drivers: no pull-ups required on data/clock

### Signal Lines

- MOSI：Direction=Master→Slave, Description=Master Out, Slave In
- MISO：Direction=Slave→Master, Description=Master In, Slave Out
- SCLK：Direction=Master→Slave, Description=Serial clock
- CS/SS：Direction=Master→Slave, Description=Chip Select (active LOW)

### Speed

- Typical: 1-40 MHz (device dependent)
- Check slave device datasheet for maximum
- Longer wires = lower reliable speed

### Clock Modes

- 0：CPOL=0, CPHA=0, Clock Idle=LOW, Data Sampled On=Rising edge
- 1：CPOL=0, CPHA=1, Clock Idle=LOW, Data Sampled On=Falling edge
- 2：CPOL=1, CPHA=0, Clock Idle=HIGH, Data Sampled On=Falling edge
- 3：CPOL=1, CPHA=1, Clock Idle=HIGH, Data Sampled On=Rising edge

**Note:** Mode 0 is most common. Check device datasheet.

### Pull-up Requirements

- **MOSI, MISO, SCLK:** Generally NOT required (push-pull drivers)
- **CS lines:** 10kΩ pull-up recommended to prevent floating during boot/reset

### Platform Notes

**Raspberry Pi:**
- SPI0 (GPIO7-11) is primary bus — CE0 (GPIO8), CE1 (GPIO7)
- SPI1 (GPIO16-21) available but **conflicts with PCM/I2S audio**
- Enable with `dtparam=spi=on` in config.txt

**ESP32:**
- VSPI (SPI3): GPIO23 (MOSI), GPIO19 (MISO), GPIO18 (SCLK), GPIO5 (CS) — **recommended**
- HSPI (SPI2): GPIO13 (MOSI), GPIO12 (MISO), GPIO14 (SCLK), GPIO15 (CS)
- **WARNING:** HSPI pins overlap strapping pins! GPIO12 can brick the module if HIGH at boot.
- Any GPIO can be used via GPIO matrix (except input-only pins for outputs)

---

## UART (Universal Asynchronous Receiver/Transmitter)

### Overview

- Two-wire asynchronous serial communication
- Point-to-point: one transmitter, one receiver per pair
- No clock line: baud rate must match on both ends
- Simple: widely supported, easy to debug

### Signal Lines

- TX：Direction=Output, Description=Transmit data (connect to peer's RX)
- RX：Direction=Input, Description=Receive data (connect to peer's TX)
- RTS：Direction=Output, Description=Request to Send (optional flow control)
- CTS：Direction=Input, Description=Clear to Send (optional flow control)

**Critical:** TX connects to RX, RX connects to TX (crossover).

### Common Baud Rates

- 9600：Use Case=Legacy devices, GPS modules
- 19200：Use Case=Some sensors
- 38400：Use Case=Bluetooth modules
- 57600：Use Case=Faster sensors
- 115200：Use Case=Most common default
- 230400：Use Case=High-speed peripherals
- 460800：Use Case=ESP32 flash programming
- 921600：Use Case=Fast data transfer

### Frame Format

Standard: **8N1** (8 data bits, No parity, 1 stop bit)

Other formats exist (7E1, 8E1, etc.) but 8N1 covers 95%+ of use cases.

### Voltage Levels

- TTL 3.3V：Voltage=0V / 3.3V, Common Devices=RPi, ESP32, modern MCUs
- TTL 5V：Voltage=0V / 5V, Common Devices=Arduino, many modules
- RS-232：Voltage=±12V, Common Devices=PC serial ports

**CRITICAL:** RS-232 levels (±12V) will **DESTROY** 3.3V GPIO instantly. Use MAX232 or similar transceiver.

### Level Shifting

- 5V TX → 3.3V RX: Use voltage divider (1kΩ + 2kΩ) or level shifter
- 3.3V TX → 5V RX: Often works directly (check VIH threshold)
- Always use bidirectional level shifter for RTS/CTS

### Platform Notes

**Raspberry Pi:**
- UART0 (GPIO14/15) is primary serial port
- **Conflicts with Bluetooth** on Pi 3/4/Zero2W — use `dtoverlay=disable-bt` or `dtoverlay=miniuart-bt`
- Pi 4/5 have additional UARTs via dtoverlay

**ESP32:**
- UART0 (GPIO1/3) is **USB serial debug** — avoid for peripherals
- UART1 and UART2 are freely available
- Any GPIO can be assigned via GPIO matrix

---

## PWM (Pulse Width Modulation)

### Overview

- Digital approximation of analog voltage
- Square wave at fixed frequency
- Duty cycle controls average voltage
- Used for: LED dimming, motor speed, servo position, audio

### Key Parameters

- Frequency：Description=Pulses per second, Typical Range=50 Hz - 100 kHz
- Duty Cycle：Description=HIGH time percentage, Typical Range=0-100%
- Resolution：Description=Steps of duty control, Typical Range=8-bit (256) to 16-bit (65536)

**Average Voltage:** Vavg = VCC × (Duty Cycle / 100)

### Frequency by Application

- LED dimming：Frequency=500-5000 Hz, Reason=>500Hz avoids visible flicker
- Servo control：Frequency=50 Hz, Reason=Standard RC servo protocol (20ms period)
- Motor control：Frequency=1-20 kHz, Reason=Higher = less audible whine
- Audio generation：Frequency=20-100 kHz, Reason=Above audible range
- Switching PSU：Frequency=50-500 kHz, Reason=Efficiency vs. noise tradeoff

### Servo Control Specifics

- Period: 20ms (50 Hz)
- Pulse width: 1ms (0°) to 2ms (180°)
- Neutral: 1.5ms (90°)
- Duty cycle: 5% (1ms) to 10% (2ms) at 50Hz

### Platform Notes

**Raspberry Pi:**
- **2 hardware PWM channels** only
- PWM0: GPIO12 (preferred) or GPIO18 (conflicts with audio)
- PWM1: GPIO13 (preferred) or GPIO19 (conflicts with audio)
- Software PWM available on any pin but less precise (jitter)
- Enable with `dtoverlay=pwm` or `dtoverlay=pwm-2chan`

**ESP32:**
- **LEDC peripheral:** 16 channels of hardware PWM
- Can output on **any output-capable GPIO**
- Cannot use input-only pins (GPIO34-39)
- Configurable resolution (1-16 bit) and frequency
- Motor Control PWM (MCPWM) for advanced motor control

---

## 1-Wire

### Overview

- Single-wire bidirectional bus
- Parasitic power option (power over data line)
- Each device has unique 64-bit ROM ID
- Multiple devices on same bus (addressed by ROM)
- Open-drain: requires pull-up resistor

### Signal Line

- DQ：Type=Bidirectional, Open-drain, Description=Data and (optionally) power

### Pull-up Requirement

**REQUIRED:** 4.7kΩ to VCC (3.3V or 5V depending on devices)

- Stronger pull-up (2.2kΩ-1kΩ) for long cables or many devices
- Parasitic power mode may need stronger pull-up during temperature conversion

### Common 1-Wire Devices

- DS18B20：Function=Temperature sensor, Notes=Most popular 1-Wire device
- DS18S20：Function=Temperature sensor, Notes=Older, 9-bit only
- DS2401：Function=Serial number, Notes=Silicon serial number
- DS2413：Function=GPIO, Notes=2-channel I/O
- iButton：Function=Various, Notes=Key fobs, access control

### Platform Notes

**Raspberry Pi:**
- Default pin: GPIO4
- Enable with `dtoverlay=w1-gpio`
- Change pin with `dtoverlay=w1-gpio,gpiopin=N`
- Kernel driver handles protocol automatically

**ESP32:**
- Any GPIO can be used via OneWire library
- GPIO4 is common convention
- Requires software library (no hardware peripheral)

---

## CAN (Controller Area Network)

### Overview

- Differential two-wire bus (noise immune)
- Multi-master: any node can initiate
- Message-based: no addresses, messages have IDs
- Priority: lower message ID = higher priority
- Error detection: CRC, ACK, bit stuffing
- Common in: automotive, industrial, robotics

### Signal Lines

- CAN_H：Description=CAN High (dominant = 3.5V)
- CAN_L：Description=CAN Low (dominant = 1.5V)

**Note:** Requires transceiver chip (GPIO cannot drive CAN directly)

### Common Transceivers

- MCP2551：Voltage=5V, Notes=Classic, widely available
- SN65HVD230：Voltage=3.3V, Notes=Good for ESP32/RPi
- TJA1050：Voltage=5V, Notes=Automotive grade

### Speed and Termination

- 125 kbps：Max Bus Length=500m, Use Case=Long distance
- 250 kbps：Max Bus Length=250m, Use Case=General purpose
- 500 kbps：Max Bus Length=100m, Use Case=Automotive
- 1 Mbps：Max Bus Length=40m, Use Case=High speed

**Termination:** 120Ω resistor at **each end** of bus (two total). Many transceiver modules have onboard termination jumper.

### Platform Notes

**Raspberry Pi:**
- No built-in CAN controller
- Requires external MCP2515 (SPI-to-CAN) + transceiver
- Enable with `dtoverlay=mcp2515-can0,oscillator=8000000,interrupt=25`
- Uses SocketCAN interface

**ESP32:**
- Built-in TWAI controller (CAN 2.0B compatible)
- Only needs external transceiver (e.g., SN65HVD230)
- Common pins: GPIO4 (TX), GPIO5 (RX) — but any GPIO works
- ESP-IDF and Arduino libraries available

---

## ADC (Analog-to-Digital Converter)

### Overview

- Converts continuous analog voltage to discrete digital value
- Key parameters: resolution, reference voltage, sample rate
- Input must not exceed reference voltage

### Key Parameters

- Resolution：Description=Bits of precision (10-bit = 1024 steps, 12-bit = 4096)
- Reference：Description=Full-scale input voltage (typically VCC or internal ref)
- Sample Rate：Description=Conversions per second (SPS)
- Input Range：Description=Allowable input voltage (0 to Vref typically)

### Platform Comparison

- Built-in ADC：Raspberry Pi=**No**, ESP32=Yes (2 ADCs)
- Resolution：Raspberry Pi=N/A, ESP32=12-bit (4096 levels)
- Channels：Raspberry Pi=N/A, ESP32=ADC1: 8ch, ADC2: 10ch
- Reference：Raspberry Pi=N/A, ESP32=0-3.3V (with attenuation)
- Sample Rate：Raspberry Pi=N/A, ESP32=Up to 2 MSPS
- WiFi Conflict：Raspberry Pi=N/A, ESP32=**ADC2 unusable with WiFi**

### ESP32 ADC Attenuation

- 0 dB：Input Range=0-1.1V, Notes=Highest accuracy
- 2.5 dB：Input Range=0-1.5V
- 6 dB：Input Range=0-2.2V
- 11 dB：Input Range=0-3.3V, Notes=Full range, lower accuracy

### External ADC Options

- ADS1115：Interface=I2C, Resolution=16-bit, Channels=4, Notes=Programmable gain, slow (860 SPS)
- ADS1015：Interface=I2C, Resolution=12-bit, Channels=4, Notes=Faster than ADS1115 (3300 SPS)
- MCP3008：Interface=SPI, Resolution=10-bit, Channels=8, Notes=Simple, cheap, fast
- MCP3208：Interface=SPI, Resolution=12-bit, Channels=8, Notes=Higher resolution MCP3008
- ADS7828：Interface=I2C, Resolution=12-bit, Channels=8, Notes=8-channel I2C option

### Input Protection

- **Never exceed reference voltage** — will damage ADC or give invalid readings
- Use voltage divider for higher voltages
- Add clamp diodes (Schottky to VCC and GND) for unknown inputs
- Add RC filter (100Ω + 100nF) to reduce noise

### Platform Notes

**Raspberry Pi:**
- No built-in ADC — external ADC required for any analog input
- MCP3008 (SPI) or ADS1115 (I2C) are most common choices
- Many HATs include ADC chips

**ESP32:**
- ADC1 (GPIO32-39): **Always available**, even with WiFi active
- ADC2 (GPIO0-27 subset): **Unusable when WiFi or Bluetooth active**
- Design rule: Use ADC1 pins for analog if project uses WiFi
- Non-linear at extremes — calibration improves accuracy

---