Initial: knowledge base
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# Protocol Quick Reference
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> **类别：reference** | **主题：通信协议快速参考** | **信源：** 多平台汇总
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## Table of Contents
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- [I2C (Inter-Integrated Circuit)](#i2c-inter-integrated-circuit)
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- [Overview](#overview)
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- [Signal Lines](#signal-lines)
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- [Speed Modes](#speed-modes)
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- [Pull-up Requirements](#pull-up-requirements)
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- [Common I2C Addresses (Collision Detection)](#common-i2c-addresses-collision-detection)
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- [Platform Notes](#platform-notes)
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- [SPI (Serial Peripheral Interface)](#spi-serial-peripheral-interface)
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- [Overview](#overview)
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- [Signal Lines](#signal-lines)
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- [Speed](#speed)
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- [Clock Modes](#clock-modes)
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- [Pull-up Requirements](#pull-up-requirements)
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- [Platform Notes](#platform-notes)
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- [UART (Universal Asynchronous Receiver/Transmitter)](#uart-universal-asynchronous-receivertransmitter)
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- [Overview](#overview)
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- [Signal Lines](#signal-lines)
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- [Common Baud Rates](#common-baud-rates)
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- [Frame Format](#frame-format)
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- [Voltage Levels](#voltage-levels)
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- [Level Shifting](#level-shifting)
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- [Platform Notes](#platform-notes)
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- [PWM (Pulse Width Modulation)](#pwm-pulse-width-modulation)
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- [Overview](#overview)
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- [Key Parameters](#key-parameters)
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- [Frequency by Application](#frequency-by-application)
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- [Servo Control Specifics](#servo-control-specifics)
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- [Platform Notes](#platform-notes)
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- [1-Wire](#1-wire)
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- [Overview](#overview)
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- [Signal Line](#signal-line)
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- [Pull-up Requirement](#pull-up-requirement)
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- [Common 1-Wire Devices](#common-1-wire-devices)
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- [Platform Notes](#platform-notes)
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- [CAN (Controller Area Network)](#can-controller-area-network)
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- [Overview](#overview)
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- [Signal Lines](#signal-lines)
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- [Common Transceivers](#common-transceivers)
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- [Speed and Termination](#speed-and-termination)
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- [Platform Notes](#platform-notes)
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- [ADC (Analog-to-Digital Converter)](#adc-analog-to-digital-converter)
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- [Overview](#overview)
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- [Key Parameters](#key-parameters)
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- [Platform Comparison](#platform-comparison)
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- [ESP32 ADC Attenuation](#esp32-adc-attenuation)
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- [External ADC Options](#external-adc-options)
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- [Input Protection](#input-protection)
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- [Platform Notes](#platform-notes)
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---
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## I2C (Inter-Integrated Circuit)
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### Overview
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- Two-wire synchronous serial bus
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- Multi-device: multiple slaves on same bus
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- Addressable: 7-bit (128) or 10-bit (1024) addresses
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- Open-drain: requires external pull-up resistors
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- Half-duplex: bidirectional on single data line
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### Signal Lines
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- SDA：Direction=Bidirectional, Type=Open-drain, Description=Serial data
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- SCL：Direction=Master→Slave, Type=Open-drain, Description=Serial clock
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### Speed Modes
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- Standard：Speed=100 kHz, Notes=Universal compatibility
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- Fast：Speed=400 kHz, Notes=Most common for sensors
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- Fast Plus：Speed=1 MHz, Notes=Requires stronger pull-ups
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- High Speed：Speed=3.4 MHz, Notes=Rarely used in hobby projects
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### Pull-up Requirements
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**REQUIRED** on both SDA and SCL lines.
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- Standard/Fast：Recommended Value=4.7kΩ, Notes=Most common choice
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- Fast Plus：Recommended Value=2.2kΩ, Notes=Stronger pull needed
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- High Speed / Long wires：Recommended Value=1kΩ, Notes=Compensates for capacitance
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**Calculation Formula:**
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```
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R = (VCC - VOL) / IOL
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R = (3.3V - 0.4V) / 3mA = 967Ω minimum
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```
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**Consequences of Wrong Value:**
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- **Too high (>10kΩ):** Slow rise times, communication errors, fails at higher speeds
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- **Too low (<1kΩ):** Excessive current draw, devices cannot pull line LOW, bus contention
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### Common I2C Addresses (Collision Detection)
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- 0x20-0x27：Device(s)=MCP23017 GPIO expander, PCF8574
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- 0x27, 0x3F：Device(s)=PCF8574 LCD backpack
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- 0x29：Device(s)=VL53L0X ToF distance sensor
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- 0x39：Device(s)=APDS9960 gesture/color sensor
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- 0x3C, 0x3D：Device(s)=SSD1306 OLED display
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- 0x40：Device(s)=INA219 current sensor, PCA9685 PWM
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- 0x48-0x4B：Device(s)=ADS1115/ADS1015 ADC
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- 0x50-0x57：Device(s)=AT24C EEPROM
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- 0x5A：Device(s)=MLX90614 IR thermometer
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- 0x60：Device(s)=Si5351 clock generator
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- 0x68：Device(s)=DS3231 RTC, MPU6050 IMU
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- 0x76, 0x77：Device(s)=BME280/BMP280 sensor
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**Note:** Many devices have address pins (A0, A1, A2) to resolve conflicts.
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### Platform Notes
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**Raspberry Pi:**
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- I2C1 (GPIO2/3) is the primary user-accessible bus
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- I2C0 (GPIO0/1) is **reserved** for HAT EEPROM detection — do not use
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- Enable with `dtparam=i2c_arm=on` in config.txt
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- Pi 4/5 have additional I2C buses via dtoverlay
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**ESP32:**
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- Any GPIO pair works via GPIO matrix — no fixed pins
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- Convention: GPIO21 (SDA), GPIO22 (SCL)
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- Do NOT use input-only pins (GPIO34-39) — they cannot drive SDA
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---
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## SPI (Serial Peripheral Interface)
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### Overview
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- Four-wire synchronous serial bus
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- Full-duplex: simultaneous send and receive
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- One chip select (CS) per slave device
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- No addressing: CS line selects device
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- Push-pull drivers: no pull-ups required on data/clock
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### Signal Lines
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- MOSI：Direction=Master→Slave, Description=Master Out, Slave In
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- MISO：Direction=Slave→Master, Description=Master In, Slave Out
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- SCLK：Direction=Master→Slave, Description=Serial clock
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- CS/SS：Direction=Master→Slave, Description=Chip Select (active LOW)
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### Speed
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- Typical: 1-40 MHz (device dependent)
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- Check slave device datasheet for maximum
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- Longer wires = lower reliable speed
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### Clock Modes
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- 0：CPOL=0, CPHA=0, Clock Idle=LOW, Data Sampled On=Rising edge
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- 1：CPOL=0, CPHA=1, Clock Idle=LOW, Data Sampled On=Falling edge
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- 2：CPOL=1, CPHA=0, Clock Idle=HIGH, Data Sampled On=Falling edge
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- 3：CPOL=1, CPHA=1, Clock Idle=HIGH, Data Sampled On=Rising edge
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**Note:** Mode 0 is most common. Check device datasheet.
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### Pull-up Requirements
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- **MOSI, MISO, SCLK:** Generally NOT required (push-pull drivers)
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- **CS lines:** 10kΩ pull-up recommended to prevent floating during boot/reset
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### Platform Notes
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**Raspberry Pi:**
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- SPI0 (GPIO7-11) is primary bus — CE0 (GPIO8), CE1 (GPIO7)
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- SPI1 (GPIO16-21) available but **conflicts with PCM/I2S audio**
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- Enable with `dtparam=spi=on` in config.txt
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**ESP32:**
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- VSPI (SPI3): GPIO23 (MOSI), GPIO19 (MISO), GPIO18 (SCLK), GPIO5 (CS) — **recommended**
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- HSPI (SPI2): GPIO13 (MOSI), GPIO12 (MISO), GPIO14 (SCLK), GPIO15 (CS)
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- **WARNING:** HSPI pins overlap strapping pins! GPIO12 can brick the module if HIGH at boot.
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- Any GPIO can be used via GPIO matrix (except input-only pins for outputs)
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---
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## UART (Universal Asynchronous Receiver/Transmitter)
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### Overview
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- Two-wire asynchronous serial communication
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- Point-to-point: one transmitter, one receiver per pair
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- No clock line: baud rate must match on both ends
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- Simple: widely supported, easy to debug
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### Signal Lines
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- TX：Direction=Output, Description=Transmit data (connect to peer's RX)
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- RX：Direction=Input, Description=Receive data (connect to peer's TX)
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- RTS：Direction=Output, Description=Request to Send (optional flow control)
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- CTS：Direction=Input, Description=Clear to Send (optional flow control)
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**Critical:** TX connects to RX, RX connects to TX (crossover).
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### Common Baud Rates
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- 9600：Use Case=Legacy devices, GPS modules
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- 19200：Use Case=Some sensors
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- 38400：Use Case=Bluetooth modules
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- 57600：Use Case=Faster sensors
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- 115200：Use Case=Most common default
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- 230400：Use Case=High-speed peripherals
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- 460800：Use Case=ESP32 flash programming
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- 921600：Use Case=Fast data transfer
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### Frame Format
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Standard: **8N1** (8 data bits, No parity, 1 stop bit)
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Other formats exist (7E1, 8E1, etc.) but 8N1 covers 95%+ of use cases.
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### Voltage Levels
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- TTL 3.3V：Voltage=0V / 3.3V, Common Devices=RPi, ESP32, modern MCUs
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- TTL 5V：Voltage=0V / 5V, Common Devices=Arduino, many modules
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- RS-232：Voltage=±12V, Common Devices=PC serial ports
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**CRITICAL:** RS-232 levels (±12V) will **DESTROY** 3.3V GPIO instantly. Use MAX232 or similar transceiver.
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### Level Shifting
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- 5V TX → 3.3V RX: Use voltage divider (1kΩ + 2kΩ) or level shifter
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- 3.3V TX → 5V RX: Often works directly (check VIH threshold)
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- Always use bidirectional level shifter for RTS/CTS
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### Platform Notes
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**Raspberry Pi:**
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- UART0 (GPIO14/15) is primary serial port
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- **Conflicts with Bluetooth** on Pi 3/4/Zero2W — use `dtoverlay=disable-bt` or `dtoverlay=miniuart-bt`
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- Pi 4/5 have additional UARTs via dtoverlay
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**ESP32:**
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- UART0 (GPIO1/3) is **USB serial debug** — avoid for peripherals
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- UART1 and UART2 are freely available
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- Any GPIO can be assigned via GPIO matrix
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---
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## PWM (Pulse Width Modulation)
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### Overview
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- Digital approximation of analog voltage
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- Square wave at fixed frequency
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- Duty cycle controls average voltage
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- Used for: LED dimming, motor speed, servo position, audio
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### Key Parameters
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- Frequency：Description=Pulses per second, Typical Range=50 Hz - 100 kHz
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- Duty Cycle：Description=HIGH time percentage, Typical Range=0-100%
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- Resolution：Description=Steps of duty control, Typical Range=8-bit (256) to 16-bit (65536)
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**Average Voltage:** Vavg = VCC × (Duty Cycle / 100)
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### Frequency by Application
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- LED dimming：Frequency=500-5000 Hz, Reason=>500Hz avoids visible flicker
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- Servo control：Frequency=50 Hz, Reason=Standard RC servo protocol (20ms period)
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- Motor control：Frequency=1-20 kHz, Reason=Higher = less audible whine
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- Audio generation：Frequency=20-100 kHz, Reason=Above audible range
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- Switching PSU：Frequency=50-500 kHz, Reason=Efficiency vs. noise tradeoff
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### Servo Control Specifics
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- Period: 20ms (50 Hz)
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- Pulse width: 1ms (0°) to 2ms (180°)
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- Neutral: 1.5ms (90°)
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- Duty cycle: 5% (1ms) to 10% (2ms) at 50Hz
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### Platform Notes
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**Raspberry Pi:**
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- **2 hardware PWM channels** only
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- PWM0: GPIO12 (preferred) or GPIO18 (conflicts with audio)
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- PWM1: GPIO13 (preferred) or GPIO19 (conflicts with audio)
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- Software PWM available on any pin but less precise (jitter)
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- Enable with `dtoverlay=pwm` or `dtoverlay=pwm-2chan`
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**ESP32:**
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- **LEDC peripheral:** 16 channels of hardware PWM
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- Can output on **any output-capable GPIO**
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- Cannot use input-only pins (GPIO34-39)
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- Configurable resolution (1-16 bit) and frequency
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- Motor Control PWM (MCPWM) for advanced motor control
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---
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## 1-Wire
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### Overview
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- Single-wire bidirectional bus
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- Parasitic power option (power over data line)
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- Each device has unique 64-bit ROM ID
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- Multiple devices on same bus (addressed by ROM)
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- Open-drain: requires pull-up resistor
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### Signal Line
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- DQ：Type=Bidirectional, Open-drain, Description=Data and (optionally) power
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### Pull-up Requirement
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**REQUIRED:** 4.7kΩ to VCC (3.3V or 5V depending on devices)
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- Stronger pull-up (2.2kΩ-1kΩ) for long cables or many devices
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- Parasitic power mode may need stronger pull-up during temperature conversion
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### Common 1-Wire Devices
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- DS18B20：Function=Temperature sensor, Notes=Most popular 1-Wire device
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- DS18S20：Function=Temperature sensor, Notes=Older, 9-bit only
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- DS2401：Function=Serial number, Notes=Silicon serial number
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- DS2413：Function=GPIO, Notes=2-channel I/O
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- iButton：Function=Various, Notes=Key fobs, access control
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### Platform Notes
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**Raspberry Pi:**
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- Default pin: GPIO4
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- Enable with `dtoverlay=w1-gpio`
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- Change pin with `dtoverlay=w1-gpio,gpiopin=N`
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- Kernel driver handles protocol automatically
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**ESP32:**
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- Any GPIO can be used via OneWire library
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- GPIO4 is common convention
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- Requires software library (no hardware peripheral)
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---
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## CAN (Controller Area Network)
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### Overview
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- Differential two-wire bus (noise immune)
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- Multi-master: any node can initiate
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- Message-based: no addresses, messages have IDs
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- Priority: lower message ID = higher priority
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- Error detection: CRC, ACK, bit stuffing
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- Common in: automotive, industrial, robotics
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### Signal Lines
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- CAN_H：Description=CAN High (dominant = 3.5V)
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- CAN_L：Description=CAN Low (dominant = 1.5V)
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**Note:** Requires transceiver chip (GPIO cannot drive CAN directly)
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### Common Transceivers
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- MCP2551：Voltage=5V, Notes=Classic, widely available
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- SN65HVD230：Voltage=3.3V, Notes=Good for ESP32/RPi
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- TJA1050：Voltage=5V, Notes=Automotive grade
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### Speed and Termination
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- 125 kbps：Max Bus Length=500m, Use Case=Long distance
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- 250 kbps：Max Bus Length=250m, Use Case=General purpose
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- 500 kbps：Max Bus Length=100m, Use Case=Automotive
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- 1 Mbps：Max Bus Length=40m, Use Case=High speed
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**Termination:** 120Ω resistor at **each end** of bus (two total). Many transceiver modules have onboard termination jumper.
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### Platform Notes
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**Raspberry Pi:**
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- No built-in CAN controller
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- Requires external MCP2515 (SPI-to-CAN) + transceiver
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- Enable with `dtoverlay=mcp2515-can0,oscillator=8000000,interrupt=25`
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- Uses SocketCAN interface
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**ESP32:**
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- Built-in TWAI controller (CAN 2.0B compatible)
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- Only needs external transceiver (e.g., SN65HVD230)
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- Common pins: GPIO4 (TX), GPIO5 (RX) — but any GPIO works
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- ESP-IDF and Arduino libraries available
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---
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## ADC (Analog-to-Digital Converter)
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### Overview
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- Converts continuous analog voltage to discrete digital value
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- Key parameters: resolution, reference voltage, sample rate
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- Input must not exceed reference voltage
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### Key Parameters
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- Resolution：Description=Bits of precision (10-bit = 1024 steps, 12-bit = 4096)
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- Reference：Description=Full-scale input voltage (typically VCC or internal ref)
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- Sample Rate：Description=Conversions per second (SPS)
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- Input Range：Description=Allowable input voltage (0 to Vref typically)
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### Platform Comparison
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- Built-in ADC：Raspberry Pi=**No**, ESP32=Yes (2 ADCs)
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- Resolution：Raspberry Pi=N/A, ESP32=12-bit (4096 levels)
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- Channels：Raspberry Pi=N/A, ESP32=ADC1: 8ch, ADC2: 10ch
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- Reference：Raspberry Pi=N/A, ESP32=0-3.3V (with attenuation)
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- Sample Rate：Raspberry Pi=N/A, ESP32=Up to 2 MSPS
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- WiFi Conflict：Raspberry Pi=N/A, ESP32=**ADC2 unusable with WiFi**
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### ESP32 ADC Attenuation
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- 0 dB：Input Range=0-1.1V, Notes=Highest accuracy
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- 2.5 dB：Input Range=0-1.5V
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- 6 dB：Input Range=0-2.2V
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- 11 dB：Input Range=0-3.3V, Notes=Full range, lower accuracy
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### External ADC Options
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- ADS1115：Interface=I2C, Resolution=16-bit, Channels=4, Notes=Programmable gain, slow (860 SPS)
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- ADS1015：Interface=I2C, Resolution=12-bit, Channels=4, Notes=Faster than ADS1115 (3300 SPS)
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- MCP3008：Interface=SPI, Resolution=10-bit, Channels=8, Notes=Simple, cheap, fast
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- MCP3208：Interface=SPI, Resolution=12-bit, Channels=8, Notes=Higher resolution MCP3008
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- ADS7828：Interface=I2C, Resolution=12-bit, Channels=8, Notes=8-channel I2C option
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### Input Protection
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||||
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||||
- **Never exceed reference voltage** — will damage ADC or give invalid readings
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||||
- Use voltage divider for higher voltages
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||||
- Add clamp diodes (Schottky to VCC and GND) for unknown inputs
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||||
- Add RC filter (100Ω + 100nF) to reduce noise
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||||
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||||
### Platform Notes
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||||
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||||
**Raspberry Pi:**
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||||
- No built-in ADC — external ADC required for any analog input
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||||
- MCP3008 (SPI) or ADS1115 (I2C) are most common choices
|
||||
- Many HATs include ADC chips
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||||
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||||
**ESP32:**
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||||
- ADC1 (GPIO32-39): **Always available**, even with WiFi active
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||||
- 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
|
||||
|
||||
---
|
||||
Reference in New Issue
Block a user