Serial & Bus Communication Protocols
Overview of common communication protocols used for interfacing sensors, peripherals, and other computers with the Raspberry Pi.
RS-232 (Serial)
RS-232 is a standard for serial communication transmission of data.
Hardware Flow Controls
Used to signal when the device is ready to send or receive data.
| DTE | Direction | DCE | Description | Active |
|---|---|---|---|---|
| RTS | → | RTS | Request to send (†) | Low |
| CTS | ← | CTS | Clear to send (†) | Low |
| DSR | ← | DSR | Data set ready | Low |
| DTR | → | DTR | Data terminal ready | Low |
† Primary flow control signals
Software Flow Control (XON/XOFF)
Uses specific characters embedded in the data stream to control the flow.
| Code | Meaning | ASCII | Hex | Keyboard |
|---|---|---|---|---|
| XOFF | Pause transmission | DC3 | 13 | Control-S |
| XON | Resume transmission | DC1 | 11 | Control-Q |
Disadvantages of Software Flow Control:
- Data Loss: Line noise can mask XOFF characters, causing buffer overruns.
- Link Lockup: Missing XON characters can prevent transmission from resuming.
- Data Collision: XON/XOFF characters cannot be used as literal data unless escaped.
I2C
I2C 是一种多主从、同步、半双工的串行通信总线，主要用于板级短距离通信。
- I2C - Inter-Integrated Circuit - 芯片间总线
- 寻址: 每个设备拥有唯一的 7 位或 10 位地址，支持一主多从或多主多从。
- 特点: 硬件连接简单（仅需上拉电阻），支持热插拔，但传输速率受总线电容限制。
| pin | for | cn |
|---|---|---|
| VCC | 3.3V or 5V | 电源 |
| GND | Ground | 接地 |
| SDA | Serial Data Line | 数据 |
| SCL | Serial Clock Line | 时钟 |
- 接线
- SDA <-> SDA
- SCL <-> SCL
- 标准 4-pin I2C
| 模式 | 速率 (Bit Rate) | 备注 |
|---|---|---|
| Standard-mode | 100 kbit/s | 最常用的标准速率 |
| Fast-mode | 400 kbit/s | 多数传感器支持的速率 |
| Fast-mode Plus | 1 Mbit/s | 增强型快速模式 |
| High-speed mode | 3.4 Mbit/s | 高速模式 |
| 电平标准 | 3.3V / 5V | 漏极开路结构，必须接上拉电阻 |
- 传感器数据采集: 如 MPU6050 (陀螺仪)、BME280 (温湿度计)。
- 系统配置存储: 读写 AT24C02 等 EEPROM 存储器。
- 实时时钟 (RTC): 与 DS3231 等时钟芯片同步时间。
- 低速外设控制: 驱动 0.96 寸 OLED 显示屏或 PCF8574 IO 扩展芯片。
SPI (Serial Peripheral Interface)
SPI is a synchronous serial communication interface used for short-distance communication, primarily in embedded systems.
- Pins: Typically uses four wires: MOSI (Master Out Slave In), MISO (Master In Slave Out), SCLK (Serial Clock), and SS/CS (Slave Select / Chip Select).
- Speed: Much higher speeds than I2C (often up to 10+ MHz).
- Usage: SD cards, LCD displays, and high-speed ADCs.
1-Wire
A device communications bus system designed by Dallas Semiconductor (now Maxim Integrated).
- Pins: Technically requires only one data line plus ground (often powered via a pull-up resistor on the data line, known as "parasite power").
- Addressing: Each device has a unique, factory-programmed 64-bit ID.
- Usage: Temperature sensors (DS18B20), electronic keys (iButton), and digital potentiometers.