T&M SPI vs I2C: Understanding Serial Communication Protocols
This article dives into the differences between two popular serial communication protocols: SPI (Serial Peripheral Interface) and I2C (Inter-Integrated Circuit). Both are used extensively in embedded systems to facilitate communication between microcontrollers and various peripheral devices, but they have distinct characteristics that make them suitable for different applications.
What is SPI?
SPI, which stands for Serial Peripheral Interface, is a synchronous serial communication protocol. Here’s a breakdown of its key features:
- Full-Duplex Communication: SPI allows for simultaneous data transmission and reception, making it a very efficient protocol.
- Synchronous Protocol: SPI relies on a clock signal (SCLK) to synchronize data transfer between the master and slave devices.
- Master-Slave Architecture: SPI operates in a master-slave setup, where one master device controls the communication and multiple slave devices respond to the master.
- Flexible Clock Rates: The master device controls the clock signal, allowing for variable clock rates which can be tailored to the needs of the connected devices. Typical rates include 1.5 MHz for master and 3 MHz for slave bit frequencies.
- Programmable Clock Polarity and Phase: This allows for fine-tuning the clock signal to accommodate various devices.
- Ideal for Microcontrollers: SPI’s design makes it well-suited for microcontrollers, particularly those with less-precise clock signals.
- Master Clock Control: Only the master can control the clock. Slaves cannot manipulate the clock signal, meaning no clock signal means no data transfer.
- Data Transfer: Data is output during the rising or falling edge of the clock signal, and latched on the opposite edge.
SPI Interface Diagram
Figure-1 illustrates a typical SPI interface with one master and three slaves.
- SCLK (Clock): The clock signal controlled by the master.
- MOSI (Master Out Slave In): The line used by the master to send data to the slaves.
- MISO (Master In Slave Out): The line used by the slave to send data to the master.
- Slave Select (SS): Individual SS lines are used to select a specific slave device to communicate with.
SPI employs three registers:
- SPCR (Serial Peripheral Control Register): Manages and controls the functioning of the SPI peripheral
- SPSR (Serial Peripheral Status Register): Indicates the status of the SPI operation (eg, errors, or flags).
- SPDR (Serial Peripheral Data Register): Stores the data to be transmitted or received.
What is I2C?
I2C, which is short for Inter-Integrated Circuit, is a popular serial communication protocol that uses just two wires. It’s also sometimes referred to as IIC or I^2^C. Here’s what you need to know about I2C:
- Two-Wire Interface: I2C uses two wires, SDA (Serial Data) and SCL (Serial Clock), to communicate.
- Multi-Device Support: The I2C protocol allows up to 127 devices to be connected on the same bus.
- Unique Addressing: Each device on the I2C bus has a unique address, which allows the master to select the desired device for communication.
- Moderate Data Rates: I2C supports data rates up to 400 KHz, making it suitable for many applications.
- Master-Slave Setup: I2C uses a master-slave setup, where one device (typically a microcontroller) acts as the master and other devices act as slaves.
- Pull-Up Resistors: The I2C bus uses pull-up resistors, typically ranging from 1.53 KOhm to 100 KOhm.
I2C Interface Diagram
Figure-2 illustrates the basic I2C interface featuring a single master and several slaves.
The I2C protocol consists of several components:
- Start Condition: Initiated by the master, it’s signaled by a falling edge on the SDA line while the SCL line remains high.
- Address Packet: Includes a 7-bit or 10-bit slave address with a read/write bit and a slave acknowledge (SLA) bit.
- Data Packets: One or more data packets consisting of 8 data bits sent, MSB (Most Significant Bit) first.
- Stop Condition: The communication is terminated when the master signals a stop condition on the SDA line.
SPI vs I2C: Key Differences
| Feature | SPI | I2C |
|---|---|---|
| Wires | 4+ (SCLK, MOSI, MISO, SS) | 2 (SDA, SCL) |
| Speed | Typically faster | Typically slower |
| Complexity | More complex for multi-slave setup | Simpler multi-slave implementation |
| Addressing | Uses separate SS lines for slaves | Uses a 7-bit or 10-bit address |
| Full Duplex | Yes | No |
| Max Devices | Limited by SS lines | Up to 127 |
| Clock Control | Master only | Master only |
| Pull-Up Resistors | No | Yes |
Conclusion
Both SPI and I2C are powerful serial communication protocols, but their design characteristics make them suitable for different applications. SPI is ideal for applications requiring high-speed, full-duplex communication, such as data acquisition, while I2C is perfect for connecting multiple devices with a minimal number of wires, such as sensors and displays. Choosing the right protocol depends on the specific needs of your embedded system.
