Serial communication protocols (found in embedded systems and audio devices)

I2S

• Use for high-speed digital audio data transfer between codecs and audio processors.

SPI

• Use for general-purpose communication with peripherals where speed is important and you have a limited number of devices.

I2C

• Use for low-speed control and configuration where you need to connect multiple devices using a minimal number of pins.

Their purpose, key characteristics, and typical uses

1. I2S (Inter-IC Sound)

• Purpose: Specifically designed for transmitting digital audio data between integrated circuits. It’s a high-speed, synchronous serial bus for audio.

• Key Characteristics:

  • Synchronous: Requires a clock signal to synchronize data transfer.
  • Serial: Data is transmitted one bit at a time.
  • Three main lines:
    • BCLK (Bit Clock or Serial Clock): The clock signal that clocks each bit of the audio data. The frequency of BCLK depends on the sample rate and bit depth.
    • WS (Word Select or LRCLK or L/R Select): Indicates which channel (left or right) is currently being transmitted (for stereo audio). Changes state at the beginning of each word.
    • SD (Serial Data or SDATA): Carries the actual audio data.
  • Master/Slave: One device is the master (typically providing the clocks) and the other is the slave. The specific master/slave roles can vary depending on the devices.
  • Point-to-Point: Usually connects only two devices directly (e.g., a codec to a DSP, or a DAC to an amplifier).
  • Framing: Data is typically transmitted in frames, with each frame containing audio data for both channels (left and right) in stereo.
  • No Addressing: Because it’s typically point-to-point, there’s no need for device addressing.
  • Common Data Formats: PCM (Pulse Code Modulation) is the most common audio data format.
  • MSB First: Data is usually transmitted most significant bit (MSB) first.

• Typical Uses:

  • Connecting audio codecs (ADC/DAC) to audio processors (DSPs).
  • Transferring audio data between a microcontroller or FPGA and an audio codec.
  • Connecting digital audio amplifiers to audio sources.
  • High-quality audio equipment.

• Advantages:

  • High speed suitable for audio data.
  • Simple and well-defined protocol.
  • Low overhead (minimal control signals).

• Disadvantages:

  • Point-to-point connection limits the number of devices that can be connected.
  • No addressing means only one slave device can be connected to a master.
  • Requires careful clock synchronization.

2. SPI (Serial Peripheral Interface)

• Purpose: A general-purpose synchronous serial communication protocol used for communication between microcontrollers and peripheral devices.

• Key Characteristics:

  • Synchronous: Requires a clock signal.
  • Serial: Data is transmitted one bit at a time.
  • Four Main Lines (typically):
    • MOSI (Master Out Slave In): Data sent from the master to the slave.
    • MISO (Master In Slave Out): Data sent from the slave to the master.
    • SCK (Serial Clock): The clock signal generated by the master to synchronize data transfer.
    • SS (Slave Select) or CS (Chip Select): A signal used by the master to select which slave device to communicate with. Each slave has its own SS/CS line.
  • Master/Slave: One device is the master, and one or more devices are slaves.
  • Full Duplex: Can transmit and receive data simultaneously.
  • Addressing: Slave selection is accomplished using individual SS/CS lines for each slave. The master asserts the SS/CS line for the desired slave.
  • Multiple Slaves: Multiple slave devices can be connected to a single SPI master, but each slave needs its own SS/CS line.
  • Configurable Clock Polarity and Phase: SPI has four different modes (CPOL, CPHA) that determine the clock polarity and phase, allowing it to work with a wide variety of devices.

• Typical Uses:

  • Communication with sensors (temperature, pressure, accelerometers).
  • Interfacing with memory devices (EEPROMs, flash memory).
  • Controlling display drivers.
  • Communication with analog-to-digital converters (ADCs) and digital-to-analog converters (DACs).
  • Interfacing with other microcontrollers or processors.

• Advantages:

  • Relatively high speed.
  • Full-duplex communication.
  • Simple protocol.
  • Multiple slave devices can be connected.

• Disadvantages:

  • Requires more pins than I2C.
  • No built-in addressing scheme (requires separate SS/CS lines for each slave).
  • No acknowledgment mechanism.

3. I2C (Inter-Integrated Circuit)

• Purpose: A two-wire serial communication protocol used for low-speed communication between integrated circuits. It’s primarily used for control and configuration rather than high-bandwidth data transfer.

• Key Characteristics:

  • Synchronous: Requires a clock signal.
  • Serial: Data is transmitted one bit at a time.
  • Two Lines:
    • SDA (Serial Data): Carries the data.
    • SCL (Serial Clock): The clock signal used for synchronization.
  • Multi-Master: Multiple devices can act as a master, although typically only one master is active at a time.
  • Addressing: Each device on the bus has a unique 7-bit or 10-bit address.
  • Open Drain: SDA and SCL lines are open-drain, requiring pull-up resistors.
  • Acknowledgment: The receiver acknowledges each byte of data that is received.
  • Arbitration: A mechanism is used to resolve bus conflicts if multiple masters try to access the bus simultaneously.
  • Master/Slave: Devices can be masters or slaves. The master initiates the communication, and the slave responds.

• Typical Uses:

  • Controlling and configuring peripheral devices (sensors, RTCs, EEPROMs).
  • Reading data from sensors.
  • Interfacing with display drivers.
  • Controlling power management circuits.
  • Accessing configuration registers in other ICs (e.g., audio codecs, DSPs).

• Advantages:

  • Only requires two wires, minimizing pin count.
  • Supports multiple master and slave devices.
  • Built-in addressing scheme.
  • Acknowledgment mechanism for error detection.

• Disadvantages:

  • Relatively slow compared to SPI and I2S.
  • More complex protocol than SPI.
  • Open-drain lines require pull-up resistors.

Summary Table:

Feature	I2S	SPI	I2C
Purpose	Audio Data Transfer	General-Purpose Serial	Control & Configuration
Synchronization	Synchronous	Synchronous	Synchronous
Lines	BCLK, WS, SD	MOSI, MISO, SCK, SS	SDA, SCL
Addressing	None	Slave Select (SS/CS)	7/10-bit Address
Master/Slave	Yes	Yes	Yes
Multi-Master	No	No (typically)	Yes
Speed	High	Medium/High	Low/Medium
Complexity	Simple	Medium	Complex
Typical Use	Audio Codec Connection	Sensors, Memory, Displays	Sensors, Configuration