Introduction
I2S (Inter-IC Sound) is a serial bus interface standard used to connect digital audio devices. It is designed to transfer PCM audio data between integrated circuits. Because the I2S bus separates clock and serial data signals, receivers are simpler than those used in asynchronous communication systems where clock recovery is required. Despite its similar name, I2S is not related to the I2C protocol.
I2S first appeared in the mid-1980s and is now widely used in smaller devices such as digital microphones. It is a dual-channel interface and consists of three main signal lines: clock, data, and word select (WS). WS indicates whether the current data belongs to the left or right audio channel.
I2S is a fully digital interface and requires no encoding or decoding. Transmission speed depends on audio sample rate and bit depth. For example, audio sampled at 44.1 kHz with 8-bit precision requires at least 352.8 kHz for mono and 705.6 kHz for stereo.
Unlike PDM audio, which requires an external codec for decimation, I2S uses an internal codec and filtering so that the audio received by the DSP is already at an acceptable rate. This reduces design complexity and component cost, making I2S suitable for compact, battery-powered products.
However, if DSP resources already exist in the design, PDM may be more efficient due to its lower pin count and lower power consumption.
Characteristics of I2S
1. Serial Data (SD)
- MSB (Most Significant Bit) is transmitted first.
- The transmitter and receiver do not need the same word length; the receiver simply uses the bits it needs.
- Data bits may be clocked out on either clock edge, but must be clocked in on the rising edge.
- No unused clock cycles exist between data words—LSB of one word is immediately followed by the MSB of the next.
2. Word Select (WS)
- WS = 0 → Left channel data
- WS = 1 → Right channel data
- WS transitions one clock period before the end of the current word.
3. Clock
- I2S does not define a maximum data rate.
- The serial clock (SCK) runs continuously.
4. Operating Modes
- Master: Provides LRCK and SCK signals.
- Slave: Uses external LRCK and SCK signals from the Master.
5. Transmitting and Receiving
- I2S supports both TX and RX modes.
- TX data is shifted out on the falling edge of SCK.
- RX data is sampled on the rising edge of SCK.
- MSB is always transmitted first.
- TX and RX can be enabled independently.
Left-Right Clock (LRCK)
- LRCK defines left and right channel sample boundaries.
- Left channel → WS low; Right channel → WS high.
- LRCK frequency equals the audio sample rate.
- In Master mode: LRCK = MCK / CONFIG.RATIO
- LRCK toggles around the falling edge of SCK.
Serial Clock (SCK)
- Pulses once per transmitted data bit.
- In Master mode: SCK = 2 × LRCK × CONFIG.SWIDTH
- In Slave mode: SCK is provided externally.
Master Clock (MCK)
- MCK is the source for generating SCK and LRCK in Master mode.
- Generated internally and must be enabled in Master mode.
- Requirements:
- SCK frequency ≤ MCK
- MCK/LRCK ratio must be a multiple of 2 × CONFIG.SWIDTH
- MCK may be routed to an external pin to power external I2S devices.
- MCK is not required in Slave mode.
Difference Between I2C and I2S Protocol
- I2C is used for general chip-to-chip communication; I2S is dedicated to audio.
- I2C supports multiple masters/slaves; I2S supports only one master.
- I2C uses two wires; I2S uses three.
- I2C supports clock stretching; I2S does not.
- I2C uses start/stop bits; I2S does not.
- I2C requires acknowledgment after every byte; I2S does not.
- I2C requires pull-up resistors; I2S does not.
Advantages of I2S
- Separate clock and data lines simplify the receiver design.
- Data synchronization is straightforward due to a single master.
- I2S microphones do not require analog front-end circuitry.
- Supports fully digital audio transmission.
Disadvantages of I2S
- Not suitable for long-distance cable transmission.
- Less supported in high-level applications.
- Synchronization issues may occur at high bit rates.
- No built-in error detection mechanism.
- Mainly suitable for on-board chip-to-chip communication.
- No standardized connectors or cabling.
Applications of I2S
- Connecting digital audio devices
- Transferring audio from DSPs or microcontrollers to audio codecs
- Used in CD players and modern audio systems
- Used in audio ADCs, DACs, DSPs, microcontrollers, and sample-rate converters
- Designed specifically for digital audio communication between ICs
- Essential for communication between microcontrollers and audio peripherals