I2C (Inter-Integrated Circuit)

📘 I2C (Inter-Integrated Circuit)

1) Concept

I2C is a two-wire serial communication bus used to connect multiple devices. It is commonly used for sensors, EEPROMs, PMICs, RTCs, touch controllers, and other low-speed peripherals.

SPI: faster, requires more wires (typically 4+)
I2C: slower, but only needs 2 wires

Therefore, I2C is preferred when wiring simplicity is more important than data rate.

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2) Bus Signals (Only 2 Lines)

Both lines use open-drain signaling, so pull-up resistors (typically 2.2kΩ–10kΩ) are required.

SCL ────┬─■(Pull-up)─ Vcc
SDA ────┤

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3) Roles: Master & Slave

A single I2C bus can have multiple slave devices, each addressed individually.

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4) Data Transfer Rules

I2C communication follows this sequence:

START
→ Slave Address (7-bit or 10-bit)
→ R/W bit (0 = Write, 1 = Read)
→ ACK
→ Data (1 byte at a time)
→ ACK after each byte
→ STOP

Important: I2C transfers 1 byte (8 bits) at a time,

and the receiver responds with 1 additional bit:

Example: Writing value 0xAB to slave 0x50:

START
0x50 + Write(0) → ACK
0xAB           → ACK
STOP

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5) Speed Modes

Compared to SPI/QSPI, I2C is significantly slower.

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6) Advantages

• Very simple wiring (only two lines)

• Easy to add multiple slave devices (address-based)

• Low hardware cost (just pull-up resistors)

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7) Disadvantages

• Slow data rate

• Signal integrity can degrade with cable length or noise

• Open-drain signaling → less clean waveforms

• If ACK/NACK handling fails, the bus may lock up (hang)

Thus, I2C Bus Recovery routines are commonly required in real firmware.

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8) Comparison with SPI/QSPI

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9) One-Line Summary

I2C is a low-speed serial bus that uses only two wires and transfers data one byte at a time,
with the receiver sending an ACK/NACK bit after each byte.

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