What is I2C?
I²C uses only two bidirectional open-drain lines, Serial Data (SDA) and Serial Clock (SCL), pulled up with resistors. Typical voltages used are +5 V or +3.3 V although systems with other, higher or lower, voltages are permitted.
The I²C reference design has a 7-bit address space with 16 reserved addresses, so a maximum of 112 nodes can communicate on the same bus. The most common I²C bus modes are the 100 kbit/S standard mode and the 10 kbit/S low-speed mode, but clock frequencies down to DC are also allowed. Recent revisions of I²C can host more nodes and run faster (400 kbit/S Fast mode, 1 Mbit/S Fast mode plus or Fm+, and 3.4 Mbit/S High Speed mode), and also support other extended features, such as 10-bit addressing.
The maximum number of nodes is limited by the address space, and also by the total bus capacitance of 400 pF, which restricts practical communication distances to a few meters.
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