The two wires on the bus are called SDA (Serial Data Line) and SCL (Serial Clock Line), once a
transmission starts, slave devices synchronise on these signals, START, STOP, Data Change, Data
Capture and ACKnowledge conditions are carefully chosen for avoiding glitches.
START: SCL High, SDA Falling Edge,
STOP: SCL High, SDA Rising Edge,
Data Capture: after SCL Rising Edge,
Data Change: after SCL Falling Edge,
ACK: slave brings SDA to Low for the next
clock pulse,
NACK: slave leaves SDA at High for the next
clock pulse.
All devices have open drain outputs, so the bus lines have to be pulled up with appropriately
sized pull-up resistors (which affects the baud rate). These bring the bus lines to a high value
during idle times and devices communicate by bringing it down to low.
On the figure below, you can see a communication example where the master transmits a
command to the slave and then it proceeds to receive data from it. The master uses NACK signal
to stop the communication. The slave can pause the communication while it executes its
interrupts.
Figure 106 : Communication frames during an I²C transfer
1.2.
There are a few defined standard baud rates for this protocol but other frequencies are allowed.
The baud rate is limited by the pull-up resistor and the bus capacitance. There are four
categories of transmission speed for the bidirectional bus:
Standard-mode: bit-rate up to 100 kbits/s,
Fast-mode: bit-rate up to 400 kbits/s,
Fast-mode Plus: bit-rate up to 1 Mbits/s,
High-speed mode: bit-rate up to 3.4 Mbits/s.
It is harder to establish a bus for high bit rates as the bus capacitance and the pull-up resistor
become more of a problem, sometimes more complex components are needed like current
sources or switched resistor circuits.
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