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I2C uses a sequence of ones and zeros for serial communication. SDA is used for the data bits while SCL is the serial clock that times the bit sequence. A logical one is sent when the SDA releases the line, allowing the pullup resistor to pull the line to a high level. A logical zero is sent when SDA pulls down on the line, setting a low level near ground. Figure 3-2 shows the representation of a digital one and zero for I2C communication.
The ones and zeros are received when SCL is pulsed. For a valid bit, SDA does not change between a rising edge and the falling edge of SCK for that bit. Changes of the SDA between the rising and falling edges of the SCL can be interpreted as a START or STOP condition on the I2C bus.