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For instance, this tutorial has these two seemingly contradictory quotes:

Because serial ports are asynchronous (no clock data is transmitted), devices using them must agree ahead of time on a data rate.

and:

Each I2C bus consists of two signals: SCL and SDA. SCL is the clock signal, and SDA is the data signal.

Can someone help me to understand how these are compatible statements? How can it have a clock signal without transmitting clock data?

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  • \$\begingroup\$ "serial port" without further qualification usually means a UART or RS-232 port, neither of which have clock lines. An I2C port is not a UART or RS-232 port so it's not a contradiction that I2C ports have clock lines. \$\endgroup\$ Jan 1, 2017 at 13:32

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Serial ports exist in both synchronous and asynchronous forms. I2C is synchronous but the more familiar UART serial is asynchronous.

Both have their own advantages. Synchronous serial ports allow for arbitrary timing, while asynchronous ports require precise timing but use less connections.

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The article you linked toward is trying to compare similarities and differences between three different serial techniques -- the common "Serial Port" that computers used extensively up to a few years ago, the complex Serial Peripheral interconnect, which "every" DSP supports, and I2C, a "faster than Serial Port, but slower than SPI" synchronous serial interconnect technology.

The article was not particularly clear in explaining that "not all serial connections are Asynchronous Serial Ports".

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The asynchronous serial ports may be better known as "RS232 serial ports" or "COM Ports" on PCs - these are normally used for communication between different pieces of equipment.

The I2C and SPI synchronous buses are used between components on the same PC board, or possibly between separate PC boards within a piece of equipment.

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