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There are many synchronous data transfer protocols. Here we introduce just two: I2C and SPI.
The I2C (Inter-IC) protocol is used with the I2C bus introduced by Phullips 20 years ago. It is still used for embedded applications. It operates in half duplex data transfer format and transmits in standard mode at 100 Kbits/sec. Half duplex means that data on the same wire goes different directions at different times. The I2C protocol is simple and can be implemented on simple and small microprocessors using software or hardware.
The SPI (Serial Peripheral Interface) can be either half duplex or full duplex. It can transfer as fast as 2.5 Mbits per second. SPI control is more complex and is usually implemented in hardware.