The bus cycle is the period during which a bus is driven within an instruction cycle when a microcontroller executes an instruction. The basic functions of the microcontroller include I/O and memory input/output, calculation, and so on. Among these, input and output use bus driving, as a read cycle and write cycle, respectively.
In the latest microcontrollers, internal processing is performed at accelerated speed, resulting in a processing speed difference between the internal bus and the external bus. In the case of execution of instructions in the internal memory of such a microcontroller, instruction fetch is possible with just 1 bus cycle.
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A bus is a set of lines used to simultaneously transfer a group of data, and is named thus by analogy to the transportation vehicle "bus" on which many people can ride at a time. The term "bus" usually indicates address busses and data busses, which enable easy connection of multiple devices.