Machine cycle and clock cycle of avr microcontroller

Machine cycle: the time to read an instruction word from the memory. Simply put, it is the time taken to execute a single-cycle instruction. The AVR microcontroller uses the Harvard structure (data and program are stored separately) and a reduced instruction set. One machine cycle is equal to one clock cycle. That is, it only takes one clock pulse to execute a single-cycle instruction.

Clock cycle: The operation of digital circuits cannot be separated from CLK, which is the clock pulse. The period of the clock pulse is called the clock cycle. Since the speed requirements of each device or functional module are different, as you said, it is divided into CPU clock cycle, ADC clock cycle, etc. Usually, the clock cycle we refer to is the CPU clock cycle. The microcontroller usually uses an external crystal oscillator to generate the clock source, so there is a saying that the clock cycle is equal to the crystal oscillator period.

CPU clock cycle: The period of the clock pulse provided to the CPU. Since the CPU is usually the fastest, the clock source directly provides clock pulses to the CPU. In a system, the CPU clock period is the shortest. The CPU clock cycle is usually referred to as the clock cycle.

ADC clock cycle: The period of the clock pulse provided to the AD converter. AVR uses a successive approximation ADC. Under default conditions, the successive approximation circuit requires an input clock from 50 kHz to 200 kHz to obtain maximum accuracy. Since an ADC that is too fast will affect accuracy, it may even fail to work at a frequency of several MHz, and we do not have the advantage of such a fast speed, so the pulse generated by the clock source is divided and provided to the ADC. This frequency division process is achieved through a frequency divider (usually a counter).