The operating speed in electronic circuits changes according to the operating (power supply) voltage used. In general, as long as the circuits are manufactured using the same process (such as the transistor size), the higher the voltage, the faster the circuit can operate (until now, the speed of operation depended on how fine the manufacturing process was).
For a long time, most logic circuits operated on 5 V. This gradually changed to 3.3 V, and now circuits can use a variety of operating voltages.
Most of today's microcontrollers using CMOS circuits, can operate on a wide range of voltages, from 1.8 V or 2.3 V up to 5.5 V.
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TTL logic ICs operates only on 5 V. However, normal CMOS logic ICs can operate on a wide range of voltages. Even obsolete CMOS products like the 4000 series were able to operate on a voltage range of 5 V to 15 V.
In early microcontrollers, the internal circuits used operating voltage of 12 V, and the external interface voltage was 5 V in order to enable connection with the TTL. In those days, since microcontrollers were just LSI devices with a simple CPU function and had to be combined with several external ICs, different power supplies were required according to parts in them. With the advent of single-chip microcontrollers, there is no longer any need to interface with external ICs, so the operating voltage can be selected freely. Some early single-chip microcontrollers had a P-ch MOS structure, which you rarely see these days, and operated on about 10 V. N-ch MOS and early CMOS single-chip microcontrollers operated on 5 V. As process technology became finer, the operating voltage became lower-some microcontrollers today operate on as low as 1.8 V.
Consequently, there are now microcontrollers in which the highest operating frequency can be determined according to the operating voltage. Unrelated to the highest operating frequency, the frequency at which the oscillator can be oscillated differs depending on the operating voltage.
The microcontroller's operating clock can now be selected from among several input (oscillated) clocks divided using different division ratios. Moreover, since the microcontroller usually operates using a clock with a low frequency and with low voltage while starting to apply the power, if the operating voltage can be detected, the clock with the frequency that best suits for your application can be selected by software, depending on the detected voltage.
On the other hand, the oscillation frequency is fixed according to the resonator used. In other words, in order to prevent the microcontroller from operating until the operating voltage reaches the right level for the oscillation frequency, the user must take some kind of hardware measure such as applying a reset.