Are there well-established terms for clock sources used with microcontrollers, to distinguish whether microcontroller CLKOUT is required?

Question

What are the best terms for crystals/resonators/oscillators used with microcontrollers that cover each of the following groups:

• clock sources that require an additional oscillator circuit (NOT gate) of the type found in many microcontrollers, and are therefore hooked up to both CLKIN and CLKOUT pins of a microcontroller
• clock sources that provide a clock signal without additional circuitry, and can therefore be hooked up to a single pin on the device (typically CLKIN or OSCIN) without requiring additional pins

Answer 1

Crystals are two-pin components and ceramic resonators are two-pin or three-pin components. Both are passive components in a sense that they don't have power supply inputs and thus don't oscillate by themselves, they need the oscillator circuitry that is typically provided inside the MCU. These can be used with a clock setting that requires "external crystal" or "external resonator" and must be connected with two pins to the MCU. If the MCU is capable of different clock settings, it must be configured to use this kind of clock source by turning on the internal oscillator circuitry.

If the MCU has a separate setting of accepting a square wave clock signal on a single input pin, this is typically just called "external clock". This selection also typically turns off the internal oscillator circuitry as it is not needed.

The square wave can however come from any source, like a "crystal oscillator" you can buy as a separate module that contains both the crystal and the oscillator circuitry and it takes in supply voltages and outputs square wave. Or any other circuit that produces square wave.

Some MCUs also have a setting for connecting external resistor and capacitor as an RC oscillator.