Clock oscillators are a pillar of electrical engineering, and these components play a vital role in devices such as: Central processing units, communication buses, tone generators, frequency synthesizers, RF systems, and peripherals.
However, one of a clock oscillator circuit's primary and critical functions is to control the speed of the digital processors in a microcontroller. You can find clock oscillators in the internal circuits of a microcontroller, controlling how quickly the processor runs. Oscillator circuits can produce square, triangle, and sine waves which vary in frequency, phase noise, jitter, reliability, and drift. We'll discuss how these components work and highlight some of the different oscillator variations found inside or used in combination with microcontrollers (MCU).
Signal frequency
The main variation in MCU oscillator circuits is the frequency with which the oscillator produces a signal. This frequency is what allows for MCU overclocking, which is the manual changing of oscillator speed to increase MCU output performance for the sake of more power consumption. Oscillators that are internally integrated into the MCU are often RC-based, making them subject to frequency variability given temperature and load. Additionally, lower-quality MCUs may utilize large-tolerance components, which also impact frequency, frequency drift, and frequency reliability.
Oscillator Frequency Stability in Microcontrollers
High-quality MCUs can avoid the potentially negative pitfalls of internal RC-based oscillators and are incredibly reliable. These MCUs often use microelectromechanical oscillators with phased-locked loops (PLLs) to create stable high-frequency signals. These oscillator circuits are comprised of a reference (usually crystal) and a voltage-controlled oscillator, which are stable and unstable oscillators, respectively. These oscillators may be internally or externally integrated into the microcontroller.
Types of Oscillators: Common oscillator technology variations
Crystal oscillators (resonators) are made from high-quality quartz crystal wafers. These wafers serve as the reference oscillator in microcontrollers. A crystal oscillator can vary in size, but thinner crystal cuts offer higher frequency operation. For example, 0.15 mm thick quartz crystal may operate at 15 MHz. Crystal oscillator frequency and stability can depend on the:
- Type of crystal
- Geometric orientation of the crystal's cuts
- Crystal dimensions
The crystal sits between two electrodes, which creates a naturally oscillating signal. This setup utilizes the piezoelectric effect to create a temperature-dependent acoustic vibration. The electrode "harvests" the vibrations to generate the oscillation signal that the rest of the oscillator circuit uses. However, crystal oscillators create relatively slow oscillating signals, operating between 0-100 MHz. Therefore, you can't directly use them as an input signal for an MCU's high-frequency digital operations.
2. SAW oscillators
Surface acoustic wave (SAW) devices, also called SAW Oscillators (SOs), utilize interdigital transducers (IDTs). Placed on a piezoelectric substrate, these oscillators create SAW. These waves are capable of stable frequency ranges from 10MHz to 2Ghz and are dependent on the dimensions of the IDT, substrate material characteristics, and subsequently changes in the substrate due to external influences such as temperature.
MEMS oscillators are semi-stable but are based on microelectronic mechanical systems (MEMS). MEMS create oscillating signals when they are electrostatically triggered. These systems can vary in size and frequency, but they may also be subject to mass loads and other similar forces―which can influence stability.
4. Voltage-controlled oscillators
Voltage-controlled oscillators can operate at higher frequencies but are much less stable than other oscillator signals. Since electronics timing continues to demand higher speed and require a stable reference signal, you can find these oscillators working in tandem with crystal oscillators, MEMS oscillators, and amplifiers to form a PLL. PLLs are capable of frequency speeds in the gigahertz range.
You can frequently find clock oscillators in the internal circuits of a microcontroller. These oscillators control how quickly various functions of the MCU work. Regardless of the oscillator technology that they utilize, oscillators are fundamental in modern electronics.