Abstract: Crystal oscillators, ceramic resonant tank circuits, RC (resistance, capacitance) oscillators and silicon oscillators are the four clock sources suitable for microcontrollers (µC). Optimizing the clock source design for specific applications depends on the following factors: cost, accuracy, and environmental parameters. This application note discusses various factors related to microcontroller clock selection and compares different types of oscillators.
Please also refer to: Overview of Microcontroller Clock Support Solutions Microcontroller clock sources can be divided into two categories: clock sources based on mechanical resonant devices, such as crystal oscillators and ceramic resonant tank circuits; clock sources based on phase-shift circuits, such as RC (Resistance, capacitance) oscillator. Silicon oscillators are usually fully integrated RC oscillators. In order to improve stability, they include clock sources, matching resistors and capacitors, and temperature compensation. Figure 1 shows two clock sources. Figure 1 shows two discrete oscillator circuits, of which Figure 1a is a Pierce oscillator configuration for mechanical resonant devices, such as crystal oscillators and ceramic resonator tanks. Figure 1b is a simple RC feedback oscillator.
Figure 1. Simple clock source: (a) Pierce oscillator (b) RC feedback oscillator The main difference between mechanical resonator and RC oscillator is based on crystal and ceramic resonator tank (mechanical) oscillators usually provide very high Initial accuracy and lower temperature coefficient. Relatively speaking, the RC oscillator can be started quickly, and the cost is relatively low, but usually the accuracy is poor over the entire temperature and operating supply voltage range, which will vary from 5% to 50% of the nominal output frequency. The circuit shown in Figure 1 can produce a reliable clock signal, but its performance is affected by environmental conditions and selection of circuit components and the layout of the oscillator circuit. The component selection and circuit board layout of the oscillator circuit need to be taken seriously. In use, the ceramic resonant tank circuit and the corresponding load capacitance must be optimized according to a specific logic series. The crystal oscillator with high Q value is not sensitive to the choice of amplifier, but it is easy to produce frequency drift (or even damage) during overdrive. The environmental factors that affect the operation of the oscillator are: electromagnetic interference (EMI), mechanical vibration and shock, humidity and temperature. These factors will increase the change in output frequency, increase instability, and in some cases, cause the oscillator to stop. Oscillator module Most of the above problems can be avoided by using an oscillator module. These modules have their own oscillators, provide low-impedance square wave output, and can guarantee operation under certain conditions. The two most commonly used types are crystal oscillator modules and integrated silicon oscillators. The crystal oscillator module provides the same accuracy as a discrete crystal oscillator. The accuracy of the silicon oscillator is higher than that of the discrete RC oscillator, and in most cases it can provide an accuracy comparable to that of the ceramic resonant tank circuit. Power consumption also needs to be considered when choosing an oscillator. The power consumption of the discrete oscillator is mainly determined by the power supply current of the feedback amplifier and the capacitance value inside the circuit. The power consumption of the CMOS amplifier is proportional to the operating frequency and can be expressed as the power dissipation capacitance value. For example, the power dissipation capacitance of the HC04 inverter gate is 90pF. When working under 4MHz, 5V power supply, it is equivalent to 1.8mA power supply current. Coupled with the 20pF crystal load capacitance, the entire power supply current is 2.2mA.
Ceramic resonant tank circuits generally have larger load capacitances and require more current accordingly.
In contrast, crystal oscillator modules generally require a supply current of 10mA to 60mA.
The power supply current of a silicon oscillator depends on its type and function, and can range from a few microamps for low frequency (fixed) devices to a few milliamps for programmable devices. A low-power silicon oscillator, such as the MAX7375, requires less than 2mA when operating at 4MHz. Conclusion In a specific microcontroller application, the selection of the best clock source requires comprehensive consideration of the following factors: accuracy, cost, power consumption, and environmental requirements. The following table gives several commonly used oscillator types and analyzes their respective advantages and disadvantages.
Table 1. Performance comparison of different types of clock sources
Please also refer to: Overview of Microcontroller Clock Support Solutions Microcontroller clock sources can be divided into two categories: clock sources based on mechanical resonant devices, such as crystal oscillators and ceramic resonant tank circuits; clock sources based on phase-shift circuits, such as RC (Resistance, capacitance) oscillator. Silicon oscillators are usually fully integrated RC oscillators. In order to improve stability, they include clock sources, matching resistors and capacitors, and temperature compensation. Figure 1 shows two clock sources. Figure 1 shows two discrete oscillator circuits, of which Figure 1a is a Pierce oscillator configuration for mechanical resonant devices, such as crystal oscillators and ceramic resonator tanks. Figure 1b is a simple RC feedback oscillator.
Figure 1. Simple clock source: (a) Pierce oscillator (b) RC feedback oscillator The main difference between mechanical resonator and RC oscillator is based on crystal and ceramic resonator tank (mechanical) oscillators usually provide very high Initial accuracy and lower temperature coefficient. Relatively speaking, the RC oscillator can be started quickly, and the cost is relatively low, but usually the accuracy is poor over the entire temperature and operating supply voltage range, which will vary from 5% to 50% of the nominal output frequency. The circuit shown in Figure 1 can produce a reliable clock signal, but its performance is affected by environmental conditions and selection of circuit components and the layout of the oscillator circuit. The component selection and circuit board layout of the oscillator circuit need to be taken seriously. In use, the ceramic resonant tank circuit and the corresponding load capacitance must be optimized according to a specific logic series. The crystal oscillator with high Q value is not sensitive to the choice of amplifier, but it is easy to produce frequency drift (or even damage) during overdrive. The environmental factors that affect the operation of the oscillator are: electromagnetic interference (EMI), mechanical vibration and shock, humidity and temperature. These factors will increase the change in output frequency, increase instability, and in some cases, cause the oscillator to stop. Oscillator module Most of the above problems can be avoided by using an oscillator module. These modules have their own oscillators, provide low-impedance square wave output, and can guarantee operation under certain conditions. The two most commonly used types are crystal oscillator modules and integrated silicon oscillators. The crystal oscillator module provides the same accuracy as a discrete crystal oscillator. The accuracy of the silicon oscillator is higher than that of the discrete RC oscillator, and in most cases it can provide an accuracy comparable to that of the ceramic resonant tank circuit. Power consumption also needs to be considered when choosing an oscillator. The power consumption of the discrete oscillator is mainly determined by the power supply current of the feedback amplifier and the capacitance value inside the circuit. The power consumption of the CMOS amplifier is proportional to the operating frequency and can be expressed as the power dissipation capacitance value. For example, the power dissipation capacitance of the HC04 inverter gate is 90pF. When working under 4MHz, 5V power supply, it is equivalent to 1.8mA power supply current. Coupled with the 20pF crystal load capacitance, the entire power supply current is 2.2mA.
Ceramic resonant tank circuits generally have larger load capacitances and require more current accordingly.
In contrast, crystal oscillator modules generally require a supply current of 10mA to 60mA.
The power supply current of a silicon oscillator depends on its type and function, and can range from a few microamps for low frequency (fixed) devices to a few milliamps for programmable devices. A low-power silicon oscillator, such as the MAX7375, requires less than 2mA when operating at 4MHz. Conclusion In a specific microcontroller application, the selection of the best clock source requires comprehensive consideration of the following factors: accuracy, cost, power consumption, and environmental requirements. The following table gives several commonly used oscillator types and analyzes their respective advantages and disadvantages.
Table 1. Performance comparison of different types of clock sources
| Clock Source | Accuracy | Advantages | Disadvantages |
| Crystal | Medium to high | Low cost | SensiTIve to EMI, vibraTIon, and humidity. Complex circuit impedance matching. |
| Crystal Oscillator Module | Medium to high | InsensiTIve to EMI and humidity. No addiTIonal components or matching issues. | High cost; high power consumption; sensitive to vibration; large packaging. |
| Ceramic Resonator | Medium | Lower cost | Sensitive to EMI, vibration, and humidity. |
| Integrated Silicon Oscillator | Low to medium | Insensitive to EMI, vibration, and humidity. Fast startup, small size, and no additional components or matching issues. | Temperature sensitivity is generally worse than crystal and ceramic resonator types; high supply current with some types. |
| RC Oscillator | Very low | Lowest cost | Usually sensitive to EMI and humidity. Poor temperature and supply-voltage rejection performance. |
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