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Analyzing the Behavior of an Oscillator and
Ensuring Good Start-up
This application note explains how an oscillator functions and which methods can be
used to check if the oscillation conditions are met in order to ensure a good start-up
when power is applied.
Oscillator Fundamentals
A microcontroller integrates on-chip an oscillator to generate a stable clock used to
synchronize the CPU and the peripherals.
Figure 1. Basic Oscillator Architecture
Noise
The basic architecture of an oscillator (regardless of its structure) is shown in Figure 1
and built around an amplifier, a feed-back and noise applied on Xtal1 input. The role of
each elements is explained hereafter:
•
Amplifier: Used to amplify the signal applied on Xtal1 and to lock the oscillations
exhibit Xtal2. The class A structure is the most popular but new ones are currently
used in order to optimize the consumption or other criterion,
•
Feed-back loop: Used to filter the output signal and to send it to the Xtal1 input.
The oscillator stability is linked to the bandwidth of the loop. The narrower the
filter, the more stable the oscillator. Crystals or ceramic resonators are generally
used because they have the narrowest bandwidth and efficiency for the stability of
the frequency.
Amplifier
Xtal1
+
Feed-back
Loop
Xtal2
G(f)
H(f)
80C51 MCU's
Application Note
Rev. 4363A–80C51–07/04
1
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Summary of Contents for Atmel 80C51
- Page 1 Analyzing the Behavior of an Oscillator and Ensuring Good Start-up 80C51 MCU’s This application note explains how an oscillator functions and which methods can be used to check if the oscillation conditions are met in order to ensure a good start-up when power is applied.
- Page 2 (a). Sometimes a resistor is inserted (b) between the amplifier output and the crystal in order to limit the power applied, avoiding the destruction of the crystal. Figure 2. Typical Oscillator Structures Xtal2 Xtal1 Xtal2 Xtal1 4363A–80C51–07/04...
- Page 3 Figure 4, consists of two resonant circuits: • C1, L1 and R1 is a series resonant circuit (fs), • In addition the series circuit, C0 in parallel forms a parallel circuit which has a parallel resonance frequency (fa) . 4363A–80C51–07/04...
- Page 4 The behavior of the crystal depends on the frequency and is summarized in Table 1. Table 1. Nature of the Impedance Versus the Frequency Frequency f < fs f=fs fs < f < fa f=fa f>fa Z(f) Capacitive Resistance Inductive Resistance Capacitive Phase(°) 4363A–80C51–07/04...
- Page 5 It should be noted that no oscillator structure is able to oscillate at the exact fa frequency. This is due to the high quality factor at fa and the difficulty to stabilize an oscillator at this frequency. 4363A–80C51–07/04...
- Page 6 It should be less than half of the maximum drive level. Excessive drive may cause correlation difficul- ties, frequency drift, spurious emissions, "ringing" wave forms, excessive ageing, and/or fatal structural damage to the crystal. 4363A–80C51–07/04...
- Page 7 G(f) Rout Xtal1 Xtal2 Xtal1 Xtal2 vout Cxtal2 Cxtal1 Figure 8. (a) Typical structure of a class-A amplifier. (b) Equivalent schematic. (c) Gain response. Next section explains the two specific amplifier areas needed to startup and lock an oscillator. 4363A–80C51–07/04...
- Page 8 Only a few microvolts or millivolts are needed but the startup time is inversely proportional to this level. Typical waveform of an oscillation is shown in Figure 10. Figure 10. Start and Lock of a Feedback Oscillator Vxtal2 Steady State Start and lock 4363A–80C51–07/04...
- Page 9 Considering the expression of fp, CL plays an important role to have the required oscil- lation frequency. CL is the loading capacitor used during the crystal calibration by the crystal manufacturer to tune the oscillator frequency. If an accurate frequency is 4363A–80C51–07/04...
- Page 10 This start-up condition depends on the product of the gain and feed-back but also on the frequency. The lock condition is controlled by the non-linear area of the amplifier output. The gain is automatically reduced while the output oscillation increased until a stabiliza- tion point is found. 4363A–80C51–07/04...
- Page 11 Xtal1 Xtal2 38pF Figure 16 plots the gain and the phase of the open-loop circuit. At 16.001MHZ the gain is greater than unity (38dB) and the phase is zero. The oscillation conditions are met ensuring a good oscillator startup. 4363A–80C51–07/04...
- Page 12 Phase > 0 G=-3dB G=0.3dB -21.4 .00100MHz 16.00188MHz 15.99503MHz 16.00796MHz V(VXtal1)/V(VXtal10)) DB(V(VXtal1)/V(VXtal10)) a) Cp1 and Cp2 are too big (56pF), b) R1 is too big = 40ohms. Table 3 resumes the case studies analyze with the spice model and tool. 4363A–80C51–07/04...
- Page 13 Amplifier Figure 18 shows in what conditions the oscillator will oscillate. To have an oscillation stable in steady condition, the lost of energy in the crystal has to be cancelled. This con- dition occurs when: R i n – 4363A–80C51–07/04...
- Page 14 - - - - - - - - - - - - - - - - - - - - × 6 28 gm is the amplifier gain. An example is given hereafter. The main characteristics of this case study is: • Amplifier: gm=0.01A/V, Cxtal1=5pf, Cxtal2=8pF, Cxtal3=5pf • Crystal: R1=80, L1=11.64mH, C1=8.5fF, C0=5pF 4363A–80C51–07/04...
- Page 15 Two methods have been presented to analyze and to check the oscillation condi- tions.They have shown the possibility to predict the added capacitors in versus the electrical characteristics of the crystal or resonator devices. It will help to specify the margin of the crystal and resonator devices. 4363A–80C51–07/04...
- Page 16 No licenses to patents or other intellectual property of Atmel are granted by the Company in connection with the sale of Atmel products, expressly or by implication. Atmel’s products are not authorized for use as critical components in life support devices or systems.
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