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8/16/32-Bit Crystal Oscillator Basics Microcontrollers Application Note 8/16/32-Bit Crystal Oscillator Basics AP56002 Application Note V1.0, 2012-08 Microcontrollers Edition 2012-08 Published by Infineon Technologies AG 81726 Munich, Germany © 2012 Infineon Technologies AG All Rights Reserved. LEGAL DISCLAIMER THE INFORMATION GIVEN IN THIS APPLICATION NOTE IS GIVEN AS A HINT FOR THE IMPLEMENTATION OF THE INFINEON TECHNOLOGIES COMPONENT ONLY AND SHALL NOT BE REGARDED AS ANY DESCRIPTION OR WARRANTY OF A CERTAIN FUNCTIONALITY, CONDITION OR QUALITY OF THE INFINEON TECHNOLOGIES COMPONENT. THE RECIPIENT OF THIS APPLICATION NOTE MUST VERIFY ANY FUNCTION DESCRIBED HEREIN IN THE REAL APPLICATION. INFINEON TECHNOLOGIES HEREBY DISCLAIMS ANY AND ALL WARRANTIES AND LIABILITIES OF ANY KIND (INCLUDING WITHOUT LIMITATION WARRANTIES OF NON-INFRINGEMENT OF INTELLECTUAL PROPERTY RIGHTS OF ANY THIRD PARTY) WITH RESPECT TO ANY AND ALL INFORMATION GIVEN IN THIS APPLICATION NOTE. Information For further information on technology, delivery terms and conditions and prices, please contact the nearest Infineon Technologies Office (www.infineon.com). Warnings Due to technical requirements, components may contain dangerous substances. For information on the types in question, please contact the nearest Infineon Technologies Office. Infineon Technologies components may be used in life-support devices or systems only with the express written approval of Infineon Technologies, if a failure of such components can reasonably be expected to cause the failure of that life-support device or system or to affect the safety or effectiveness of that device or system. Life support devices or systems are intended to be implanted in the human body or to support and/or maintain and sustain and/or protect human life. If they fail, it is reasonable to assume that the health of the user or other persons may be endangered. Crystal Oscillator Basics AP56002 Device1 Revision History: V1.0 2012-08 Previous Version(s): Page Subjects (major changes since last revision) – This is the first release … Trademarks We Listen to Your Comments Is there any information in this document that you feel is wrong, unclear or missing? Your feedback will help us to continuously improve the quality of this document. Please send your proposal (including a reference to this document) to: [email protected] Application Note 3 V1.0, 2012-08 Crystal Oscillator Basics AP56002 Table of Contents Table of Contents 1 Introduction . 5 2 Oscillator-Inverter . 5 3 Oscillator Circuitry . 6 3.1 What is a Crystal? . 6 3.2 Fundamental Mode Circuitry . 7 3.2.1 External Load Capacitors . 7 3.2.2 On-Chip Load Capacitors . 8 4 Oscillator Start-up Time . 9 4.1 Definition of Oscillator Start-up Time (tst_up) . 9 4.2 Definition of Oscillator Off Time (toff) . 10 5 Drive Level . 11 5.1 Drive Current Measurement Method . 11 5.2 Drive Level Calculation for Fundamental Mode . 12 6 Start-up and Oscillation Reliability . 14 6.1 Principle of the Negative Resistance Method . 14 6.2 Measurement Method of Start-up and Oscillation Reliability . 15 6.2.1 General Description . 15 6.2.1.1 Quick Negative Resistance Test . 17 6.2.1.2 Comprehensive Negative Resistance Test . 18 6.3 Trouble Shooting . 19 6.3.1 Parasitic RC or LC Oscillation . 19 6.3.2 Pull down Resistor RX1 . 19 6.3.3 Feedback Resistor Rf . 20 6.4 Assessment of the Results . 20 6.5 General Hints . 22 7 Oscillator Circuitry Layout Recommendations . 23 7.1 Ground Island . 23 7.2 Ground Connection of the Crystal Package . 23 7.3 Ground Supply . 23 7.4 Avoid Capacitive Coupling . 23 7.5 Avoid Parallel Tracks of High Frequency Signals . 23 7.6 Correct Module Placement . 23 7.7 Layout Example . 24 8 List of Abbreviations . 25 Application Note 4 V1.0, 2012-08 Crystal Oscillator Basics AP56002 Introduction 1 Introduction This Application Note provides recommendations for the selection of quartz crystals and for the circuit composition for each oscillator. The cooperation between the IC oscillator and the quartz crystal does not always work properly because of problems in the composition of external circuits. This application note provides users with the appropriate knowledge to help ensure trouble-free oscillator operation. Note: The content of this document relating to measurements to find the right external circuits is generic information and can be used for all Pierce oscillators using an oscillator-inverter. 2 Oscillator-Inverter Microcontrollers include the active part of the oscillator, called the oscillator-inverter. For historical and evolutionary reasons, different oscillator-inverters are implemented in different Infineon microcontroller product families. The main differences are in: • oscillator gain • oscillation amplitude • oscillator supply voltage • frequency range XTAL1 is the oscillator-inverter input. XTAL2 is the output. Some devices include a 32 kHz oscillator. This is a real-time clock oscillator-inverter, where XTAL3 is the oscillator-inverter input and XTAL4 is the output. Note: Details about electrical parameters are described in the Data Sheet of each particular device. The on-chip oscillator-inverter can either run with an external crystal and appropriate external oscillator circuitry (also known as the passive part of the oscillator), or it can be driven by an external oscillator. The external oscillator directly connected to XTAL1, leaving XTAL2 open, feeds the external clock signal to the internal clock circuitry. The oscillator input XTAL1 and output XTAL2 connect the internal CMOS Pierce oscillator to the external oscillator circuitry. The oscillator provides an inverter and a feedback element, with the resistance of the feedback element typically in the range of 0.2 MΩ to 1 MΩ. Depending on the Infineon microcontroller family, the oscillator is either enabled at power-on or, because of power consumption issues, it is disabled at power-on and has to be enabled by user software. Note that also for reasons of power consumption, some oscillators allow gain to be reduced after stable oscillation of the oscillator circuitry. Note: The oscillator-inverter can be used in combination with quartz crystals and also with ceramic resonators. Application Note 5 V1.0, 2012-08 Crystal Oscillator Basics AP56002 Oscillator Circuitry 3 Oscillator Circuitry The standard microcontroller oscillator circuitry typically consists of • the on-chip oscillator-inverter, • a quartz crystal, • two load capacitors, • a series resistor RX2 to limit the current through the crystal. Use of this resistor depends on the crystal used and the required resonance frequency. The crystal is used as the system frequency reference, typically in the range from 4 MHz to 25 MHz (40 MHz). This reference frequency is used by the on-chip PLL to provide system and CPU frequencies higher than the crystal frequency. 3.1 What is a Crystal? A quartz crystal consists of piezoelectric material equipped with electrodes and housed in a hermetically sealed package. In an oscillator circuit the crystal is mechanically vibrating on its resonance frequency fOSC, and provides a stable reference oscillation signal to the microcontroller and is used as input reference clock. The resonance frequency of a quartz crystal is the rate of expansion and contraction of the crystal and is determined by the size and cut of the crystal. Depending on the crystal resonance frequency, different cuts in the crystal structure are used for the crystal blank. AT-cut crystals vibrating in thickness shear, fundamental mode are typically used for microcontroller oscillator circuits for the frequency range from 4 MHz to 40 MHz. Real-time clock oscillators with 32 kHz resonance frequency use tuning fork crystals. Quartz crystals used in oscillator circuits are produced synthetically in autoclaves. When a crystal is running on its resonance frequency an equivalent circuit diagram can be used for simulation and calculation. C1 L1 R1 Q C0 Figure 3-1 Equivalent Circuit of a Quartz Crystal • The oscillating mass of the crystal hardware blank represents the dynamic inductance L1. • The elasticity of the crystal hardware blank represents the motional dynamic capacitance C1. • The molecular friction, the damping by the mechanical mounting system and acoustical damping by the gas filled housing, is represented by R1. •C0 is the shunt capacitance which is given by the electrodes on the quartz crystal. The total frequency deviation of a quartz crystal is effected by different factors such as temperature range or Application Note 6 V1.0, 2012-08 Crystal Oscillator Basics AP56002 Oscillator Circuitry capacitive load, and is typically in a range of |+/-100 ppm| to |+/-300 ppm| for microcontroller applications using AT-cut crystals. Table 3-1 Typical AT-Cut Quartz Crystal Characteristics Initial frequency tolerance at 25°C +/-30 ppm Pullability +/-15 ppm/pF Temperature stability +/-0.5 ppm/°C Aging +/-2.0 ppm/Year 3.2 Fundamental Mode Circuitry The external crystal circuitry can be prepared for fundamental mode or 3rd overtone mode. For a microcontroller frequency range from 4 MHz to 40 MHz, crystals are used in fundamental mode. 3.2.1 External Load Capacitors Most of today’s microcontroller oscillator circuitries use external load capacitors. Besides the typical oscillator components, a test resistor RQ may be temporarily inserted to measure the oscillation allowance (negative resistance) of the oscillator circuitry. The circuitry is shown in Figure 3-2. How to check start-up reliability is discussed in the chapters that follow. Oscillator Module Shaper fosc Rf XTAL1 XTAL2 external Components Crystal R RQ X2 CX1 CX2 Figure 3-2 Oscillator Circuit Block Diagram with External Load Capacitors (Fundamental Mode) Application Note 7 V1.0, 2012-08 Crystal Oscillator Basics AP56002 Oscillator Circuitry 3.2.2 On-Chip Load Capacitors New designs of oscillator-inverters offer the possibility to use on-chip load capacitors to save external components and optimize the BOM (Bill Of Materials). The on-chip load capacitors should only be used in combination with the Amplitude Regulation Feature of the oscillator.
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