AN1783 Determining Oscillator Start-up Parameters Freescale

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AN1783 Determining Oscillator Start-up Parameters Freescale

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AN1783
Determining Oscillator Start-up Parameters
Freescale Semiconductor, Inc.
Stuart Robb David Brook, East Kilbride, Scotland Andreas Rusznyak, Geneva, Switzerland
1.0, December 1998
Introduction
Many microcontrollers (MCUs) incorporate inverting amplifier with external crystal ceramic resonator Pierce oscillator configuration. This paper describes calculate minimum gain (transconductance) amplifier required ensure oscillation with specific external components, also measure amplifier transconductance establish whether minimum gain requirement met.
Oscillator Circuit
STOP Internal OSC1 External Components OSC2
Figure Standard Pierce Oscillator 1MHz Operation Figure shows standard Pierce oscillator configuration typically used MCUs frequencies range 1MHz 20MHz. oscillator pins labelled OSC1, OSC2 MC68HC05
Motorola, Inc., 1999
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Oscillator Circuit
MC68HC08 families MCUs EXTAL, XTAL, respectively, MC68HC11 MC68HC12 families. some MCUs (e.g. MC68HC05B MC68HC05X families), resistor integrated on-chip, which case external resistor required. This circuit applicable some members MC68HC12 family MCUs which employ power oscillator, e.g. MC68HC12D60.
Internal Circuit
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circuit internal shown simplified form NAND gate followed inverter. NAND gate inputs; connected called OSC1 other input connected inverted internal STOP signal. There conditions under which oscillator required start oscillating; when power applied (called power-on reset) other when STOP signal de-asserted. Following power-on reset, oscillation will start soon supply voltage, VDD, reached level where oscillator loop gain greater than unity. reliable operation, oscillator must oscillating time reached minimum specified operating value. Most MCUs have power STOP mode. STOP mode entered when software executes STOP command result STOP signal asserted stop oscillator. longer clocked only current consumed `leakage'. external interrupt reset release STOP signal allow oscillator re-start. remainder this paper will ignore STOP input treat NAND gate simple inverter. output signal OSC2 typically distorted sine wave whose amplitude even exceed supply rail voltages. following inverter provides additional voltage gain produce approximately square wave signal which turn drives internal clock generation circuitry.
External Circuit
current designs p-channel n-channel transistors inverter contribute approximately equally total gain provided that Vout VDD/2. Resistor ensures that this optimal condition oscillation start-up. circuit oscillate, there must positive feedback closed loop gain must greater than unity. Resistor results negative feedback which increases open loop gain requirement amplifier. usually made large possible minimise feedback whilst still overcoming leakage currents start-up. operation between 1MHz 20MHz value range typically used. humid dirty environments good practice lacquer oscillator components tracks after they have been cleaned prevent leakage
AN1783 Rev. MOTOROLA
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Freescale Semiconductor, Inc. Application Note
currents condensation dirt accumulating printed circuit board (PCB). Care should taken when laying components PCB. components should positioned close possible traces should kept short possible. other traces should kept away possible avoid coupling. often worthwhile surrounding components with shield trace connected ground careful create loops) ground plane. designer should ensure that input OSC1 preferably also output OSC2 placed between `quiet' pins carrying signals. ceramic resonator used with capacitors integrated into common package, manufacturer recommend optimal value resonator capacitors form resonant circuit. represent external capacitors stray capacitance parallel. stray capacitance should measured estimated included values used Equations
Freescale Semiconductor, Inc.
Figure Crystal Equivalent Circuit crystal ceramic resonator small signal equivalent circuit shown Figure called `series resistance', called motional series inductance capacitance, respectively. shunt capacitance, represents low-frequency parallel plate capacitance resonator stray capacitance crystal holder. Equations additional stray capacitance between OSC1 OSC2 pins should included into this value. Values particular crystal specified data sheet usually available from crystal manufacturer. order measure these values, manufacturer must apply signal crystal, i.e. values obtained particular level power dissipation crystal. However, start-up oscillator, only signal across crystal thermal (Johnson) noise power dissipation crystal extremely low. known that effective value increase power dissipated crystal decreases levels. maximum value therefore estimated crystal manufacturer. this estimated maximum value which should used equations
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Calculating Minimum Required Transconductance
Calculating Minimum Required Transconductance
V1.gm
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Figure Simplified Oscillator Equivalent Circuit Figure shows simplified small signal equivalent circuit oscillator. inverter modelled current source with output current equal V1.gm where input voltage transconductance inverter. total output conductance i.e. output conductances p-channel n-channel transistors inverter start-up. components resonant circuit have been described above. developed impedance resonance circuit comprising resonator capacitors given
(Eqn
where being frequency resonance. represents total capacitance parallel with series components resonator:
(Eqn
frequency oscillation given good approximation
(Eqn
quartz resonators term neglected. minimum transconductance required inverter sustain oscillation this circuit given approximately
AN1783 Rev. MOTOROLA
(Eqn
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Freescale Semiconductor, Inc. Application Note
gds, this simplified
(Eqn
validity this simplification checked measuring gds, described later this paper. RPQ, equation further reduced
(Eqn
Freescale Semiconductor, Inc.
(Eqn
Measuring Amplifier Characteristics
recommended circuit measuring transconductance amplifier shown Figure [2]. circuit simple implement should powered with reset held ensure amplifier stays active does execute code. unused inputs should connected left floating. Note that diagram correctly indicates that OSC1 OSC2 connected together. transconductance does vary significantly frequencies below oscillator's maximum design frequency. However, recommended measure transconductance intended operating frequency, effects stray capacitances will make measurements inaccurate. frequency range 10kHz 100kHz recommended. signal around 500mVpp less should used with terminating resistor coupling capacitor ensure that amplifier input output remain their linear region. essential that high impedance measuring instrument, such oscilloscope with capacitance, high input resistance probe (<1pF, >10M) used measure with respect ground. value resistance should
OSC1
OSC2
signal generator 500mVpp
Figure Measuring Amplifier Transconductance
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Measuring Amplifier Characteristics
transconductance amplifier depends process parameters varies with supply voltage temperature. Measurements should taken worst process parameter devices available) over expected range supply voltage temperature. worst case (lowest) figure expected combination minimum expected supply voltage highest expected operational temperature. Based measurement results, transconductance output conductance
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(Eqn
this stage unknown separate measurement must made determine then neglected. noticed that this method determining available takes into account reduction additional n-channel transistor which exists series with inverter real NAND gate implementation. recommended circuit measuring output conductance shown Figure with measurement transconductance, should powered held reset state. frequency range 10kHz 100kHz recommended again high impedance measuring instrument required.
OSC1
OSC2
Rm=1k
signal generator 500mVpp
100n
Figure Measuring Amplifier Output Conductance determine available gain worst case output conductance should measured under same conditions under which minimum value gm+gds been determined. circuit Figure signal input inverter practically zero, which enables output conductance measured:
(Eqn
that known, calculated subtracting (eqn from gm+gds (eqn addition, validity simplification Equation checked.
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lowest value found within expected range supply voltage temperature called worst case value gmwcs. oscillation `gain margin' evaluated calculating ratio worst case transconductance minimum required transconductance calculated from Equations
gain margin
(Eqn
Freescale Semiconductor, Inc.
gain margin must greater than unity oscillator oscillate general rule thumb, gain margin greater than would considered reasonable ensure reliable start-up operation. insufficient gain margin found, main options are: Reduce size capacitors (shouldn't less than 10pF), different resonator with lower series resistance. summary, four steps required check reliable oscillator start-up: Measure worst case output conductance, Measure worst case transconductance, Calculate minimum required transconductance, Calculate gain margin.
Example
following measurements were made with 4.5V 23°C. Using circuit Figure 0.496 Vpp, 0.478 From Equation
0.496 0.478 0.478 1000
Using circuit Figure 0.488 Vpp, 0.448 From Equation
0.488 0.448 0.855 0.448
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following data obtained crystal: 8MHz 2.0pF 7.0fF stray capacitance parallel resonator will assumed well parallel resistance this parallel resistance integrated on-chip). 15pF represent 12pF capacitors combined with stray capacitance on-chip structures, package capacitances traces. With these values minimum transconductance calculated with Equation
Freescale Semiconductor, Inc.
0.149
0.855 gain margin 0.149
This result indicates sufficient gain margin 23°C, would advisable measure transconductance highest expected operating temperature verify gain margin.
References
A.Rusznyak: Start-Up Time CMOS Oscillators (IEEE Trans. Circuits Systems, March 1987, 259.268). P.Renard: Problem Oscillator Start-Up (Motorola Internal Report, System Eng. Group Geneva, Oct. 1996).
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reach USA/EUROPE/Locations Listed: Motorola Literature Distribution, P.O. 5405, Denver, Colorado 80217, 1-800-441-2447 1-303-675-2140. Customer Focus Center, 1-800-521-6274 JAPAN: Motorola Japan Ltd.: SPD, Strategic Planning Office, 141, 4-32-1 Nishi-Gotanda, Shinagawa-ku, Tokyo, Japan. 03-5487-8488 ASIA/PACIFIC: Motorola Semiconductors H.K. Ltd., Ping Industrial Park, Ting Road, N.T., Hong Kong. 852-26629298 MfaxTM, Motorola Back System: RMFAX0@email.sps.mot.com; http://sps.motorola.com/mfax/; TOUCHTONE, 1-602-244-6609; Canada ONLY, 1-800-774-1848 HOME PAGE: http://motorola.com/sps/ Mfax trademark Motorola, Inc. Motorola, Inc., 1999
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AN1783/D

AN1783 Determining Oscillator Start-up Parameters Freescale

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