Harris Microprocessor Design Clock Generators with COSMAC Micropr

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Harris Microprocessor Design Clock Generators with COSMAC Micropr

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AN6565.1
Harris Microprocessor
Design Clock Generators with COSMAC Microprocessor CDP1802
Author: Hillman
Introduction
Clock signal generation CDP1802 COSMAC Microprocessor simple straightforward. CDP1802 features static operation, single-phase clock input, on-chip oscillator amplifier make practical lowcost, highly stable, crystal-controlled oscillator clock generator. design external oscillators this purpose, crystal controlled, equally straightforward they require only minimal circuitry. addition oscillator amplifier, CDP1802 incorporates necessary start/stop logic on-chip. This application note describes clock generator designs suitable various applications. Crystal Oscillator Design basic oscillator circuit CDP1802 consists on-chip amplifier external feedback network illustrated Figure oscillation occur, gain amplifier times attenuation feedback network must greater than equal one. addition, total phase shift through amplifier feedback network must equal times degrees, where integer. Oscillations occur system which amplified signal returned phase amplifier after being attuned less than originally amplified. frequency stability oscillator primarily dependent upon phase-changing properties feedback network. Because their high inherent frequency stability, quartz crystals commonly used feedback network. parallel resonant oscillator circuit shown Figure phase angle type feedback network shown this figure extremely sensitive change frequency, condition necessary stable oscillation. equivalent resistance crystal fact zero (infinite change phase angle feedback circuit would cause change oscillator frequency. Therefore, oscillator highest stability, crystal should high possible. general, increases with increasing frequency.
OPTIONAL FEEDBACK NETWORK BUFFERED CLOCK (1/6 CD4069B)
CRYSTAL 180o PHASE SHIFT
OSCILLATOR AMPLIFIER CDP1802
VOUT
(GAIN (ATTENUATION FEEDBACK NETWORK 180o PHASE SHIFT
FIGURE PARALLEL RESONANT OSCILLATOR CIRCUIT
FIGURE BASIC OSCILLATOR CIRCUIT
crystal load capacitance, defined series Higher values crystal load capacitance generally improve frequency stability, also increase power consumption. choice equivalent load capacitance (usually specified crystal suppliers) only fixes
Copyright
Harris Corporation 1997 7-67
Application Note 6565
CHANGE FREQUENCY (f/V) PARTS MILLION
series capacitors value amplifier output capacitance should fixed. trimmer should connected parallel with, used place fixed output capacitor permit compensation variation stray capacitance circuit component values. required capacitance range oscillator trimmer capacitor determined variation oscillation frequency with load capacitance. total trimming range mainly function crystal characteristics. more detailed analysis, Reference Practical Oscillator Circuits amplifier, feedback network, crystal considerations discussed preceding paragraphs combined design crystal-controlled oscillator CDP1802. majority microprocessor applications require frequency oscillation exact require oscillator trimming. "untrimmed" crystal oscillator will within specified crystal frequency. most microprocessor applications following simple guidelines used. crystal should connected between terminals CDP1802. crystal frequencies between 100KHz 6.4MHz, feedback resistor should used parallel with crystal. Capacitors required value between 20pF 30pF each recommended improve stability. should noted that on-chip oscillator timing generator capable operating frequencies higher than microprocessor maximum operating frequency. reliable operation, crystal frequency must always less than equal maximum operating frequency specified CDP1802 data sheet. practical example, CDP18S020 Evaluation oscillator, consists feedback resistor 2MHz crystal, both connected parallel across terminals CDP1802. (Crystal: Part X023303; CL=15pF; Series holder, series HC330; made Turotel, Inc., 13402 Highway, Grandview, Missouri 68030.) Provisions oscillator capacitors made Evaluation Kit, their required. increase oscillator stability with respect supply voltage that obtained adding capacitors shown Figure
CRYSTAL 2MHz, 100K,
LOAD CAPACITANCE, (pF)
FIGURE STABILITY CDP1802 CRYSTAL OSCILLATOR FUNCTION LOAD CAPACITANCE VALUE
amplifier stability also depends upon value resistor feedback network. Figure shows relationship between feedback resistor value oscillator stability. curve indicates that adequate value feedback resistor.
CHANGE FREQUENCY (f/V) PARTS MILLION
CRYSTAL 2MHz, 100K,
0.01
FEEDBACK RESISTANCE,
FIGURE STABILITY CDP1802 CRYSTAL OSCILLATOR FUNCTION FEEDBACK RESISTANCE VALUE
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Application Note 6565
External Clock Generators low-frequency applications (less than 500kHz) costeffective approach external RC-controlled oscillators. Three simple RC-controlled oscillators that used clock CDP1802 shown Figure When external clock used high-noise environments, 20pF 30pF capacitor between terminal (XTAL) CDP1802 ground used increase microprocessor noise immunity.
(VTR) (VDD -VTR) (VDD VDIODE)2
selection should compatible with system requirements. capacitor should non-polarized have leakage. There upper limit either values maintain oscillation. However, should larger than inherent stray capacitance. must larger than output impedance COS/MOS device, which typically hundreds ohms. addition, with very large values some short-term instability with respect time noted. Based these considerations recommended values these components are: greater than 100pF, practical value greater than 10k, less than
(1/3 CD4069B)
FIGURE INVERTER TYPE OSCILLATOR (SEE REFERENCES
CD4047A
With large values circuit Figure 5(c) used. This circuit, because hysteresis, eliminates multiple output pulses caused noise input waveform. more detailed analysis, References Clock Buffering some applications desirable supply clock signal other system components. such cases loading oscillator circuit should minimized buffering clock through COS/MOS inverter, shown Figure loading presented inverter will mainly capacitive, about 5pF, usually neglected non-critical designs. buffer should located close crystal order minimize stray capacitance. When crystal oscillator being trimmed desired frequency, buffered clock technique should also used prevent oscillator from being loaded frequency counter.
FIGURE OSCILLATOR USING DIGITAL CD4047A (SEE REFERENCES
References
Harris documents available web, http://www.semi.harris.com/ Harris AnswerFAX (407) 724-7800. CDP1802 Data Sheet, Harris Semiconductor, AnswerFAX Doc. 1305. CD4047A Data Sheet, Harris Semiconductor, AnswerFAX Doc. 623. CD4093B Data Sheet, Harris Semiconductor, AnswerFAX Doc. 836. AN6086 Application Note, Harris Semiconductor, "Timekeeping Advances Through COS/MOS Technology", AnswerFAX Doc. 96086. AN6230 Application Note, Harris Semiconductor, "Timekeeping Advances Through COS/MOS Technology", AnswerFAX Doc. 96230. AN6267 Application Note, Harris Semiconductor, "Astable Monostable Oscillators Using Harris COS/MOS Digital Integrated Circuits", AnswerFAX Doc. 96267. MPM-201 Harris Semiconductor, "User Manual Harris CDP1802 COSMAC Microprocessor".
(1/4 CD4093B)
FIGURE SCHMITT-TRIGGER-TYPE OSCILLATOR (SEE CD4093B DATA SHEET) FIGURE THREE SIMPLE RC-CONTROLLED OSCILLATOR CIRCUITS SUITABLE EXTERNAL CLOCK CDP1802 MICROPROCESSOR (OUTPUT CONNECTED THROUGH EVALUATION P2-W)
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