TN235 External 32.768 kHz Oscillator Circuits
Contents
1. The oscillator is constructed using low cost single gate logic An unbuffered gate is used for the oscillator because buffered inverters have a tendency to oscillate at higher frequencies and are prone to startup problems The output of the oscillator is fed to the Rabbit through a Schmitt trig ger buffer The Schmitt trigger serves two primary functions First it prevents power supply or high frequency switching noise primarily from address lines from getting coupled into the slow rising clock signal generated by the oscillator and second it buffers the output of the oscillator to generate fast rising falling 4 ns square waves Internal and External 32 768 kHz Oscillators The 32 768 kHz oscillator circuit implemented in Rabbit based systems may vary depending on the Rabbit processor s revision and version low power requirements and the type of crystal used Table 1 lists the types of crystal oscillator circuits that can be used with each type of Rabbit microprocessor Table 1 32 768 kHz Crystal Oscillator Circuit Types 32 768 kHz Oscillator Internal Microprocessor Schmitt Internal External Trigger Rabbit 2000 A C Yes Yes Yes Rabbit 3000 No Yes No Rabbit 3000A No Yes Yes External oscillator is used in low power applications with battery backup The Schmitt trigger is part of the on chip oscillator buffer Note that the Rabbit 2000 family of microprocessors contain an internal 32 72. NOTE The 0 04ppm C parabolic curvature constant is a maximum value Actual tests of the crystal yield a drift of 140 ppm 12 13 seconds day at the temperature extremes 40 C and 85 C 022 0084 Rev E 10 Crystal Drive Level Typical 32 768 kHz crystals are specified for a maximum drive level of 1 uW A modest over drive perhaps 100 over this limit will most likely not have any adverse effect except to cause the crystal to age more rapidly Aging in a crystal is exhibited as a gradual change of frequency about 3 parts per million and is most significant in the first few months of operation The drive power can be computed from P I R where I is the rms AC current and R is the effective resistance of the crystal Typical values for R are 25 kQ for 32 768 kHz turning fork crys tals Maximum values are often specified as 35 kQ or 50 kQ If the effective resistance is 25 kQ then 1 uW of power is reached when I 6 3 uA rms It is logical to use the typical effective resistance rather than the maximum total resistance in computing drive power If a particular crys tal has a higher resistance it requires more power to sustain the same amplitude of physical flex ure of the quartz This indicates that the stress on the quartz will not be greater even though the drive power is greater for a unit that happens to have an effective resistance of 35 kQ rather than the typical value of 25 KQ In calculating the current through th
3. Rabbit Semiconductor has published an application note on con formal coating Technical Note TN303 Conformal Coatings 022 0084 Rev E 3 Figure 3 shows the external battery switchover circuits used in Rabbit 2000 based systems SRAM VRAM Chip Select T R11 100 KQ R10 NAVV ICSRAM 02 R7 RESET AM 22 kQ Q1 MMBT3904 Q4 2N7002 RESET Add resistor R8 to bypass Q2 Add resistor R10 to bypass Q3 Figure 3 Rabbit 2000x Battery Switchover Circuit 022 0084 Rev E Rabbit 3000 Based Oscillator and Battery Backup Circuits Figure 4 shows the external 32 768 kHz oscillator battery backup and battery switchover circuits used in Rabbit 3000 based systems found in Z World and Rabbit Semiconductor board level products VBAT CLK32K RESOUT RTC Circuit Power to VRAM Switch 3 3 V VRAM R1 NC7SP14 00 SN74AHC1GU04 U L C4 10 nF FDV302P 22 MQ RESOUT R Y1 330 kQ CL 7 pF Hi 32 768 kHz e C1 33 pF C1 values may vary or C1 may be eliminated 1 CONFORMAL D1 COATING AREA 14 VBAT_EXT 150 kQ BAT54 gt VRAM Figure 4 32 768 kHz Oscillator and Battery Backup Circuit for Rabbit 3000 Based Systems The circuit in Figure 4 consumes about 8 uA for a Rabbit 3000 with U2 present and VBAT_EXT 3 0 V Rabbit 3000A based systems have special power up requirements In t
4. ited by the operating voltage The value of R has to be large enough to prevent the crystal from being overdriven but not too large to kill the swing going back into the oscillator An excessively large R may also cause the circuit to oscillate at an overtone other than that of the fundamental frequency Moderate overdrive of the crystal may be acceptable However excessive overdrive may increase the aging of the crystal and may possibly damage the crystal It is somewhat difficult to predict a suitable value for R with which to begin As a starting point select a value for R such that it has the same impedance as C2 at the operating frequency From this point the value can then be modified to achieve the desired drive level or voltage swing 1 a 2nf C2 osc C1 C2 For parallel resonant circuits the phase shift load capacitors provide the phase shift and load capacitance necessary for the oscillator to operate at the tuned frequency The values of C1 and C2 can be modified to adjust the oscillator frequency 022 0084 Rev E 8 The value of the load capacitors can be calculated in the following manner C1 C C2 L ae eee t C1 C C2 In the above equation C represents the input capacitance of the oscillator buffer roughly 6 to 6 5 pF C represents the specified load capacitance of the parallel resonant frequency crystal and C represents the stray circuit capacitance which is usually in the range of 2 t
5. maximum operating voltage e The capacitors are used to tune the crystal frequency This is called pullability and is a function of the load capacitors R1 R2 For low power applications these two resistors limit the power consumption of the oscillator buffer U1 in Figure 4 by limiting the crossover current during switching The slower the switching speed the longer the transistors stay in the transition region and thus the greater the crossover current Note that the Schmitt trigger does not consume as much current because of its fast switch ing speed The key to controlling the current through the oscillator buffer is to limit the amount of switching current by placing resistors in series with the power and ground of the inverter These resistors not only limit the current but also affect the gain of the oscillator the startup and stop voltages the output duty cycle and output rise and fall times The circuit also becomes more sus ceptible to noise necessitating the use of the Schmitt trigger The layout of the oscillator circuit is therefore extremely important when dealing with such low current low gain high input impedance circuits The distances between the Rabbit processor oscillator buffer and Schmitt trigger must be minimized to prevent noise from getting coupled into the circuit 022 0084 Rev E 9 Crystal The 32 768 kHz crystal used in Rabbit based systems is the same type of crystal as the tuning fork quartz cryst
6. 00A Based Systems 022 0084 Rev E Component Selection Guidelines R The bias resistor R biases the oscillator buffer amplifier to operate in the linear region Vpp 2 When biased this way the amplifier has a high gain and will oscillate at the specified frequency The recommended value for Rp is between 10 MQ and 25 MQ As the value of Rp increases the gain of the amplifier will also increase enabling the oscillator to start faster and continue operat ing at a lower voltage R also limits the short circuit current when the CMOS gate is switching and thus the overall cur rent consumption It is important to note that the 32 768 kHz oscillator circuit draws a very low operating current and has a high input impedance The circuit is thus susceptible to noise from nearby high speed switching traces and board level contaminants such as dirt and moisture It is therefore necessary to protect the oscillator circuit from high speed switching signals by keeping the oscillator traces short and using guard traces and copper pours appropriately Furthermore the exposed circuit traces should be conformally coated to protect the circuit from environmental contaminants Refer to technical Note TN303 Conformal Coating for more information R The purpose of R is to increase the output impedance of the oscillator buffer and limit its drive current R also affects the amplitude of the voltage swing going into the crystal and is thus lim
7. 68 kHz oscillator Refer to Chapter 14 of the Rabbit 2000 Microprocessors User s Manual for more information on circuit requirements The internal circuit does not offer the same flexibility as the external circuit for low power operation mainly because resistors cannot be placed in series with the power or ground of the oscillator to limit the switching crossover current The rest of this technical note will concentrate on external oscillator circuits 022 0084 Rev E 2 Rabbit 2000 Based Oscillator and Battery Backup Circuits Figure 2 shows the external 32 768 kHz oscillator and battery backup and battery switchover cir cuits used in Rabbit 2000 based systems The circuits were designed for low power operation EXT_OSC Rabbit 2000x LA1 pin 40 CONFORMAL R COATING AREA 330 kQ ie 32 768 kHz Figure 2 Rabbit 2000x 32 768 kHz Oscillator and Battery Backup Circuits The current consumption of the circuit is about 4 uA with a 2 V supply Using this circuit oscilla tion continues even when the voltage drops to 0 8 V and oscillation is still very strong at 1 2 V Note that the internal Schmitt trigger of the Rabbit 2000 family of processors does not operate reliably at voltages below 0 9 V Furthermore the oscillator should have its exposed circuit traces conformally coated to prevent the possibility of loading the circuit by conduction on the PC board surface in a moist atmosphere
8. Technical Note fw A Digi International Company External 32 768 kHz Oscillator Circuits An external 32 768 kHz clock is an essential part of any Rabbit based system Besides driving the real time clock the 32 768 kHz clock is used by various processor and peripheral subsystems that are used extensively by Dynamic C software It is therefore recommended that an external 32 768 kHz oscillator circuit always be implemented It is possible to operate the Rabbit without a 32 768 kHz clock but several key features will not be available Without the 32 768 kHz clock the real time clock the watchdog timer the periodic interrupt and asynchronous remote bootstrap will not function Neither will any of the low power features that run off the 32 768 kHz clock Figure 1 shows the basic concept behind the external CMOS crystal oscillator circuits used in Rabbit based products The crystal used in the circuit is a parallel resonant crystal VBAT R1 and R2 control the power consumed by the R1 unbuffered inverter SN74AHC1GU04 U1A U2A NC7SP14 C 5 12 pF I 32 768 kHz 2 C1 values may vary or TT C1 may be eliminated Figure 1 Basic 32 768 kHz Oscillator Circuit NOTE The value of C1 may vary from system to system or C1 may be completely eliminated depending on the crystal C the amount of frequency deviation from 32 768 kHz and the measured drive through the crystal 022 0084 Rev E 1
9. als used in wristwatches Table 2 outlines the specifications for these 32 768 kHz crystals Table 2 32 768 kHz Crystal Specifications Type Through Hole or SMD Tuning Fork Crystal Nominal Frequency F 32 768 kHz Frequency Tolerance at 25 C df F 20 ppm Load Capacitance CL 7 0 12 5 pF Series Resistance RS 50 kQ max Drive Level P 1 uW max Quality Factor Q 50 000 min Turnover Temperature TT 25 C 5 C Parabolic Curvature Constant K 0 04 ppm C max Shunt Capacitance Co 1 4 pF typical Capacitance Ratio Co Cl 400 typical Motional Capacitance Cl 0 0035 pF typical Aging df F First year 3 ppm max at 25 C Operating Temperature Range TO 40 C to 85 C Storage Temperature Range TS 50 C to 125 C Shock df F 5 ppm max Vibration df F 3 ppm max Cut X Cut X cut crystals have a parabolic temperature curve The maximum frequency variation in tuning fork crystals is roughly 0 04 ppm C The frequency tolerance at 25 C is typically 20 ppm Frequency drift per day at 85 C According to the parabolic temperature curve the change in frequency at 85 C is 144 ppm Since 1 day 86400 seconds 86400 seconds day 144 ppm 12 44 seconds day Frequency drift per day at 45 C According to the parabolic temperature curve the change in frequency at 45 C is 196 ppm 86400 seconds day 196 ppm 16 93 seconds day
10. circuit in Figure 4 is used for low power systems If a Rabbit 3000A based system is not battery backed and the oscillator power consumption is not an issue the circuit can be sim plified as shown in Figure 5 below C1 values may vary or C1 may be eliminated Vcc VBAT CONFORMAL COATING AREA SN74AHC1GU04 Rs 330 kQ 32 768 kHz C 7 pF Figure 5 32 768 kHz Circuit for Applications not Battery Backed For low power circuits an alternative circuit can be designed that does not exhibit the startup issue present in the standard circuit shown in Figure 4 022 0084 Rev E 6 The circuit in Figure 6 provides separate supplies for the oscillator VOSC SRAM VRAM and RTC VBAT The circuit consumes about 6 5 uA for VBAT_EXT 3 0 V and oscillation starts at 1 25 V This solution does not have the startup issue but is more expensive primarily because of the extra PMOS transistors 3 3 V CLK32K RESOUT CONFORMAL 3 m COATING AREA oe VBAT_EXT p 220 kQ C3 T 10 nF R A l D gt R2 22 kQ RESOUT VRAM R4 R l p 22 kQ 22 MQ R Coram 10 nF Y1 BE K5 J C 7 pF 3 3V 32 768 kHz s D ol 1 C2 C1 T 33 pF RESOUT S VBAT R5 S in j 330 kQ C1 values may vary or C4 C1 may be eliminated T 10 nF Figure 6 Alternative Low Power Circuit for Rabbit 30
11. crystals with a load capacitance of 7 pF Summary of Values for Rabbit Based 32 768 kHz Oscillators Component Value Notes Rp 10 25 MQ Affects gain R 330 680 KQ Limits drive current crystal drive level 1 uW CL 6 0 12 5 pF Parallel resonant crystal load capacitance The values can be used to tune the oscillator frequency and may Cl 0 15 pF vary depending on the crystal load capacitance used Appropriate P values can be determined through calculations and optimized through experimentation C2 15 33 pF R1 R2 2 22 KQ Approved Manufacturers List Component Manufacturer Part Number Contact ECS ECS 0327 6 17 http www ecsxtal com Crystal ILSI IL3R HX5F7 32 768 http www ilsiamerica com Seiko Instruments SSPT7 032768 7pF http www siielectroniccomponents com Texas Instruments SN74AHC1GUO4DBVR http www ti com irchi NC7SU04M5 Unbuffered Pence http www fairchildsemi com Semiconductor NC7SZU04P5 Inverter On NL17SZU04DF http www onsemi com home Semiconductor Schmitt Trigger Fairchild Semi NC7SP14P5 http www fairchildsemi com 022 0084 Rev E References Marvin E Ferking Crystal Oscillator Design and Temperature Compensation Van Norstrand Reinhold Com pany New York 1978 Benjamin Parzen Design of Crystal and other Harmonic Oscillators John Wiley and Sons Inc New York 1983 Norman L Rogers Rabbit Semicond
12. e crystal the output capacitance of the buffer is not relevant because the resistor R isolates it from the crystal C1 however is very important If C1 is made smaller this will increase the voltage swing on the gate input of the oscillator buffer and will allow the oscillator to operate at a lower voltage This oscillator will start at about 1 2 V and oper ate down to about 0 75 V The current can be measured directly with a sensitive current probe but it is easier to calculate the current by measuring the voltage swing at the gate input with a low capacitance oscilloscope probe The rms voltage at this point is related to the rms current by the relationship T Vims O Crot where Crot C1 Cin Corobe 27 32768 Vims 0 707 V p p If Cio 12 pF assuming C 1 pF and the effective resistance is 25 KO then the current in uA and the drive power in uW are given by the following approximation 1 2 5 Vins PSO1 W aio or I 1 75 V p p P 0 05 V pp Based on the above equations and calculations P 1 25 uW for a 5 0 V p p swing P 0 65 uW for a 3 6 V p p swing and P 0 45 uW for a 3 0 V p p swing 022 0084 Rev E 11 From the above analysis it is clear that the value of C1 greatly affects the crystal drive level The value of C1 depends on the crystal load capacitance C For this reason Rabbit based systems use crystals with low C requirements Currently Rabbit 2000 and 3000 based systems use
13. hese systems the oscillator may not start oscillating when the battery is connected for the first time The input to the internal Schmitt trigger gets stuck in a region where the Schmitt trigger is unable to latch the data high or low Since the oscillator is not running the output gets stuck somewhere in the linear region because of R This cycle continues until some amount of random noise disrupts the stability of the system and kick starts the oscillator The stuck condition results in a drop in the battery volt age and an increase in current draw For the circuit in Figure 4 the current draw measured at R8 increases to 13 uA with the majority of the current going through VBAT This occurs because R8 is large and is used to provide current to the SRAM oscillator and VBAT The Schmitt trigger requires a large amount of current at startup and R8 limits the amount of current available to the circuit 022 0084 Rev E 5 This is not a problem with the circuit in Figure 4 because powering a system only at VBAT_EXT for a prolonged period doesn t make any sense and is not normally done If for some reason a sys tem is only powered at VBAT_EXT the first time for a long period of time the current draw will not drain the battery significantly Once main power is applied to the system the oscillator begins operating and when main power is removed the circuit will switch over to the battery and will continue to operate reliably Note that the
14. o 5 pF Note that Cin is not constant but rather is a function of frequency any measurements of Ci should be done using a sine wave generator operating at 32 768 kHz Ideally C1 and C2 would have equal values because the inverter output introduces a phase shift of 180 and the combination of C1 C2 and the crystal would provide the additional 180 phase shift required for the phase shift of the loop to equal 360 However in reality the inverter also intro duces a phase delay which creates a phase shift that is somewhat greater than 180 The capaci tors compensate for this phase difference by changing their impedance This change in impedance can only occur if the circuit oscillates at a slightly higher frequency than that of the series resonant frequency of the crystal which is about 32 765 kHz In effect the capacitors pull the oscillation frequency The capacitors serve several functions e First and foremost they provide the appropriate load capacitance for the crystal to oscillate at the correct frequency e The capacitors provide the correct amount of phase shift for the circuit to oscillate Note that oscillation will not occur if the loop gain is not greater than and if the loop phase shift does not add up to 360 e The RC circuit and the input capacitance of the oscillator buffer control the swing into the buffer and the input side capacitance also affects the crystal drive This affects the power con sumption and the
15. uctor David Salt HY Q Handbook of Quartz Crystal Devices Van Norstrand Reinhold UK Co Ltd 1987 Z World Inc 2900 Spafford Street Davis California 95616 6800 USA Telephone 530 757 3737 Fax 530 757 3792 www zworld com Rabbit Semiconductor 2932 Spafford Street Davis California 95616 6800 USA Telephone 530 757 8400 Fax 530 757 8402 www rabbitsemiconductor com 022 0084 Rev E 13
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