AN96103 X-tal oscillators on 8
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1. ohm No oscillation area Rx Oscillation area CO pF Figure 5 5 The drive level issue Once we have oscillation is there something else to worry about Most of the information so far is based on experience with the standard 5 Volt microcontrollers Their oscillator stage was designed to drive a crystal with about 1 mW This means that the crystal itself should be specified to perform proper resonation when it is driven at this power level This crystal parameter is called the drive level and itis specified with the crystal parameters Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 6 Oscillator circuits in practice All the 5V microcontrollers have a Pierce oscillator circuit Xtal1 Xtal2 that will oscillate with an external X tal and two capacitors This standard circuit is presented in most data sheets In some cases there may be good reasons to modify the standard circuit e g to reduce interference and or to compensate external influences 6 1 Reducing amplitude Reducing the oscillator amplitude is a frequently used approach for e g reducing interference The circuit figure 5 is based on the Recommendations to reduce the interference of 558 oscillator see References This circuit generates an almost sine wave shape signal on the Xtal2 pin This is achieved by using asymmetrical capaci tors and using the largest capa2. EXTERNAL OSCILLATOR RESONATOR PART FREQUENCY LOAD OPTION NUMBER MHZ SMD LEADED CAPACITANCE Cie C2 PF gmL CSA3 58MG310VA 3 58 Leaded 0 gmL CSBxxxxd 1 00 1 25 Leaded 100 gmL CSAxxxxMK 1 26 1 99 Leaded 30 gmL CSTxxxMG 2 00 2 44 Leaded 0 gmL CSTxxxMGW 2 45 6 00 Leaded 0 gmM CSTxxxMTW 6 01 8 00 Leaded 0 gmH CSTxxxMTW 8 01 13 0 Leaded 0 gmH CSTxxxMXWDC3 13 1 16 0 Leaded 0 Note 1 The xxxx in the muRata resonator part number corresponds to the frequency chosen In addition to the resonators presented Table 6 muRata is also offering resonators for SMD mounting including the new CSTCC MG series TABLE 7 Kyocera Recommended ceramic PXE resonators for PCF84Cxx and PCD33xx micros EXTERNAL OSCILLATOR RESONATOR PART FREQUENCY LOAD OPTION NUMBER MHZ SMD LEADED CAPACITANCE C1g 2 PF gmL KBR3 58MSATRPC10 3 58 Leaded 0 gmL PBRC3 58ARPC10 3 58 SMD 0 gmL KBR6 0MSA 6 0 Leaded 0 gmL PBRC6 0A 6 0 SMD 0 gmM KBR10 0MSA 10 0 Leaded 0 gmH KBR8 0M 8 0 Leaded 0 gmH PBRC8 0A 8 0 SMD 0 gmH KBR12 0M 12 0 Leaded 0 gmH KBR12 0M 12 0 Leaded 0 gmH KBR12 0M 12 0 Leaded 0 gmH KBR14 31MY 14 31 Leaded 0 gmH KBR16 0MY 16 0 Leaded 0 23 Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 TABLE 8 Kyocera Recommended cer
3. APPLICATION NOTE X tal oscillators on 8 bit microcontrollers AN96103 Philipe Semiconductors PHILIPS Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 Abstract lt Almost since the introduction of microcontrollers as electronic components there always has been an oscillator circuit on the device to make it work From application point of view only some external components were required to make it work However to make sure that it will always work required more effort This report is based on feedback from the market from customers applying 8 bit mircrocontrollers gt Philips Electronics N V 1996 All rights are reserved Reproduction in whole or in part is prohibited without the prior written consent of the copy right owner The information presented in this document does not form part of any quotation or contract is believed to be accurate and reliable and may be changed without notice No liability will be accepted by the publisher for any consequence of its use Publication thereof does not convey nor imply any license under patent or other indus trial or intellectual property rights Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 APPLICATION NOTE X tal oscillators on 8 bit microcontrollers AN96103 Author s Andre Pauptit Systems Laboratory Eindhoven The Netherlands amp Erik
4. 24 Tik asho the POCF8583 HTG s coros eod e ego RAR ek SOR A Sob Y SE e EORR Ro gue id 24 11 2 A few questions and answers on 8583 24 12 RMelerelcas peri Copies umet uer ete Tanga sh ae Wane cht ne aee Lei oe 25 Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 1 What this note IS and what it is NOT In the early days of electronics it was quite a challenge to design a circuit that did NOT oscillate All kinds of un for seen component characteristics were the main reason for this as well as limited knowledge of the oscillation phenomena Electronics have come a long way since those days and today components characteristics are well defined Oscillation has become a science and integrating oscillators even more On the subject of oscillation and of integration of crystal X tal oscillators there are many scientific publications and courses that provide high level knowledge on this subject This report will NOT repeat that In many digital circuits oscillator circuits are also integrated on the same chip just to provide the clock signal for the digital electronics Usually only the active parts of the oscillator part is embedded and not the passive fre quency determining parts These parts are usually traditional components supplied by other manuf
5. but in many microcontroller applications this is hardly an issue Be careful with micro s with embedded time critical hardware e g UART DTMF etc The parameters like Rx CO amp drive level are not always specified in the datasheet and these have tolerances on them that are considerably higher than with X tals For all this reasons a practical approach for applicating ceramic resonators is to connect them to a microcontrol ler and evaluate the signals at Xtal1 and Xtal2 pins This was done for a 87C750 and a 87C552 microcontroller and two resonator types from muRata CSA type and CSU type For more information on ceramic resonators and recommendations on resonators to be used together with the Philips low voltage microcontrollers see chapter 10 8 1 muRata CSA Ceramic resonators Figure 12 shows a microcontroller C750 using a muRata ceramic resonator CSA 12 0 MTZ Measuring the voltages shows that voltage on Xtal2 5 2 Vtt and Xtal1 4 2 Vtt and the waveform is almost a sine wave Indicating proper oscillation Replacing the resonator by a CSA16 0MZX type also shows proper oscillation with a little lower voltage levels VXtal2 4 8 Vtt VXtal1 3 8 Vtt muRata datasheets indicate that capacitors have to be changed according the fre quency when using an MZX type Detailed values however are not specified When C1 amp C2 are decreased to 15 pF the output voltage V Xtal2 increases to 5 2 Volts Applying a feedback resistor of 1MQ
6. PCD33xx PCF84Cxx 80CL51 P83CLxxx cores These microcontrol lers have a wide supply range 1 8 to 6 0 V for the PCD33xx and P83CLxxx families and 2 5 to 5 5 V for the PCF84Cxx family 10 1 Oscillator circuitry The on chip oscillator on these microcontrollers consists basically of an inverter stage which includes a feedback resistor and on chip load capacitors see Fig 14 In most applications a quartz crystal will be connected between XTAL1 and XTAL2 Alternatively a low cost ceramic resonator PXE or an inductor may also be used as a timing element Figure 14 to internal timing circuits VDD XTAL2 hee Bic I In the Power down Stop mode the oscillator is stopped and XTAL1pin is pulled HIGH The oscillator inverter is switched off to ensure no current will flow regardless of the voltage at XTAL1 To drive the device with an external clock source apply the external clock signal to XTAL1 and leave XTAL2 unconnected There are no special requirements on the duty cycle of the external clock since the input to the internal clocking circuitry is buffered by a flip flop The transconductance gm of the inverter stage can be mask programmed on certain types thereby optimizing the oscillator for a specific frequency range and resonator crystal and thereby achieving a minimum power consumption For the PCF84Cxx and certain PCD33xx micros three standard transconductance options referred to as gmL gmM and gmH can
7. amplitude or as clean a signal as the clock in pin For this and similar case here are some attention points 1 Make sure the connection to the crystal and the capacitor are as short as possible 2 Make sure there are no digital signals passing below the crystal and to the other wires 3 Check if the crystal has a higher series resistance as before 4 Check Vdd to Vss noise 5 Is the trimming capacitor connected to Vdd Is the trimming capacitor of good mechanical quality 11 2 A few questions and answers on PCF8583 RTC 1 Q Is is possible to drive the OSCI input externally with a 32 768 signal A Yes you can feed the OSCI input with 32k externally 2 Is there any data on the MAX Iddo at 3V A At 3V you should expect approximately 15 to 20 uA 3 In the data sheet under Features it lists Clock operating supply voltage 0 to 70C 1V to 6V Q Can the PCF8583 operate down to 40 deg C if the Vdd is a little higher for example between 2 5V to 6V 24 Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 A The PCF8583 operating between 0 to 70 degrees C and 1 to 6V means with a crystal connected The critical point here is the combination low voltage and low temperature When Vdd is 2 5V or higher there is normally no problem 12 References Integrated X tal Oscillators Philips Research Laboratories Centre for Technical Training Recommendations to reduce the interf
8. and or drive other circuits 9 2 Low drive level Miniaturisation of components goes on also for crystals Nowadays several crystal manufactures offer very small SMD type crystals These kind of crystals usually have a considerable lower drive level then the commonly used components Most 5 Volt micro s are driving 1 mW drive level crystals while modern crystals allow only 50 100 uWatts These kind of crystals should not be driven by a 5 Volt microcontroller without modifying the cir cuit By putting to much power to the crystal it will degrade faster The standard circuit can be modified to limit the voltage over the crystal by using series resistor or even shottky diodes in parallel 9 3 ACO Amplitude Controlled Oscillator is another development in embedded oscillator circuitry These designs are capa ble of maintaining a constant amplitude level of the oscillator output signal This way it allows tolerances on crys tals or ceramic resonators without affecting the output signal Another important feature of the ACO is that it minimizes the power consumption of the oscillator by adapting the transconductance to the crystal resonator used Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 10 Oscillators on the low voltage microcontroller families In addition to 5V microcontrollers mainly 80C51 Philips Semiconductors also offers a range of low voltage microcontrollers based on the 8048
9. be chosen from by the user depending on the device selected The transconductance options for the P83CLxxx micros types 8xCL31 51 410 580 781 782 only are named OSC1 OSC2 OSC3 and OSCA Examples of the transconductance values for the various options are shown in table and The required option is to be stated directly on the type specific order entry form when ordering With C1i C2i 8 to 10 pF typical values internal load capacitance external capacitors are mostly not required if a quartz is used However for adequate frequency stability ceramic PXE resonators may in certain cases need 19 Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 external capacitors in addition They would be in the order of the static resonator capacitance CO such as external 1 C2e 20 to 100 pF The exact value depends on the recommendations made by the resonator manufacturer 10 2 Considerations for oscillation For proper oscillator start up the transconductance gm of the inverter stage must fulfil relationship 1 and 2 shown below In this context see also Table 1 and Figures 14 and 16 a3 nos xm 1 Im lt x9 69 Cp 9 C1xC2 2 ES 2 1 Ry Cy C 9 Rp X tal _ Co Ee ces Rx Figure 15 Table 1 Notation to relationship see Figs 14 and 15 SYMBOL DEFINITION Rx reso
10. the oscillator stage Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 This tranconductance indicated with gm is defined as the amount of current change as a result of the input volt age change The unit is A Volt or S S Siemens pa Figure 2 Rbias e 2 2 The Pierce oscillator The standard circuit for the oscillator is given in figure 3 A Crystal is connected between the output Xtal2 and the input Xtal1 Output and input have a capacitor connected to the ground This is basically a Pierce oscillator circuit With a proper dimensioning of the external components the circuit should generate an almost sine wave shape signal on the Xtal2 output pin One of the parameters related to the oscillator stage that affects the oscilla tion is the transconductance A certain value of gm is needed to assure start up During power on a noise signal or a transient should result in an amount of energy fed in to the X tal to make it start and resonate This is one of the basic requirements for any oscillator stage microcontroller Vdd Xtal1 Xtal2 48 X tal vss i 4 c2 Figure 3 mm Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 3 The external components The frequency determining element in the external components is the crystal X tal
11. Basically a crystal behaves as an LC circuit for serial resonance Figure 4 shows a commonly used equivalent circuit The resonant fre quency is determined by the value of L and C so this is series resonance CO represents the total parallel capac itance of the crystal and its value is usually much higher then the one of C However its influence on the resonating frequency is very small Some typical values for these equivalent components based on a 10 MHz crystal are L 0 01 H C 0 026 pF Rx 10 ohms Co 8 5 pF X tal Note that in an application the total equivalent value of Co is also highly influenced by the two external capacitors in the basic Pierce oscillator circuit figure 3 In fact the two capacitors in series shunt the crystal meaning the Co is in fact increased Figure 4 4 Oscillation condition An oscillator stage and external components are supposed the generate the clock signal Is just connecting the external components to the oscillator stage the only condition for oscillation Again there are theories on the oscillation condition and the Barkhausen rule covers the oscillation condition basics In a practical situation how ever there are many circuit parameters that will determine whether an oscillator circuit will show reliable oscilla tion Here are just some of them Vdd supply voltage fosc oscillator frequency gm oscillator stage transconductance Rx
12. Nordberg Telecom Products Group Zuerich Keywords lt 80C51 P83CLxxx P33xx P84Cxxx X tal Oscillator gt Date 20 september 1996 draft Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 Summary Designing in 8 bit microcontrollers occasionally raises questions regarding the crystal oscillator circuit The sup port groups in Eindhoven and Zuerich have gained some experience in responding to these kind of customer questions This report reflects some of this experience For engineers in the field as well as development engi neers involved in microcontroller based products that do not have specific oscillator knowledge reading this report may result in some awareness of the oscillator issues making it easier to approach questions on this sub ject Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 Contents 1 What this note IS and what itis NOT 7 2 Fhe oscillator stage o9 EE op OM a PY Gees Se Aven E Xr oe 7 2 1 Gus does hs ce pose ber Bee wees Boots Bet 7 2 2 The Pierce Oscillator uuu Row Sade OR amp deb eua tm eo dete 8 3 The external components aa sss 9 4 Oscillation 9 5 Thedrivelevelissue c oec amp da ERR
13. RR RUE inm a oe tos tee NEQUE ams ee Dd 10 6 Oscillator circuits in practice 11 6 1 Reducing amplitude xv e beer ede a ed Sce am peu 11 6 2 DG offSets 4 elu dS d E AIL e IN a NUM NEE da t R ay dd 11 6 3 Driving other 13 Emulation and the X tal oscillator ss 15 7 1 PDS 51 approach so doter aer ms ER Pe a bcd ad Sh M ars ee ay GO DAR 15 7 2 05 750 EB 51 approach les 15 8 CeramiciresonatorsS 330 5098 ek as PS ee BRO ee ee Bt ove A 16 8 1 muRata CSA Ceramic 16 8 2 muRata CSU Ceramic 17 9 Developments and future 18 9 1 sys ah ce La cay ofan ke ee bag TO a ahs AE P 18 9 2 Low drive devel dor at Bese ue be Eat Bie cet a deme 18 9 3 pco MEM 18 10 Oscillators on the low voltage microcontroller 19 10 1 Oscillator circuitty s espe Ge eg bag ta Ro eee Bare Ee be eee i 19 10 2 Considerations for oscillation 20 10 3 Example deae a dee UE Sa eS S Eu PED CERO D ree dg 22 10 4 Recommended resonators ll ll ee Rss 23 11 Related circuits with an X tal 5
14. across the resonator is recommended by the manufacturer and it slightly decreases the voltage swing A microcontroller doesn t really require this additional resistor because a feedback resistor is already inte grated 87 750 or 87c552 XTAL1 XTAL2 11 35 10 34 T 35 34 _ Vdd A 12 0MTZ 1 Vss C1 C2 L F 12 T30pF T 30pF 999 16 Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 8 2 muRata CSU Ceramic resonators The muRata CSU type resonators have three terminals and built in capacitors for realizing a Pierce oscillator cir cuit With the same microcontollers used and no additional capacitors this shows an equal performance as the circuit in figure 12 This probably means that internal capacitors have an equivalent value of about 30 pF for both 12 MHz and 16 MHz types Note that these resonators have defined input and output terminal figure 13 87 750 or 87c552 Xtal1 Xtal2 11 35 10 34 CSU CST type 8 IUI 1 3 Figure 13 2 These types of resonators are attractive because of the internal capacitors which saves components and PCB space in the target system However it should be considered that when applying these components this elimi nates the possibility for reducing the capacitor values which sometimes may be a solution in case the microcon troller is not oscillating Philips Se
15. acturers This situation is also valid for most currently supplied microcontrollers How to assure oscillation a very legal question from the application point of view This report IS based on application feedback from the field providing some background and practical knowledge to come closer to the ultimate answer to this question A crystal or quartz in a circuit diagram is very often indicated as X tal 2 The oscillator stage Most microcontroller devices have an oscillator circuit Xtal1 Xtal2 that will oscillate with an external crystal and external capacitors The oscillator stage is basically an inverter type gate consisting of a N channel and a P Vdd cm Rbias Xtal1 Xtal2 e ra Figure 1 Vss channel transistor The main difference with a digital inverter stage is an integrated bias resistor also called feedback resistor connected between output and input This semiconductor resistor feeds back the output voltage to the input which will balance bias the stage in its analog working area In the quiescent situation this will generate a DC input and output level of about 1 2 Vdd for CMOS devices For TTL compatible versions it is just a little less 2 1 Transconductance When this oscillator stage is driven with an input voltage variation then this will result in an output current varia tion through an external load This relation dVi dlo is defined as the transconductance of
16. amic PXE resonators for P83CLxxx micros EXTERNAL OSCILLATOR RESONATOR PART FREQUENCY LOAD OPTION NUMBER MHZ SMD LEADED CAPACITANCE C1_ C2 PF OSC2 KBR455BKTS 0 455 Leaded 220 OSC2 KBR455Y 0 455 SMD 220 OSC2 KBR3 58MSATRPC10 3 58 Leaded 0 OSC2 PBRC3 58ARPC10 3 58 SMD 0 OSC2 KBR6 0MSA 6 0 Leaded 0 OSC2 PBRC6 0A 6 0 SMD 0 OSC3 KBR4 0MSA 4 0 Leaded 0 OSC3 PBRC4 0A 4 0 SMD 0 OSC3 KBR8 0M 8 0 Leaded 0 OSC3 PBRC8 0A 8 0 SMD 0 OSC3 KBR10 0M 10 0 Leaded 0 OSC3 KBR12 0M 12 0 Leaded 0 OSCA KBR14 31MY 14 31 Leaded 0 OSCA KBR16 0MY 16 0 Leaded 0 11 Related circuits with an Xtal oscillator Besides microcontrollers there are other devices using embedded crystal oscillators Much of the story told in this app note is also valid for these circuits As an example some Questions amp Answers info on the PCF8583 Real Time Clock are provided below 11 1 using the PCF8583 RTC In many previous designs the PCF8583 was used with no problems Occasionally in new designs using the PCF8583 customers are having some problems The main problem is that the clock accuracy is drifting within a short time after building the product The calibration method used is measuring the frequency at the clock out pin this accounts for probe capacitance On previous designs this method was used with no problems Observing the signal at the clock out pin on an oscilloscope it seems noisy containing jitter and not as high an
17. citor on the INPUT XTAL1 In this situation the voltage on XTAL1 is decreased The input voltage for the amplifier stage is lower so the stage will not go into saturation and therefore produce a signal with much less harmonics XTAL1 XTAL2 Vdd XTAL1 XTAL2 Vdd 52 51 52 51 lt lt 4 470 470 Vss 560K MSS E E Figure 6 198 pF 18 pF Figure 5 a a dil C1 2 C2 A customer modified the circuit of figure 5 to the circuit of figure 6 Apparently the loop gain was changed by swapping the two capacitors and then compensated the by shunting a resistor to the X tal This circuit also works but generates an output signal with more distortion Increasing the output capacitor will result in more current through the output XTAL2 6 2 DC offsets The main reason for modifying the circuit with a capacitor in series with the input see circuit of figure 7 had to do with the influence of moisture on the oscillator circuit components This could effect the basic circuit of figure 5 because due to the conductance of moisture a DC offset is introduced to the XTAL1 input resulting in an asym metric bias of the oscillator stage which may affect start up and or event prevent the stage from oscillating When this is due to the PCB tracks and or oscillator circuit external components it could be eliminated by using an extra capacitor with a non critical value when i
18. equivalent resistor value CO equivalent total parallel capacity Closed loop gain Only for the crystal there are more then ten parameters For a practical evaluation it is almost impossible to include all of them A second reason for this is that many parameter values are not known to those engineers applying the component Does this mean that oscillation will be a matter of luck Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 Without going in much theoretical detail the impedance approach can be used to give an indication on the expected oscillation The idea behind it is that the effective transconductance 1 ohm should at least make up for the load ohm Figure 5 shows a characteristic that indicates a relation between the Rx CO and oscillation When the coordi nates of Rx and CO are in the upper area oscillation conditions are not met When the coordinates cross at the characteristic or are in the lower area there will be proper oscillation Now the difficulty arises in putting actual values on the Rx and CO axis as well as determining the right shape and position of the characteristic Practical experience however indicates that a circuit like in figure 3 with C1 C2 30 pF will be in the safe area when CO is about 10 pF and Rx is about 100 ohm or smaller A lower value of Rx allows a higher value of CO and visa versa
19. erence of 558 oscillator by Mr Schutte PCALE PDS51 Development System for Philips 80C51 and Derivatives Philips Semiconductors user manual P83CL434 Data Sheet Preliminary specification 1995 Mar 02 Philips Semiconductors AN456 Using LC oscillator circuits with Philips microcontroller William Houghton Application note 25
20. miconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 9 Developments and future Most information in this note so far is based on the basic 5V version oscillator stage As in many fields of the electronics industry developments keep continuing and microcontrollers are no exception A significant trend for microcontollers is to go to low voltage and low power A future that has in fact already started since Philips cur rently already supplies a range of low voltage microcontoller Chapter 10 handles more detailed with the X tal oscillator issues for the current range of low voltage microcontrollers Some other future developments related to the X tal oscillator are summarizes here 9 1 CCO A Current Controlled Oscillator is already applicated in a new microcontroller This type of oscillator is replacing the X tal oscillator in the P83C434 to generate the CPU clock This type of oscillator design doesn t use any external components and is fully on chip Since no bonding wires of the oscillator make a connection to the pack age it is extremely EMC friendly Its frequency is stable and it can be tuned by software The frequency values however are not fixed and vary from device to device So when used for critical timing the oscillator needs a ref erence In the P83C434 there is also a simple 32KHz oscillator for a low cost crystal Note however that these kind of oscillators can not be driven by an external signal
21. nator series resistance Co static resonator capacitance Ro resonator loss resistance Rp Ro Re Re feedback resistor C C1 x C2 C1 C2 where C1 C1 C1 and C2 C2 C2 int ext load capacitance Cr parasitic feedback capacitance typically 2 pF on chip external value depends on printed circuit board wiring 0 2 The oscillator options offered for the PCF84Cxx PCD33xx families are shown in table 2 Please note that for certain PCD33xx micros only one option corresponding to gmL is available Please refer to the product data sheet for the exact transconductance values available 20 Philips Semiconductors X tal oscil lators on 8 bit microcontrollers TABLE 2 Oscillator options for PCF84Cxx family and PCD33xx family Application Note AN96103 SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNIT OmL LOW transconductance Vpp 5 V 0 2 0 4 1 0 mS mM MEDIUM 09 16 32 mS transconductance HIGH transconductance 3 0 4 5 9 0 mS C1 C2 Input output capacitance 10 pF Re feedback resistor 0 3 1 0 3 0 MQ Figure 16 PCF84xx PCD33xx Typical transconductance as a function of supply voltage TABLE 3 Oscillator options for the P83CLxxx family OSCILLATOR APPLICATION Oscillator 1 For 32 kHz clock applications with external trimmer for frequency adjustment An external 4 7MQ bias resistor is needed for use in parallel with the cr
22. nt into the pulldown transistor it is recom mended to insert a resistor in the interconnection to the Xtal4 input Figure 11 shows this situation Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 microcontroller 1 microcontroller 2 Xtal4 Xtal3 Xtal4 Xtal3 100K 32K 2 561 c2 Figure 11 mm Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 7 Emulation and the X tal oscillator Developing microcontroller applications are usually carried out with the use of an emulator device In a carefully configured oscillator circuit around a microcontroller the microcontroller is removed and replaced by a connector with a cable connection to an emulating device which hopefully has identical oscillator stage characteristics Even when it has inserting a cable can dramatically influence the oscillator behaviour especially at higher fre quencies Two low cost emulators each have different ways to handle with this issue 7 1 PDS 51 approach In the PDS51 system this problem is approached by not using the target oscillator hardware The system has a separate oscillator circuit and the target X tal has to be removed from the application and then inserted on the PDS51 daughter board When the application requires the signal Xtal2 to be present on the target for driving other circui
23. s of the chosen muRata resonator are found in table 5 see also data book for muRata resonators The PCD3351A microcontroller has only one oscillator option available corresponding to gmL as shown in table 2 TABLE 5 Electric equivalent parameters of muRata CSA3 58MG310VA resonator FREQUENCY MHZ muRata 3 58 37 9 10 8 37 9 CSA3 58MG310VA Assuming RO of the resonator being very large yields a worst case RP RF 0 3 RESONATOR Co PF Ry Q Cy PF Assume also C1e C2e 0 pF i e no separate external load capacitances connected and C1i C2i 10 pF this yields a CL 5 pF according to Table 1 According to item 10 2 Considerations for oscillation equation 1 yields a minimum required transconductance gm min of 60 3 uS Equation 2 yields a maximum allowed transconductance gm max of 4 2 mS Looking at the actually provided transconductance of the PCD3351A option gmL in table 2 showing gmL min 0 2 mS and gmL max 1 0 mS it can be concluded that the chosen resonator fits the requirements based on the assumptions made above 22 Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 10 4 Recommended resonators An overview of various recommended ceramic resonators from the manufacturers Kyocera and muRata are given in the tables 6 7 and 8 TABLE 6 muRata Recommended ceramic PXE resonators for PCF84Cxx and PCD33xx micros
24. t is much larger then the value C1 and C2 see figure 7 Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 XTAL1 XTAL2 Vdd 52 51 4 Vss E 56 CiT pF 18 pF C2 Figure 7 Another possibility of reducing the sensitivity for environmental conditions like DC offsets is shunting the oscilla tor stage with a resistor rather than shunting the X tal This way the input sensitivity is reduced but also the loop gain It can also be used for amplitude reduction It may be necessary to modify the value of the C1 See figure 8 XTAL1 XTAL2 Vdd 52 51 4 0 56 1 470 Vss Cr 18 56 pF 18 pF Figure 8 C2 All these circuit examples were tested with a 559 device They all start oscillating already at about 3 Volts Vdd Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 6 3 Driving other circuitry Another frequently asked question rises when more microcontrollers are used in the same application and or microcontrollers are used with other IC s that require a clock signal In many of these cases it is possible to use one crystal only microcontroller 1 microcontroller 2 Xtal Xtal2 Xtal Xtal2 LCi C2 Figure 9 EXE Figure 9 shows this si
25. try there is a jumper to be set Note that the signal from the PDS51 is then usually a better square wave shape signal then the original Xtal2 output signal When the target microcontroller is not using its own oscillator but gets a signal on the Xtal1 input from an other source then this signal can be used for the emulator when it has TTL drive capability 7 2 DS 750 EB 51 approach Two other popular third party low cost emulator board are the DS 750 and EB 51 Both systems have several fixed internal oscillator frequencies of which one is to be selected by the user This will probably cover the used frequencies in most applications When the application uses a frequencies that is not available on these board then there is an option to go external meaning that the emulating microcontrolller on the emulator board will be using the oscillator components on the target application As stated before this means that there is a cable inserted between microcontroller oscillator stage and the external components In this situation be aware that the induction and capacitance values of cables and connectors can affect oscillator behaviour dramatically especially at higher frequencies as well as in those cases where the Xtal2 signal also drives other circuitry For other emulators there may be other restrictions Always consult the emulator documentation on this issue Resuming It can be concluded that the X tal oscillator usually needs some special attention
26. tuation when using the standard oscillator circuit The Xtal2 output micro 1 is directly con nected to the Xtal1 input of the second microcontroller The Xtal2 output from the second microcontroller remains not connected The circuit for the microcontroller 1 may be modified as described earlier in this report In these cases always connect the input of the microcontroller 2 to the Xtal2 output of microcontroller 1 rather than direct on the X tal This is to minimize the effects of the extra load on the oscillator circuit To avoid any DC offset for microcontroller 2 a DC offset may result in more asymmetric duty cycle of the signal on Xtal2 the connection between uC1 and 2 may be DC decoupled by using a capacitor Figure 10 shows the situation microcontroller 1 microcontroller 2 D Xtal f gt 100 pF N C 470 Figure 10 loi c2 1 Both circuits applicable for most microcontrollers Some controllers however have two on board oscillators one for a direct clock signal and one for a PLL clock In this last case the X tal frequency is normally 32 kHz This oscillator stage has pulldown devices on the input and output This means that transistors are added on chip which are switched on under certain conditions e g when the oscillator is disabled These transistor then connect Xtal1 and Xtal2 to the ground To avoid the flow of too much curre
27. when using an emu lator Many designers may be focused on the functionality of their application program so this issue may easily be overlooked Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 8 Ceramic resonators Another question often asked is Can the expensive X tal be replaced by an inexpensive ceramic resonator The main reason for this question is usually the price which may have considerable impact on total systems costs Probably not surprising but this question can hardly be answered by a simple yes or no Basically a ceramic resonator can be approached in the same way as the crystal described in the first part of this application note We can use the same equivalent circuit model see figure 4 page 8 Practical experience with ceramic resonators points out some differences in relation with crystals These differences are mainly the compo nent values in the equivalent circuit e g L C Rx CO and the determined by L and C is lower for a ceramic res onator than for a crystal The major differences between resonators and crystal are As already indicated the price is significantly lower than with conventional X tals and sometimes the smaller physical dimensions may reduce the required PCB space As the Q of a resonator is much lower than the X tal Q the oscillator stage will need more gain than with a X tal The frequency stability is a less accurate than when using an X tal
28. ystal Oscillator 2 For low power low frequency applications Oscillator 3 For medium frequency range applications Oscillator 4 For high frequency range applications 21 Philips Semiconductors X tal oscillators on 8 bit microcontrollers Application Note AN96103 Table 4 P83CLxxx family oscillator equivalent circuit parameters SYMBOL PARAMETER OPTION CONDITIONS MIN TYP MAX UNIT Om transconductance OSC1 Tamp 25 C 15 uS Vpp 4 5 V transconductance OSC2 25 0 2 0 6 1 0 mS Vpp 4 5 V transconductance OSC3 Tamb 25 C 0 4 1 5 4 0 mS Vpp 4 5 V Om3 transconductance OSC4 Tamb 25 C 1 0 4 0 10 0 mS Vpp 4 5 V C1 input capacitance OSC1 3 0 pF 1 input capacitance OSC2 3 4 8 0 pF C2 output OSC1 23 pF capacitance C2 output OSC2 3 4 8 0 pF capacitance Re feedback resistor OSC1 Note 1 MQ Re feedback resistor OSC2 3 4 0 3 1 0 3 0 MQ Note 1 The OSC1 option does not include any internal feedback resistor between pin XTAL1 and XTAL2 10 3 Example Below an example is shown illustrating the calculations necessary to check if a specific quartz or resonator can be used together with a certain oscillator option The parts to be checked are the PCD3351A microcontroller to be used at 3 58 MHz with the resonator CSA3 58MG310VA from muRata The parameter
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