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Crystal oscillators

Posted: 29 Aug 2000 ?? ?Print Version ?Bookmark and Share

Keywords:atmel? crystal oscillator? oscillator? cell ic? asic?


Crystal Oscillators WARNING! USE THE STYLE SHEET THAT CAME WITHIN THIS DOCU- MENT'S DIRECTORY. CHANGE THE TEXT OF THIS MESSAGE TO WHITE USING THE "TAG' TOOL BEFORE PRINTING, THEN REVERT BACK TO BLACK BEFORE ARCHIVING. Cell-Based IC (CBIC) Application Note 0693B Crystal Oscillators Introduction This Engineering Application Note (EAN) describes the use of the Crystal Oscillator parts in the ATMEL Libraries. Overview Crystal oscillator cells use an external crystal to generate clock signals for de- signs that require high stability and a precise frequency. Crystal Theory The mechanical impedance of a crystal for one particular series resonant fre- quency k (the fundamental and its over- tones) can be electrically modeled by a series RLC resonant circuit. See Fig. 1. The parasitic capacitances of the crys- tal's electrodes and the package are modelled by the capacitor C0. Together with the motional inductances Lx(k) this capacitor forms resonant circuits that also produce parallel resonant frequen- cies fp(k). They cannot be used for os- cillation with the type of oscillators pre- sented here. Cell-Based IC 7-45 For a realistic crystal model, the quality factor Q(k) for the series resonators decreases for the overtone frequen- cies: Q (k + 1) If the parasitic capacitance C3 is negligible, the formula for Zc can be simplified to: C1, C2>>C3: -Re[z_c ] = gm C1 C2 (gm C3) 2 + 2 C12C22 -Im[z_c ] = gm 2 C3 + 2 (C1 + C2)C1C2 ((gm C3) 2 + 2 C12C22) C3 = 0: -Re[z_c ] = gm 2 C1C2 -Im[z_c ] = C1+C2 C1C2 I0 TA C1 C2 Zc C3 Crystal TA: Active Transistor I0: DC Bias Current C1: Functional Capacitance C2: Functional Capacitance C3: Parasitic Capacitances -Rmax -Rx Re Im gm= 0 gm= 8 pgm Zx(p) Zc(gm) Zc: Oscillator Impedance Zx: Crystal Impedance OP Figure 2. Basic Three Point Oscillator Circuit Figure 3. Complex Plane Representation of the Imped- ances Zc and Zx The complex plane representation also shows that there is a maximum negative resistance Rmax for Rx. Above this value no operating point can be achieved because the two curves no longer intersect: -Rmax = 1 2 C3 (1+C3 C1C2 C1+C2 ) In the range from Rx to Rmax the frequency pulling de- pends only on the capacitances of the circuitry and can be decreased by an increase of the two functional capaci- tances C1 and C2: Px = CX 2 (C3+ C1C2 C1+C2 ) Implementation Figure 4 shows schematically the implementation of the three point oscillator circuit. It can be used to calculate the necessary external capacitances, the impedance Zc and all other values using the electrical parameters of the cell and the crystal. The cells OSC2 and OSC3 are imple- mented as the normal 3 point circuits, using one active transistor to obtain the negative resistance. OSC1 uses a CMOS inverter instead of the single transistor, but the schematic diagram for the impedance is not different from those of the other cells. OSC3 has an additional amplitude regulation circuit to allow a safe startup and to decrease the power dissipation by limiting the biasing current to a value that is necessary to obtain the operating point. The functional capacitors C1 and C2 are partially realized in the cells on chip. The internal C3int contains the parasitics of the oscillator circuitry, C3ext parasitic crystal, board and package capacitances. C1ext and C2ext have to be exter- nally connected to the circuitry to achieve the desired total values of C1 and C2. R is an internal resistance required to force the transistor into the active mode. Its value is very high to avoid degrading the frequency stability and increasing the current. Test Mode For the latest processes (ECLP07, ECDM05 and after), a test mode has been implemented on the oscillators. The pin TEST must be activated (TEST = 1) during a test se- quence to be able to apply signals on the XIN pin. In nor- mal use, when a quartz crystal is connected to the XIN pin, the TEST signal must be set to low. Another application of this `test mode' is the possibility of applying an external clock directly on the XIN pin without using a quartz crystal. This must be done with TEST set high so that the clock is applied directly to the gates on the output stage of the oscillator. Cell-Based IC 7-47 References 1) Vittoz, E. A.,`Quarz Oscillators for Watches',Proc. Int. Congress Chronometry, 1979, pp. 131 - 140. 2) Vittoz, E. A., Degrauwe, M. G. R., Bitz, S.,`High-Per- formance Crystal Oscillator Circuits: Theory and Applica- tion', IEEE J. Solid-State Circuits, Vol. SC-23, June 1988, pp. 774 - 783. 3) Tietze, U., Schenk, Ch.,`Halbleiterschaltungstechnik', neunte AuflageSpringer Verlag, Berlin, Heidelberg, New York, 1989. gm*Vgs C1ext C3ext C2ext C1int C3int C2int R Rx Lx Cx XIN XOUT Off Chip On Chip Vgs Figure 4. Schematic Circuit Diagram of the Oscillator Used for Calculations 7-48 Cell-Based IC

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