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PDF AN1783 Data sheet ( Hoja de datos )
| Número de pieza | AN1783 | |
| Descripción | Determining MCU Oscillator Start-Up Parameters | |
| Fabricantes | Motorola Semiconductors | |
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.comMotorola Semiconductor Application Note
Order this document
by AN1783/D
www.DataSheet4U AN1783Determining MCU Oscillator Start-up Parameters
mBy Stuart Robb & David Brook, East Kilbride, Scotland
Andreas Rusznyak, Geneva, Switzerland
Rev 1.0, December 1998
.coIntroduction
t4UMany microcontrollers (MCUs) incorporate an inverting amplifier for use
with an external crystal or ceramic resonator in a Pierce oscillator
econfiguration. This paper describes how to calculate the minimum gain
(transconductance) of the amplifier required to ensure oscillation with
especific external components, and also how to measure the amplifier
htransconductance to establish whether the minimum gain requirement is
met.
taSOscillator Circuit
a STOP
.D Internal
to MCU
w OSC1
External
w Components
R0
Q
OSC2
w .comC1 C2
t4UFigure 1 Standard Pierce Oscillator for > 1MHz Operation
heeFigure 1 shows the standard Pierce oscillator configuration typically
taSused on MCUs for frequencies in the range 1MHz to 20MHz. The
oscillator pins are labelled OSC1, OSC2 on the MC68HC05 and
www.Da© Motorola, Inc., 1999
AN1783 Rev. 1.0
1 page  
Application Note
If gm >> gds, this can be simplified to:
gmmin
≈
(---C----1----+-----C----2---)--2-
C1C2
----1------ + --1----
RPQ R0
(Eqn 5)
The validity of this simplification can be checked by measuring gds, as
described later in this paper.
If R0 >> RPQ, equation 5 can further be reduced to:
gmmin
≈
(---C----1----+-----C----2---)--2-
C1C2
----1------
RPQ
(Eqn 6)
or, if C1 = C2, to
gmmin
≈
----4------
RPQ
=
4Rω2
C0
+
C-----1-
2
2
(Eqn 7)
Measuring Amplifier Characteristics
The recommended circuit for measuring the transconductance of the
amplifier is shown in Figure 4 [2]. The circuit is simple to implement and
should be powered up with the MCU reset pin held at 0V to ensure the
amplifier stays active and the MCU does not execute any code. All unused
inputs should be connected to 0V or VDD and not left floating. Note that the
diagram correctly indicates that OSC1 and OSC2 are connected together.
The transconductance does not vary significantly at frequencies below the
oscillator’s maximum design frequency. However, it is not recommended to
measure the transconductance at the intended operating frequency, as the
effects of stray capacitances will make the measurements inaccurate. A
frequency in the range of 10kHz to 100kHz is recommended. A signal of
around 500mVpp or less should be used with a 50Ω terminating resistor
and a 1µF coupling capacitor to ensure that the amplifier input and output
remain in their linear region. It is essential that a high impedance measuring
instrument, such as an oscilloscope with a low capacitance, high input
resistance probe (<1pF, >10MΩ) is used to measure Vin and Vg with
respect to ground. The value for the resistance Rm should be 100Ω to 1kΩ.
OSC1
OSC2
Rm
50 ohm signal generator
1µ 500mVpp
Vin Vg
50
Figure 4 Measuring Amplifier Transconductance
AN1783 Rev. 1.0
5 MOTOROLA
5 Page | ||
| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet AN1783.PDF ] | |
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