A universal 0.03-mm2 one-pin crystal oscillator in CMOS JSSC

A Universal0.03-mm2One-Pin
Crystal Oscillator in CMOS
John A.T.M.van den Homberg
Abstract—A one-pin crystal oscillator with an integrated load
capacitance of15pF has been realized in a standard0.35- m
CMOS technology.Due to the structure of the oscillator and the
use of MOS gate capacitance for the load capacitors,the chip area
can be very small.The total active area including load capacitors
is less than0.03mm2.The design can be operated with supply
voltages in the range of1.4–3.6V and allows crystal frequencies in
the range of3–30MHz.The current consumption of the oscillator
core is180 A at10MHz with3.3-V power supply.It produces a
rail-to-rail output swing,regulated by an amplitude control loop,
and has the sameflexibility and ease of frequency tuning as a
common Pierce oscillator.As no special IC process options are
required,the design is very suitable for clock generation in digital
very-large-scale-integration chips.
Index Terms—Amplitude control,CMOS,frequency tuning,
one-pin oscillator,quartz crystal.
I.I NTRODUCTION
C RYSTAL oscillators are widely used in electronic sys-
tems to generate an accurate timing reference.In many digital IC’s,a crystal oscillator is embedded to provide the clock signal.The most common configuration is a Pierce oscillator whose crystal and load capacitors are connected between two pins of the IC.To minimize the number of external components,the load capacitors and bias resistor may be integrated on-chip.In a Pierce oscillator,both capacitors refer to ground,facilitating the use of MOS gate capacitance and thereby greatly reducing the chip area.The resonance frequency can be trimmed by implementing the load capacitors as capacitor banks switched by pass gates[1],[2].Due to the limited number of pins on an IC package,it is desirable to have the same function with a single pin and the crystal referring to ground.Several one-pin oscillators have been reported,but they all need at least onefloating capacitor.This leads to large chip areas or expensive IC process options and difficulties with respect to tuning by means of switched-capacitor banks. This paper presents a one-pin oscillator that overcomes these problems by using a different structure,in which all capacitors refer to ground.Furthermore,it requires smaller capacitors and a
lower
(1) The locus
of is half a circle in the lower half of the complex plane.A
ratio
can be estimated in the following way.By
neglecting
and
is sufficiently high.In that case,the voltage at node2 will follow the voltage at node1.In equilibrium,no voltage
difference between nodes1and2is present,and the output
current of
both
is positive,indicating that the unity-gain frequency of the reference branch is higher than
that of the positive feedback branch.In a situation
where
Fig.6.Total open-loop gain in the oscillator.
branch,respectively.A differential ac source has been applied between the positive and negative input of
the
equals
.Now we can calculate the impedance seen
by the resonant branch
as
can be estimated in the same way as
in the three-point oscillator.
Neglecting
(5)
and setting the real part
of
is half the value
of
and
and a PMOS transistor connected
to
I2and
M18
Fig.9.Capacitor-bank implementation.
C.Slave Mode
When an external rail-to-rail clock signal is supplied to the XTAL pin,the control loop will regulate
the
m standard CMOS technology.The total active area including the15-pF load capacitor and the5-pF reference capacitor
is
(a)(b)
Fig.10.Measured oscillator current consumption(a)including comparator and(b)without
comparator.
Fig.11.Jitter measurement result.
less than0.03
mm。