Crystal oscillator having frequency adjustment responsive to power supply voltage

Frequency compensation for a crystal oscillator circuit whose oscillation frequency is dependent on source voltage applied thereto. When a capacitor is added to the frequency compensation terminal of the crystal oscillator circuit, the circuit changes the oscillation frequency thereof on the basis of the capacitance of the capacitor. A source voltage detector compares the source voltage being applied to the crystal oscillator circuit with a predetermined reference voltage and produces a control signal matching whether the source voltage is higher or lower than the reference voltage. On receiving the control signal, a control switch turns on or off the contact thereof to add or not to add the capacitor to the frequency compensation terminal. Assuming that the characteristic of the crystal oscillator circuit is such that the oscillating frequency decreases with the decrease in the source voltage, when the source voltage is low, the capacitor for frequency compensation is not added so as to increase the oscillation frequency of the crystal oscillator. Conversely, when the characteristic is such that the oscillation frequency increases with the decrease in the source voltage, the capacitor is added so as to decrease the oscillation frequency.

BACKGROUND OF THE INVENTION 
The present invention relates to a crystal oscillator and, more 
particularly, to a crystal oscillator for generating a reference clock 
signal which clocks a timepiece included in a battery-powered portable 
apparatus. 
A battery-powered portable telephone or similar portable apparatus is often 
provided with a timepiece function for user's convenience. Circuits for 
the timepiece function are mounted on a one-chip integrated circuit (IC), 
for a miniature and inexpensive configuration. Preferably, a portable 
apparatus should be so constructed as to turn off its power source when 
the expected function thereof is not effective, thereby promoting battery 
saving. The timepiece, however, does not function correctly unless it is 
powered at all times. To insure accurate operations, the timepiece is 
usually clocked by a reference clock signal which is generated by a 
crystal oscillator. It follows that reducing the current to be consumed by 
the crystal oscillator is decisive in reducing the current consumption of 
the entire portable apparatus. More specifically, reducing the current 
consumption of the crystal oscillator is successful in enhancing battery 
saving. In light of this, it is a common practice to drive the crystal 
oscillator by a comparatively low voltage when the portable apparatus is 
not serving the expected function. 
However, the problem is that the oscillation frequency of the crystal 
oscillator changes with the drive, or source, voltage. Should the source 
voltage be lowered for the battery saving purpose, the oscillation 
frequency of the crystal oscillator would be changed to prevent the 
timepiece from achieving sufficient accuracy. 
SUMMARY OF THE INVENTION 
An object of the present invention is, therefore, to provide a 
generally-improved crystal oscillator device for use in a timepiece of a 
battery-powered portable apparatus. 
Another object of the present invention is to provide a crystal oscillator 
device whose change in oscillation frequency ascribable to a change in a 
source voltage is compensated for. 
A crystal oscillator device of the present invention includes a source 
terminal to which power of different voltages may be applied, and a 
crystal oscillator circuit connected to the source terminal and whose 
oscillation frequency is dependent on the source voltage. The crystal 
oscillator also includes a capacitor for frequency compensation and a 
control switch for selectively adding or not adding the capacitor to the 
crystal oscillator circuit. The crystal oscillator circuit changes the 
frequency thereof on the basis of the capacitance of the capacitor when 
the latter is added thereto. Connected to the control switch, a source 
voltage detector compares the source voltage being applied to the crystal 
oscillator circuit with a predetermined reference voltage and produces a 
control signal on the basis of whether the source voltage is higher or 
lower than the reference voltage. In response to the control signal, the 
control switch is turned on or off to add or not to add the capacitor to 
the crystal oscillator circuit. Assuming that the crystal oscillator 
circuit has such a characteristic that the oscillation frequency decreases 
with the decrease in source voltage, when the source voltage is low, the 
capacitor is not added to the crystal oscillator circuit so as to increase 
the oscillation frequency of the crystal oscillator. Conversely, when the 
characteristic is such that the oscillation frequency increases with the 
decrease in source voltage, when the source voltage is low, the capacitor 
is added to the crystal oscillator circuit to lower the oscillation 
frequency of the crystal oscillator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a crystal oscillator device 10 includes a crystal 
oscillator circuit 1 having terminals 12 and 13 connected to opposite ends 
of a crystal 2. The crystal oscillator circuit 1 may be included in a 
one-chip microprocessor, for example, in .mu.PD75008 manufactured and 
marketed by NEC Corporation, the present assignee. To better understand 
the present invention, the mode-by-mode current consumption of .mu.PD75008 
which is applicable to a portable telephone will first be described. This 
IC has a portable telephone function, a timepiece function and other 
similar functions. The major component of the IC is a central processing 
unit (CPU) which needs a source voltage of 5 volts (V) and executes 
control by digital signals. To implement the timepiece function, the IC 
has an active circuit assigned to the crystal oscillator device 10. A 
portable telephone is operable in any one of three different modes, i.e., 
an operation mode which is used for only a short period of time, a standby 
mode used for several milliseconds within 0.5 seconds, and a timepiece 
mode used at all times. In the operation mode and standby mode, the IC has 
to interchange data with other CPUs and, therefore, needs the source 
voltage of 5 V. In the timepiece mode, a source voltage simply high enough 
to drive the crystal oscillator circuit, e.g. 3 V, suffices since the IC 
does not have to interchange data with other CPUs. Regarding the necessary 
voltage and current, the operation mode needs 5 V and 2.5 mA while the 
standby mode needs 5 V and 90 .mu.A. In the case of the timepiece mode, a 
current of 25 .mu.A is needed when a 5-V power source is used, but only 
5.5 .mu.A suffices when use is made of a 3-V power source. As these data 
indicate, using the 3-V power source during the timepiece mode which 
occupies major part of the operation time of the apparatus contributes a 
great deal to battery saving. 
However, simply changing the source voltage of the crystal oscillator 
circuit results in the inaccurate operation of the timepiece, because the 
oscillation frequency of the crystal oscillator circuit 1 is dependent on 
the source voltage. In the following description, let it be assumed that 
the oscillation frequency of the circuit 1 increases with the increase in 
the voltage being applied to a source terminal 11 by way of example. A 
source voltage detector 3 compares the source voltage being applied to the 
circuit 1 with a predetermined reference voltage. The source voltage 
detector 3 generates a first control signal when the source voltage is 
higher than the reference voltage or a second control signal when the 
source voltage is lower than the reference voltage. A capacitor 4 is 
connected at one end to the terminal 12, or frequency control terminal, of 
the crystal oscillator circuit 1 and at the other end to one end of a 
control switch 5. The other end of the control switch 5 is connected to 
ground. 
The first control signal from the source voltage detector 3 is applied to 
the control terminal 51 of the control switch 5 to close the switch 5. On 
the closure of the control switch 5, the other end of the capacitor 4 is 
connected to ground with the result that the capacitor 4 is added to the 
frequency control terminal 12 of the crystal oscillator circuit 1. The 
crystal oscillator circuit 1 has such a characteristic that the 
oscillation frequency thereof decreases when the capacitor 4 is added to 
the frequency control terminal 12 as mentioned above. The capacitance of 
the capacitor 4 is selected in such a manner as to provide the crystal 
oscillator device 10 with a required degree of stability by taking account 
of the source voltage dependency and additional capacitance dependency of 
the crystal oscillator circuit 1. 
The second control signal from the source voltage detector 3 opens the 
control switch 5. In this condition, the other end of the capacitor 4 is 
not connected to ground so that the capacitor 4 is not added to the 
crystal oscillator circuit 1. As a result, the oscillation frequency of 
the circuit 1 is no compensated. 
In the illustrative embodiment, the crystal oscillator device 10 is 
considered to be in a standard condition when the source voltage is low. 
In such a condition, the oscillation frequency of oscillator device 10 is 
not compensated. As the source voltage and, therefore, the oscillation 
frequency of crystal oscillator circuit 1 increases, the capacitor 4 is 
added to the frequency control terminal 12. This is successful in lowering 
the oscillation frequency of the circuit 1 by an amount corresponding to 
the change (increment) in the frequency ascribable to the change in the 
source voltage. 
In FIG. 1, the oscillation frequency of crystal oscillator circuit 1 is 
assumed to increase with the increase in source voltage. Conversely, when 
the circuit 1 has such a characteristic that the oscillation frequency 
decreases with the increase in source voltage, the first and second 
control signals from the source voltage detector 3 each may be inverted in 
logical level so as to add the capacitor 4 when the source voltage is 
lower than the reference voltage. 
To summarize the above-described operation, when the oscillation frequency 
of the crystal oscillator circuit 1 changes in response to a change in the 
source voltage, the source voltage detector 3 detects the change in source 
voltage and then outputs a control signal matching it. On receiving the 
control signal, the control switch 4 adds or does not add the frequency 
compensating capacitor 4 to the circuit 1. As a result, the change in the 
oscillation frequency of the crystal oscillation ascribable to the change 
in the source voltage is compensated for. 
FIG. 2 shows a specific construction of the crystal oscillator circuit 1. 
As shown, the circuit 1 has an inverter 14 which is an active circuit. A 
resistor 15 has high resistance and is connected between the input and 
output terminals of inverter 14 to define the bias voltage of inverter 14. 
The inverter 14 is operable in a linear range adjoining the defined bias 
point. The inverter 14 is connected between the source terminal 11 and 
ground and applied with the source voltage. A resistor 16 is connected at 
one end to the output terminal of the inverter 14 and at the other end to 
one end of the crystal 2 via the terminal 13, serving to adjust the drive 
current to be fed to the crystal 2. A capacitor 17 is connected between 
the terminal 13 and ground while a capacitor 18 is connected between the 
frequency control terminal 12 and ground. The frequency control terminal 
12 is also connected to the input terminal of the inverter 14. The 
inverter 14 and resistors 15 and 16 may be included in a one-chip 
microprocessor or similar IC, as mentioned earlier. 
The crystal oscillator circuit 1 is a Pierce nonadjustment oscillator 
circuit using the crystal 2 as an inductive reactance. The capacitors 17 
and 18 constitute part of a tuning circuit of the crystal oscillator 
circuit 1 in cooperation with the crystal 2. The generated signal appears 
on the output of the inverter 14. The crystal oscillator circuit 1 changes 
the oscillation frequency thereof in response to a change in the source 
voltage being applied to the inverter 14 via the source terminal 11. More 
specifically, the oscillation frequency of the circuit 1 lowers as the 
source voltage lowers, and vice versa. 
The oscillation frequency of crystal oscillator circuit 1 shown in FIG. 2 
has voltage dependency, as shown in FIG. 3. FIG. 3 shows a characteristic 
with respect to a one-chip microprocessor .mu.PD75008 which includes the 
inverter 14 and resistors 15 and 16 of the circuit 1. The oscillation 
frequency of the circuit 1 is 32.768 kHz when the source voltage is 5 V. 
On the decrease in the source voltage to 3 V, the oscillation frequency is 
lowered by 4.5 PPM (10.sup.-6). The characteristic shown in FIG. 3 was 
obtained with the capacitor 17 having a capacitance of 33 pF and the 
capacitor 18 having a capacitance of 22 pF, and at a temperature of 
25.degree. C. 
FIG. 4 indicates how the oscillation frequency of crystal oscillator 
circuit 1 having the voltage-to-frequency characteristic of FIG. 3 changes 
in association with the capacitance of the capacitor 18 connected to the 
frequency control terminal 12. As the graph shows, when the source voltage 
applied to the circuit 1 is 5 V and the capacitance of the capacitor 18 is 
22 pF, the circuit 1 oscillates at a frequency of 32.768 kHz. As the 
capacitance of the capacitor 18 decreases to 17 pF, the oscillation 
frequency of the circuit 1 increases by 4.55 PPM. Hence, assuming that the 
circuit 1 has the construction shown in FIG. 2, that the required 
oscillation frequency is 32.768 kHz, and that the source voltage of 3 V is 
standard, it is necessary, in the standard condition, to provide the 
capacitor 18 with a capacitance of 17 pF. When the source voltage is 5 V, 
the capacitance of the capacitor 18 has to be increased by 5 pF from the 
standard condition to 22 pF. In other words, the capacitor 4 in the 
circuitry of FIG. 1 needs a capacitance of 5 pF. Stated another way, if 
the capacitor 4 included in the embodiment of FIG. 1 is provided with a 
capacitance of 5 pF, the voltage dependency of the oscillation frequency 
of the crystal oscillator device 10 can be lowered to 1 PPM or less. 
Referring to FIG. 5, the specific constructions of the source voltage 
detector 3 and control switch 5, FIG. 1, are shown. The control switch 5 
includes an NPN transistor 52. The NPN transistor 52 has a base serving as 
the control terminal 51 of the control switch 5 and an emitter which is 
connected to ground. The collector of the NPN transistor 51 is connected 
to the capacitor 4, FIG. 1. When the first control signal which is in a 
high level is applied to the base of the NPN transistor 52, i.e., the 
control terminal 51, the transistor 52 is turned on with the result that 
the other end of the capacitor 4 connected to the collector is connected 
to ground. Consequently, the capacitor 4 is added to the frequency control 
terminal 12 of the crystal oscillator circuit 1. Conversely, when the 
second control signal which is in a low level is fed to the base of the 
NPN transistor 52, the transistor 52 is turned off to prevent the 
capacitor 4 from being added to the frequency control terminal 12. 
The source voltage detector 3 includes a comparator 31 which is implemented 
by an operational amplifier having non-inverting and inverting input 
terminals provided with the voltage appearing on the source terminal 11 
and the reference voltage which is applied from a battery 32, 
respectively. The power source and one input to the comparator 61 are fed 
from the source terminal 11. The comparator 31 compares the voltages 
applied to the input terminals to produce a control signal. More 
specifically, assuming that the output voltage of the battery 32 is 4 V, 
the comparator 31 produces a (logical) ONE when the voltage on the source 
terminal 11 is 5 V. When the voltage on the source terminal 11 is 3 V, the 
comparator 31 produces a (logical) ZERO. In this manner, the source 
voltage detector 3 detects the source voltage of the crystal oscillator 
circuit 1 in terms of two levels by comparing it with the reference 
voltage from the cell 32, feeding the result of detection to the control 
terminal 51 of the control switch 5. When the inverted logical output of 
the source voltage detector 3 is needed, an inverter may be inserted 
between the output of the comparator 31 and the terminal 51. The 
comparator 31 will consume a minimum of current if implemented by a 
complementary metal oxide semiconductor (CMOS) circuit. Also, the service 
life of the battery 32 will be increased since it does not have to output 
a current. 
Referring to FIG. 6, a crystal oscillator device 10A according to an 
alternative embodiment of the present invention is shown. The device 10A 
includes capacitors 71 and 72 in place of the capacitor 4, FIG. 1. The 
capacitor 71, like the capacitor 4, is connected between the frequency 
control terminal 12 and the collector of NPN transistor 52. The capacitor 
72 is connected between the collector of NPN transistor 52 and ground. In 
this embodiment, when a high-level control signal is fed to the base of 
NPN transistor 52, i.e., the control terminal 51, opposite ends of the 
capacitor 72 are connected to ground. As a result, the capacitor 71 is 
added to the frequency control terminal 12 of crystal oscillator circuit 1 
to change the oscillation frequency of the circuit 1. For example, when 
the circuit 1 is implemented by .mu.PD75008, the oscillation frequency is 
lowered. When a low-level control signal is applied to the base of NPN 
transistor 52, the capacitor 72 is serially connected to the capacitor 71 
to reduce the capacitance to be added to the frequency control terminal 
12. Consequently, when use is made of .mu.PD75008, a high-level control 
signal is applied to the control terminal 51 to increase the oscillation 
frequency. 
It will be seen that when the crystal oscillator circuit 1 is of the type 
reducing the oscillation frequency thereof in response to an increase in 
source voltage, the circuitry shown in FIG. 6 is practicable with the 
first control signal which turns on the control switch 5 when the source 
voltage is high, as in the embodiment of FIG. 1, i.e., the alternative 
embodiment does not have to be supplied with an inverted logical level.