Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
1999-04-01
2001-01-09
Kinkead, Arnold (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S1160FE, C331S183000, C331S158000
Reexamination Certificate
active
06172575
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an oscillation circuit, more particularly, to an oscillation circuit having less current consumption and a method of obtaining an oscillation signal.
This application is a counterpart of Japanese patent application, Ser. No. 180259/1998, filed Jun. 26, 1998, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
In general, the operation of a semiconductor integrated circuit, such as a microcomputer, is controlled by a clock signal. Basically, the operating speed of an entire system which includes the microcomputer depends upon a frequency of the clock signal (clock rate). A crystal oscillation circuit having a CMOS type inverter is well known as an oscillation circuit which produces such a clock signal.
An example of a conventional oscillation circuit is shown in FIG.
5
.
The oscillation circuit has oscillation terminals OSC
0
and OSC
1
, a crystal resonator
1
(or a quartz resonator
1
), a CMOS inverter
2
having a P-channel transistor TR
1
and an N-channel transistor TR
2
, a set of capacitors C
1
and C
2
, and a feedback resistor FR. Input and output terminals of the CMOS inverter
2
are respectively connected to the oscillation terminals OSC
0
and OSC
1
. The oscillation circuit further includes current limiting resistors LR
1
and LR
2
which are connected to the CMOS inverter
2
. The oscillation terminals OSC
0
and OSC
1
, the feedback resistor FR, the CMOS inverter
2
, and the current limiting resistors LR
1
and LR are located inside a chip and the remaining elements are positioned outside the chip.
The current limiting resistors LR
1
and LR
2
limit an operating current to an appropriate amount while oscillating. Thus, a current consumption of the oscillation circuit can be reduced. Especially in the case where the resonant frequency of the crystal resonator
1
is relatively low, the current consumption can be substantially decreased by increasing a resistance of the current limiting resistors LR
1
and LR
2
.
However, in the conventional oscillation circuit, a gain of the oscillation circuit decreases with increasing resistance of the resistors LR
1
and LR
2
. Therefore, if a relatively large resistance is simply applied to the oscillation circuit having the crystal resonator
1
of a relatively high resonant frequency, it is not easy to achieve oscillation just after a power supply voltage is applied thereto. In fact, in this oscillation circuit, there is a possibility that oscillation is not achieved at all. On the other hand, if a relatively high resistance is simply applied to the oscillation circuit, there is a possibility that the oscillation circuit, which has the crystal resonator
1
of a relatively low resonant frequency, will exhibit an abnormal oscillation.
There are two ways to overcome this problem. One way is to prepare two kinds of oscillation circuits, one of which has resistors of relatively small resistance for a high frequency oscillation and the other of which has resistors of relatively large resistance for a low frequency oscillation. However, this reduces the mass production efficiency.
The other way is the so called mask option where extra masks are prepared to change resistors. However, this also reduces mass production efficiency.
Consequently, there has been a need for an improved oscillation circuit that may correctly oscillate at both low and high frequencies.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an oscillation circuit that may oscillate at both low and high frequencies.
It is an object of the present invention to provide an oscillation circuit that may reduce current consumption.
It is an object of the present invention to provide an oscillation circuit that has higher reliability with respect to an oscillation start up time.
It is another object of the present invention to provide an oscillation circuit which can be produced without using extra masks.
It is another object of the present invention to provide an oscillation circuit which may select appropriate resistors automatically.
According to one aspect of the present invention, for achieving the above object, there is provided an oscillation circuit for coupling with a resonator to produce an oscillating signal, which includes an oscillation terminal for connecting to the resonator, and an inverter which is coupled to the oscillation terminal and which outputs an oscillation signal according to a resonant frequency of a resonator. The oscillation circuit also includes resistors having respectively different resistance values, and a select circuit which sequentially operatively connects between the inverter and a voltage source in response to the oscillation signal.
The above and further objects and novel features of the invention will more fully appear from the following detailed description, appended claims and the accompanying drawings.
REFERENCES:
patent: 4387350 (1983-06-01), Bessolo et al.
patent: 64-64403 (1989-03-01), None
patent: 2-122705 (1990-05-01), None
patent: 61-140204 (1994-12-01), None
Jones Volentine, LLC
Kinkead Arnold
OKI Electric Industry Co., Ltd.
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