JPH104347A - Oscillation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the oscillation circuit in which malfunction of an oscillator is prevented because of start of operation of the oscillator from an oscillation unstable state and oscillation is made possible in a shortest oscillation stability wait time regardless of dispersion in the characteristic of the oscillator or the like. SOLUTION: The circuit is provided with a counter 17 counting an oscillated clock from an oscillator 1 and a CR oscillator 9 whose oscillation is made stable in a shorter time than that of the oscillator 1. In this case, the count/reset operation of the counter 17 is controlled based on an oscillated output of the CR oscillator 9, the oscillated clock from the oscillator 1 is counted for a prescribed period and when it is detected that the oscillated clock reaches a predetermined frequency, an AND gate 21 provides an output of the clock.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an oscillation circuit,
In particular, it relates to an oscillation circuit using a crystal oscillator or a ceramic oscillator.

[0002]

2. Description of the Related Art As a conventional oscillation circuit of this type, for example, Japanese Patent Application Laid-Open No. 4-160960 discloses that an oscillation output of an oscillation circuit including a crystal oscillator has a sufficiently large level and the oscillation output reaches a stable state. By supplying an internal clock after detecting that an error has occurred, a malfunction due to an abnormal clock in the semiconductor integrated circuit can be eliminated, and a configuration that reduces the chip size by eliminating the use of multi-stage transistor elements has been developed. Proposed. In this conventional technique, a clock output signal of an oscillator is detected by using a level detection circuit when the amplitude of oscillation reaches a predetermined level, and the stability of the oscillation clock is determined.

[0003]

However, in the above prior art, a malfunction due to noise is likely to occur.
There is a problem that. The reason for this is that, in the prior art, the level of the oscillation output is detected by a level detection circuit to determine whether or not the oscillation output is stable. In this case, it is erroneously determined that the oscillation is normal even though the oscillation is not stable.

Further, the above-mentioned prior art has a second problem that it cannot be confirmed whether or not the oscillation output has reached a specified frequency. The reason for this is that in the prior art, the stability of the oscillation output is detected only by the output level, and there is no way to check the oscillation frequency.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made in view of the above circumstances, and an object of the present invention is to prevent a system including an oscillator from malfunctioning by starting operation while the oscillator is in an oscillation unstable state. It is an object of the present invention to provide an oscillating circuit that can perform such operations.

Another object of the present invention is to provide an oscillation circuit which can oscillate with a minimum oscillation stabilization wait time due to variations in characteristics of an oscillator or the like.

[0007]

In order to achieve the above object, an oscillation circuit according to the present invention comprises means for counting an oscillation clock of an oscillator for a predetermined period to detect that the oscillation clock has reached a predetermined frequency. And outputting a clock when the oscillation clock reaches the frequency.

In the present invention, preferably, in addition to the counter for counting the oscillation clock of the oscillator, the oscillator further includes a counter for stably oscillating in a shorter time than the oscillator and for determining a period of the certain period. And wherein the counter controls a count operation / reset operation based on an oscillation output of the second oscillator. In the present invention, preferably, the second oscillator comprises a CR oscillator.

Further, in the present invention, while the count value of the counter does not reach a predetermined value, the oscillation clock of the oscillator is masked, and the count value of the counter becomes a predetermined value. A circuit that outputs an oscillation clock of the oscillator as a clock signal when the clock has reached the threshold.

As described above, according to the present invention, the oscillation frequency is counted for a certain period of time, and the oscillation clock is output when the oscillation is stabilized. More specifically, it has a counter (17 in FIG. 1) for counting the clock output of the oscillator (1 in FIG. 1) for a certain period of time, and a CR oscillator (9 in FIG. 1) for resetting the counter.

According to the present invention, the counter for counting the oscillation frequency of the oscillator is reset by the oscillation clock of the CR oscillator, and within a half oscillation cycle of the CR oscillation clock at which the counter can count (high or low level period). In step (2), a stable state of the oscillation frequency is determined by detecting an overflow of the counter. Therefore, a clock is output when the oscillation reaches the oscillation stable state, and a stable oscillation frequency is obtained.

[0012]

Embodiments of the present invention will be described in detail below with reference to the drawings. Hereinafter, a case where a crystal oscillator is used as an oscillator will be described. FIG. 1 is a diagram illustrating a configuration of an oscillation circuit according to an embodiment of the present invention.

Referring to FIG. 1, an embodiment of the present invention comprises a crystal oscillator 1, a CR oscillator 9, a Schmitt buffer 16 for shaping the clock output waveform of the crystal oscillator 1, a counter 17, AND gates 19 and 21, and a set. A reset flip-flop (referred to as “SR flip-flop”) 20 is provided;

The crystal oscillator 1 comprises an external crystal oscillator 2, a resistor 5, a clocked inverter 7, an N-channel transistor 6, and an inverter 8, and a CR oscillator 9
Is a resistor 12, a clocked Schmidt inverter 10,
N-channel transistor 14, inverters 11, 15,
And a capacitor 13.

The oscillation output / stop of both the crystal oscillator 1 and the CR oscillator 9 is controlled by the clock stop signal S1.

The counter 17 is composed of an n-bit binary counter 18, and the output signal S3 of the CR oscillator 9 is H
Reset at the time of high level (Hi), and Low
During the level (Lo), the clock signal S4 output from the Schmitt buffer 16 is counted.

The AND gate 19 receives a predetermined m-bit to n-bit output of the counter 17 as an input, and when both the m-bit to n-bit outputs become "1" (High level), a high signal is output as the coincidence signal S8. Output level.

The SR flip-flop 20 inputs the clock stop signal S1 to the reset (R) terminal, and
The output of the gate 19 is input to the set (S) terminal, and the clock stop signal S1 is reset while the clock stop signal S1 is at the high level (the output Q is at the low level).
While 1 is at the low level, the coincidence signal S8 from the AND gate 19 is set at the edge from the high level to the low level.

The AND gate 21 has one input terminal connected to the output terminal Q of the SR flip-flop 20, the other input terminal connected to the output of the Schmitt buffer 16, and a signal S9 at the output terminal Q of the SR flip-flop 20. Is a gate signal, and when the signal S9 changes from the low level to the high level, an oscillation clock from the crystal oscillator 9 is output.

FIG. 1 shows the operation of the embodiment of the present invention.
This will be described with reference to FIG. FIG. 2 is a timing waveform chart for explaining the operation of the embodiment of the present invention.
S1 to S10 correspond to signals at each node in the circuit configuration shown in FIG.

The operation from the start of oscillation of the crystal oscillator 1 until the AND gate 21 outputs the crystal oscillator clock signal will be described with reference to the timing chart of FIG.

Referring to FIG. 2, during a period from time T0 to T1, clock stop signal S1 is at a high level, SR flip-flop 20 is reset, output terminal Q (signal S9) is at a low level, and AND gate 21 Block (do not output) the signal S4, which is the output signal of the Schmitt buffer 16. That is, the time T0
During T1, the clock signal output S10 is at a low level.

At this time (period from time T0 to time T1), the clock stop signal S
The N-channel transistor 6 having 1 as a gate input is on, and the clocked inverter 7 for inputting a signal obtained by inverting the clock stop signal S1 via the inverter 8 to the output control terminal is in the off state.
The output signal S2 (potential of the drain terminal of the N-channel transistor 6) of the crystal oscillator 1 becomes Low level.

Similarly, the CR oscillator 9 turns on the N-channel transistor 14 which receives the clock stop signal S1 as a gate input, and outputs the clock stop signal S1 to the inverter 1
1 is input to the output control terminal, and the Kschmitt clocked inverter 10 is in the off state, so that the oscillation is stopped and the output signal S of the CR oscillation circuit 9 is output.
3 (a signal obtained by inverting the drain potential of the N-channel transistor 14 by the inverter 15) becomes High level. Therefore, the counter 17 is reset.

Next, at time T1, the clock stop signal S
1 changes to Low level. As a result, the crystal oscillator 1 turns off the N-channel transistor 6, turns on the clocked inverter 7, and starts oscillating. At this time, the CR oscillator 9 turns off the N-channel transistor 14 and turns on the Schmitt inverter 10 to start oscillating.

Here, the CR oscillator 9 includes a capacitor 13
Since the CR oscillator is made of the resistor 12 and the resistor 12, the time T
Normal oscillation starts from 1.

The output S2 of the crystal oscillator 1 has a small oscillation amplitude during the period from time T1 to T3, but from time T3, the oscillation amplitude changes to the threshold voltage V _H (high level) of the noise malfunction preventing Schmitt buffer 16. Since the threshold value exceeds V _L (low-level threshold), the Schmitt buffer 16 outputs the result of shaping the waveform of the output S2 of the crystal oscillator as a signal S4 (clock signal).

During the period from time T2 to T5, the counter 17
Counts the clock signal S4, but during the period from time T2 to T4, since the clock signal S4 to the counter 17 is abnormal, the outputs S5 to S7 from the m bits to the n bits of the counter 17 are not all High. (In the figure, only the signal S5 is High), and therefore the AND gate 19 outputs a low level as the coincidence signal S8.

On the other hand, in the period from the time T6 to the time T7, the input of the clock signal S4 is fully counted in the counter 17, so that the counter 17 outputs all the outputs S5 to S7 from m bits to n bits. age,
The AND gate 19 outputs a High level as the coincidence signal S8. Note that the counter 17 starts counting from time T5 to time T6.
Output signal S3 of the CR oscillation circuit 9 during the period
The signal is reset to the h level, and the output signal S4 of the Schmitt buffer 16 is counted from the next time T6 to T7.

In response to the change of coincidence signal S8 to High, SR flip-flop 20 is set, and SR flip-flop 20 is set.
The output signal S9 of the flip-flop 20 becomes High level (the coincidence signal S8 of the AND gate 19 is set to High at the edge changing from High to Low), and the AND gate 21 outputs the output signal S4 of the Schmitt buffer 16. Thus, stable oscillation of the crystal oscillator 1 can be output.

In the embodiment of the present invention, as described above, a standard time is generated by a CR oscillator having a longer oscillation frequency than a crystal oscillator and stably oscillating immediately after the start of oscillation.
The counter counts the output signal of the crystal oscillator within a standard time, and determines that specified oscillation has occurred.

In addition, the specified oscillation can be determined with an accuracy of ± 2 ^(m-1) .

Although the above embodiment has been described with reference to the case of a crystal oscillator, it is needless to say that a similar effect can be obtained with a ceramic oscillator. Also,
As the coincidence signal S8 for determining whether or not the specified oscillation has occurred, an overflow bit of a counter that outputs the occurrence of overflow may be used.

[0034]

As described above, according to the present invention,
It is configured to determine whether the oscillation frequency has reached the specified value by counting the oscillation clock signal for a certain period. It has the effect that it can be supplied.

Further, according to the present invention, when the oscillation stabilization time is short, the output of the oscillation clock signal can be controlled in accordance with the output of the oscillation clock signal. There is an advantage that the optimum waiting time can be set according to the oscillation stabilization time variation.

[Brief description of the drawings]

FIG. 1 is a diagram showing a circuit configuration according to an embodiment of the present invention.

FIG. 2 is a timing chart for explaining an operation of the exemplary embodiment of the present invention.

[Explanation of symbols]

Reference Signs List 1 crystal oscillator 2 crystal oscillator 3 terminal 4 terminal 5 resistor 6 N-channel transistor 7 clocked inverter 8 inverter 9 CR oscillator 10 Schmitt clocked inverter 11 inverter 12 resistor 13 capacitor 14 N-channel transistor 15 inverter 16 Schmitt buffer 17 counter 18 binary Counter 19 AND gate 20 Set / reset flip-flop (SR flip-flop) 21 AND gate

Claims (4)

[Claims] A means for detecting that the oscillation clock has reached a predetermined frequency by counting the oscillation clock of the oscillator for a predetermined period, and outputting a clock when the oscillation clock has reached the frequency. An oscillation circuit characterized by the above. 2. A counter that counts an oscillation clock of the oscillator, and a second oscillator that oscillates stably in a shorter time than the oscillator and determines a period of the fixed period, in addition to the oscillator. 2. The oscillation circuit according to claim 1, wherein the counter controls a count operation / reset operation based on an oscillation output of the second oscillator. 3. When the count value of the counter does not reach a predetermined value, the oscillation clock of the oscillator is masked, and when the count value of the counter reaches a predetermined value, 3. The oscillation circuit according to claim 2, further comprising a circuit that outputs an oscillation clock of the oscillator as a clock signal. 4. The oscillation circuit according to claim 2, wherein said second oscillator comprises a CR oscillator comprising a capacitor and a resistor.