JPH0783353B2 - Clock switching circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention selects and outputs either an external clock input from the outside or an internal clock generated internally. The present invention relates to a clock switching circuit that selects an internal clock when a clock input interruption occurs.

[Conventional technology]

Generally, there is a method of forming a synchronous network in a communication system. In this method, a reference clock is transmitted to each terminal station, and the terminal station performs communication control in synchronization with this reference clock. A communication device compatible with this method is usually configured to select an external clock or an internal clock by a clock switching circuit and operate based on the selected clock.

FIG. 2 shows a conventional clock switching circuit. Normally, the switching circuit 1 selects the external clock 2. When the external clock 2 input from the outside or the internal clock 5 output from the internal clock generation circuit 4 is selected by the clock selection setting switch 3, a clock selection instruction 6 indicating the clock is switched from the clock selection setting switch 3. It is output to the circuit 7, and the switching circuit 7 switches to a predetermined clock side. As a result, the selected clock can be output as the output clock 8.

When the external clock 2 is disconnected due to a failure in the transmission system or the like, the external clock input disconnection detection circuit 9 detects it and outputs the detection signal 10. This detection signal 10 is
It is output to an external alarm device (not shown) and the switching circuit 1.
Upon receiving this detection signal 10, the switching circuit 1 causes the internal clock 5
The internal clock 5 as the output clock 8
Will be output.

Further, when the internal clock 5 is selected,
Since it is inconvenient if the detection signal 10 is output to an external alarm device, the switch 11 shuts off the output of the detection signal 10 to the outside.

[Problems to be Solved by the Invention]

However, in the above-mentioned technique, when the clock selection is switched, both the clock selection setting switch 3 and the switch 11 must be switched, which causes a problem in operability.

In particular, when switching from the internal clock 5 to the external clock 2, it is easy to forget to switch the switch 11 because it operates normally without switching the switch 11.
In this case, the alarm device does not operate when the input of the external clock 2 is cut off.

In view of such problems, it is an object of the present invention to provide a clock switching circuit having excellent operability during clock selection switching.

[Means for Solving the Problems]

The present invention provides a detection circuit that monitors an input state of an external clock input from the outside and outputs a detection signal when there is no input, an internal clock generation circuit that generates an internal clock, and a clock that is output to the outside. A switch to select, the switch is selecting an external clock, and
A gate circuit that outputs a control signal for generating an alarm only when the detection circuit outputs a detection signal, and the gate circuit outputs a control signal, or the switch selects the internal clock. In this case, the switching means outputs the internal clock, and otherwise outputs the external clock.

[Action]

That is, in the clock switching circuit of the present invention, when the switch operated by the operator selects the external clock and the input of the external clock is interrupted, the internal clock is automatically output to the outside. At the same time, a control signal for generating an alarm is output. Then, the output of the control signal is continued until the operator switches the switch to the internal clock. Also, when the operator switches the switch to the external clock selection state with the external clock input disconnected, the clock output to the outside remains the internal clock and the control signal for outputting the alarm is output. It

Thus, according to the present invention, even if the switch is erroneously switched when the external clock input is cut off, the control signal for issuing an alarm is output, so that the operator can recognize the erroneous operation.

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a clock switching circuit according to an embodiment of the present invention. The switching circuit 1 selects and outputs one of the external clock 2 and the internal clock 5, and normally selects the external clock 2 and selects the internal clock 5 by receiving the detection signal 10. To do. Similarly, the switching circuit 7 selects one of the external clock 2 and the internal clock 5 and outputs it as the output clock 8, and performs the switching operation based on the clock selection instruction 6.

The internal clock generation circuit 4 outputs the internal clock 5. The clock selection setting switch 3 inputs a clock selection instruction 6 indicating selection of either an external clock or an internal clock. The external clock input disconnection detection circuit 9 detects an input disconnection of the external clock 2,
The detection signal 10 is output. The gate circuit 12 controls the opening and closing of the output of the detection signal 10 to the external alarm device and the switching circuit 1, receives the clock selection instruction 6 indicating the selection of the external clock, and takes the open state to select the clock indicating the internal clock. It receives the selection instruction 6 and takes the closed state.

Next, the operation of this clock switching circuit will be described.

The operator can select the output clock 8 by selecting the external clock 2 or the internal clock 5 with the clock selection setting switch 3.

When the external clock 2 is selected, the clock selection instruction 6 indicating the external clock selection is displayed on the clock selection setting switch 3
Is output to the switching circuit 7. Normally, the switching circuit 1 is
The external clock 2 is output to the switching circuit 7, and the switching circuit 7 outputs the external clock 2 as an output clock 8 to the outside.

Here, when the input disconnection of the external clock 2 occurs due to a failure of the transmission system or the like, the external clock input disconnection detection circuit 9 detects the input disconnection and outputs the detection signal 10. Gate circuit 12
Is in the open state upon receiving the clock selection instruction 6 indicating the above-mentioned external clock selection, and the detection signal 10 is output to the external alarm device and the switching circuit 1. As a result, an alarm is issued by the alarm device, the switching circuit 1 selects the internal clock 5, and the internal clock 5 is output as the output clock 8.

In addition, the internal clock 5 is set by the clock selection setting switch 3.
Is selected, the clock selection instruction 6 indicating the internal clock selection is output from the clock selection setting switch 3 to the switching circuit 7.
Is output to. Upon receiving this clock selection instruction 6, the switching circuit 7 outputs the internal clock 5 as an output clock 8 to the outside.

Further, the clock selection instruction 6 described above is also output to the gate circuit 12, and upon receipt of this clock selection instruction 6, the gate circuit 12 receives the clock selection instruction 6.
12 is closed. When the internal clock 5 is selected, it is expected that the input of the external clock 2 is disconnected, but even if the detection signal 10 is output from the external clock input disconnection detection circuit 9, it is blocked by the gate circuit 12. ,
The detection signal 10 is not output to the alarm device.

As described above, according to the present embodiment, the gate circuit 12 operates based on the clock selection instruction 6 to open and close the output of the detection signal 10, so that the operator selects only the clock selection setting switch 3 when switching the clock selection. All you have to do is operate it.

〔The invention's effect〕

As described above, according to the present invention, the detection signal output blocking unit blocks the output of the detection signal from the detection unit by receiving the internal clock selection instruction. It is only necessary to operate and input a predetermined clock selection instruction.

Therefore, there is no need to switch the switch for turning off the detection signal, which has the advantage of improving operability. In addition, it is possible to eliminate the situation that the alarm device does not operate when the input of the external clock is interrupted due to forgetting the switch switching operation, and there is an advantage that the reliability is improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a clock switching circuit according to an embodiment of the present invention, and FIG. 2 is a block diagram showing a conventional clock switching circuit. 1 ... Switching circuit, 2 ... External clock, 3 ... Clock selection setting switch, 4 ... Internal clock generation circuit, 5 ...
… Internal clock, 6 …… Clock selection instruction, 7 …… Switching circuit, 8 …… Output clock, 9 …… External clock input break detection circuit, 10 …… Detection signal, 12 …… Gate circuit.

Claims (1)

[Claims] 1. A detection circuit that monitors the input state of an external clock input from the outside and outputs a detection signal when there is no input, an internal clock generation circuit that generates an internal clock, and a clock that outputs to the outside. A switch for selecting, and a gate circuit that outputs a control signal for generating an alarm only when the switch selects an external clock and the detection circuit is outputting a detection signal, The gate circuit is outputting a control signal, or
A clock switching circuit comprising: a switching unit that outputs the internal clock when the switch selects the internal clock, and outputs the external clock otherwise.