A METHOD AND A DEVICE FOR A CHANGEOVER OF ASYNCHRONOUS CLOCK SIGNALS

Abstract: The invention relates to a method of and a device for providing redundacy for an asynchronous clock signal (MC), in which the redundant asynchronous clock signals (M1, M2) are changed over for being used as a clock signal by means of a separate selection signal (MS). According to the invention, a changeover is performed by setting the outgoing clock signal (MC) at the edge of the used clock signal (M1 or M2) to a constant state, and the clock signal to be used (M1 or M2) is activated to constitute the outgoing clock signal (MC) at such an edge of itself which is at least the second in order counted from the moment at which the outgoing clock signal (MC) is set to the constant state.

Description: A method and a device for a changeover of asynchro¬ nous clock signals The invention relates to a method of providing redundancy for an asynchronous clock signal, in which method redundant asynchronous clock signals are changed over for being used as a clock signal by means of a separate selection signal. When redundant asynchronous clock signals are used, a random spurious pulse appears in connection with a change of the clock signal to be used, which pulse may cause malfunctions of the device using the clock. Due to these disturbances, transmission errors occur in the traffic to be transmitted in a digital telephone exchange using the changeover, as well as unnecessary alarms in the clock controlling units. A known method of controlling the change proce¬ dure of a clock signal to be used is to generate the clock signal independently and to lock the phase of the generated signal to the phase of a selected ref¬ erence signal. Implementation of a phase-lock is, however, complicated and, in general, the operation thereof has to be controlled separately. The object of this invention is to provide a device, by means of which asynchronous clock signals may be changed over in a simple manner without dis¬ turbances. The method according to the invention is characterized in that the changeover is performed by setting the outgoing clock signal at the edge of the used clock signal to a constant state and that the clock signal to be used is activated to constitute the outgoing clock signal at such an edge of itself which is at least the second in order counted from the moment at which the outgoing clock signal is set to the constant state. Advantages of the method according to the in¬ vention are an undisturbed changing over from using one clock signal to using another and a simple method of implementation. The other preferred embodiments of the invention and the device according to the inven¬ tion are characterized in what has been set forth below in the claims. In the following, the invention will be ex¬ plained in greater detail by means of an example re- ferring to the attached drawing, in which Figure 1 shows a switch diagram of a device according to the invention, Figure 2 shows a timing plan of the invention. A device for a changeover of clock signals ac- cording to the invention is shown in Figure 1. An asynchronous clock selection signal MS and the inver¬ sion thereof are sampled by mutually asynchronous clock signals Ml and M2. The samples are taken by two sequentially switched D-flip-flop circuits la, lb and 2a, 2b, respectively, at the rising and falling edge of the clock signals Ml and M2 in order to eliminate an interference peak from sampled selection signals SI and S2, which are the outputs at pin 5 of the cir¬ cuit lb and at pin 9 of the circuit 2b. The inversion of the selection signal MS is sampled through an OR gate 4b by the clock signal M2, which will be introduced. The inversion of the selec¬ tion signal SI sampled by the Ml is led to one input of the OR gate. Thanks to this feedback signal, the selection signal S2 to be obtained from the D-flip- flop 2b changes to zero state and the clock signal M2 will not be introduced until after the used clock signal MC has been stopped in the 1-state. The clock signal M2 will be introduced at the first suitable edge changing to zero state. According to the invention, two sampled selec¬ tion signals SI and S2 are thus generated from the selection signal MS at successive edges of two clock signals. The first sample signal SI is fed back to the circuit sampling by the second clock signal M2 and the second sample signal S2 is fed back to the circuit sampling by the first clock signal Ml. When the second clock signal is introduced, the outgoing clock signal MC is passivated, i.e. left in a con- stant state by the first selection signal SI. On ac¬ count of the feedback, the second clock signal M2 cannot be introduced until the change of state of the first sample signal SI has been sampled by the second clock signal M2. In the initial situation of the timing plan of Figure 2, the MS is in zero state and the clock sig¬ nal Ml has been selected to be used as the clock sig¬ nal MC. With the change of the selection signal MS to the 1-state, the output 5 of the D-flip-flop lb also changes to the 1-state after the first suitable fall¬ ing and rising edge of the signal Ml. The output of the OR gate 3a and the clock signal MC to be used, selected by the AND circuit 6, also remain in the 1- state. The circuit according to the invention pre- vents pulse lengths shorter than those of incoming clock signals from occurring in the selected clock signal MC. When the clock signal M2 is changed over to use, the clock signal MC to be used shows a prolonged pulse in the 1-state. This prolongation depends on the phase difference of the clock signals and is be¬ tween zero and the duration of one period of the clock signal M2. When the clock signal Ml is changed over to use, the circuit operates on a corresponding prin- ciple. Here, the clock signal MC also shows a pro¬ longed pulse in the 1-state. The length of this pro¬ longation is between the duration of one period of the clock signal M2 and the double value thereof. A propagation delay of the inverter 5 is longer than the propagation delay of the D-flip-flop 2b. This prevents a spurious pulse at the output (pin 8) of the OR gate 3b when the clock signal Ml is to be used. Without this arrangement, the spurious pulse would occur at a moment when a 1-state is clocked for the first time at the output (pin 9) of circuit 2b. For one skilled in the art it is clear that the various embodiments of the invention are not re¬ stricted to the above example, but they may vary within the scope of the claims presented below.

Claims: Claims : 1. A method of providing redundancy for an asynchronous clock signal (MC), in which method re- dundant asynchronous clock signals (Ml, M2) are changed over for being used as a clock signal by means of a separate selection signal (MS), c h a r¬ a c t e r i z e d in that the changeover is per¬ formed by setting the outgoing clock signal (MC) at the edge of the used clock signal (Ml or M2) to a constant state and that the clock signal to be used (Ml or M2) is activated to constitute the outgoing clock signal (MC) at such an edge of itself which is at least the second in order counted from the moment at which the outgoing clock signal (MC) is set to the constant state. 2. A method according to claim 1, c h a r a c¬ t e r i z e d in that sample signals (SI, S2) are generated from the selection signal (MS) at the ris- ing and falling edge of the redundant asynchronous clock signals (Ml, M2) and that the sample signals (SI, S2) are fed back crosswise to the sampling cir¬ cuits of the clock signals (Ml, M2), whereby, at changeover of the clock signal, the outgoing clock signal (MC) is set to a constant state by the sample signal of the used clock signal and that the feedback of the sample signals (SI, S2) does not activate the asynchronous clock signal to be used as the outgoing clock signal (MC) until the change of state of the sample signal of the used clock signal has been sam¬ pled by the clock signal to be used. 3. A device for providing redundancy for an asynchronous clock signal (MC), constituted by a changeover switch of redundant asynchronous clock signals (Ml, M2), which switch is operated by a sepa- rate selection signal (MS), c h a r a c t e r i z¬ e d in that the device comprises means (lb, 2b) for performing a changeover by setting the outgoing clock signal (MC) at the edge of the used clock signal (Ml or M2) to a constant state and means (la, lb and 2a, 2b) for activating the clock signal (Ml or M2) to be used to constitute the outgoing clock signal (MC) at such an edge of itself which is at least the second in order counted from the moment at which the outgo- ing clock signal (MC) is set to the constant state. 4. A device according to claim 3, c h a r a c¬ t e r i z e d in that for each redundant clock sig¬ nal there are two sequentially switched D-flip-flops (la, lb and 2a, 2b) for sampling the selection signal (MS) at the rising and falling edge of the asynchron¬ ous clock signals (Ml, M2) to generate corresponding sample signals (SI, S2), and that the sample signals (SI, S2) are fed back crosswise to the sampling cir¬ cuits (la, 2a) of the clock signals (Ml, M2), where- by, at a change of state of the selection signal (MS), the outgoing clock signal (MC) may be set to a constant state by the sample signal of the used clock signal, and that the feedback of the sample signals (SI, S2) is arranged not to activate the asynchronous clock signal to be used as the outgoing clock signal (MC) until the change of state of the sample signal of the used clock signal has been sampled by the clock signal to be used. AMENDED CLAIMS [received by the International Bureau on 21 March 1994 (21.03.94); original claims 1 " and 2 and 3 and 4 replaced by amended claims 1 and 2 respectively (2 pages)] (amended on March 14, 1994 under Article 19) 1. A method of providing redundancy for an asynchronous clock signal (MC) , in which method re- 5 dundant asynchronous clock signals (Ml, M2) are changed over for being used as a clock signal by means of a separate selection signal (MS) , and where the changeover is performed by setting the outgoing clock signal (MC) at the edge of the used clock sig- 10 nal (Ml or M2) to a constant state, and the clock signal to be used (Ml or M2) is activated to consti¬ tute the outgoing clock signal (MC) at such an edge of itself which is at least the second in order coun¬ ted from the moment at which the outgoing clock sig- 15 nal (MC) is set to the constant state, c h r a c¬ t e r i z e d in that sample signals (SI, S2) are generated from the selection signal (MS) at the ris¬ ing and falling edge of the redundant asynchronous clock signals (Ml, M2) and that the sample signals 20 (SI, S2) are fed back crosswise to the sampling cir¬ cuits of the clock signals (Ml, M2) , whereby, at changeover of the clock signal, the outgoing clock signal (MC) is set to a constant state by the sample signal of the used clock signal and that the feedback 25 of the sample signals (SI, S2) does not activate the asynchronous clock signal to be used as the outgoing clock signal (MC) until the change of state of the sample signal of the used clock signal has been sam¬ pled by the clock signal to be used. 30 2. A device for providing redundancy for an asynchronous clock signal (MC) , constituted by a changeover switch of redundant asynchronous clock signals (Ml, M2) , which switch is operated by a sepa¬ rate selection signal (MS) , the device comprising 35 means (lb, 2b) for performing a changeover by setting AMENDED SHEET (ARTICLE 19; the outgoing clock signal (MC) at the edge of the used clock signal (Ml or M2) to a constant state and means (la, lb and 2a, 2b) for activating the clock signal (Ml or M2) to be used to constitute the out- going clock signal (MC) at such an edge of itself which is at least the second in order counted from the moment at which the outgoing clock signal (MC) is set to the constant state, c h a r a c t e r i z e d in that for each redundant clock signal there are two sequentially switched D-flip-flops (la, lb and 2a, 2b) for sampling the selection signal (MS) at the rising and falling edge of the asynchronous clock signals (Ml, M2) to generate corresponding sample signals (SI, S2) , and that the sample signals (SI, S2) are fed back crosswise to the sampling circuits (la, 2a) of the clock signals (Ml, M2) , whereby, at a change of state of the selection signal (MS) , the outgoing clock signal (MC) may be set to a constant state by the sample signal of the used clock signal, and that the feedback of the sample signals (SI, S2) is arranged not to activate the asynchronous clock signal to be used as the outgoing clock signal (MC) until the change of state of the sample signal of the used clock signal has been sampled by the clock sig- nal to be used.

Inventors: RUUSKANEN MARKKU, RUUSKANEN, MARKKU
Applicants: NOKIA TELECOMMUNICATIONS OY, RUUSKANEN MARKKU, RUUSKANEN, MARKKU
Application year: 1993
Main classification: H03K5
IPC codes: G06F1, H03K5