DIGITAL FREE-RUNNING CLOCK SYNCHRONIZER

Abstract: A digital free-running clock oscillator com­ prises a circuit synchronizing the operation of the oscillator with an asynchronous timing signal from an external source, and is provided with a protec­ tion circuit for preventing a logic race condition in the synchronizing circuit during a period of coincident transition of the oscillator output and the external timing signal.

Description: Background of the Invention The invention relates to oscillators, and more particularly to a free-running oscillator synchro­ nized with an external asynchronous pulse. Computer systems are required to work effi­ ciently with data stores of different speeds within the same system. If a semiconductor data store is used, the characteristics of the dynamic storage devices require that data in the storage elements be refreshed periodically. If store refresh opera­ tions are accomplished internally of the data store, the response time of the store will vary, and, accordingly, fully synchronous operation of a data store and the system central processor is not practical or desirable. Nevertheless, data trans­ fer between the two units must be synchronized. One means of effecting synchronization between asynchronously operating units of a computer system involves resynchronization or restarting of a clock signal source of one unit, for example a free-­ running oscillator forming a part of such unit, with a clock signal from another unit. However, clock synchronizing circuits using logic gates are particularly susceptible to logic race conditions inasmuch as the asynchronous clock signals of the various units of the system drift with respect to each other. Such logic race conditions can result in the generation of clock signals having pulse widths of insufficient duration for proper system operation. Accordingly, it is an object of the invention to provide an improved synchronizing circuit for a free-running clock signal generator. Another object of the invention is to provide an improved free-running clock signal generator having a digital logic circuit for synchronizing the generation of the clock signals with an asyn­ chronous clock signal from an external source. Another object of the invention is to provide an improved digital-logic clock signal synchro­ nizing circuit which provides protection against logic race conditions. Summary of the Invention In accordance with the present invention, a free-running clock signal oscillator comprises a logic element supplying a clock-signal output con­ nected in driving relation to a precision delay line, wherein the output of the delay line is connected back to the input of the logic element whereby the delay line serves as the timing element of the oscillator. The logic element also receives as an input a control signal from an asynchronously operating external source with which the clock signal output is to be synchronized. This control signal turns the oscillator on and off, thereby synchronizing the operation of the oscillator with the control signal. The control signal is coupled to the aforementioned logic element through a logic-race protection circuit which prevents a transition of the control signal when the clock signal output is in transition and when a potential logic-race condition exists in the logic element of the oscillator. Brief Description of the Drawings While the invention is set forth with particu­ larity in the appended claims, other objects, fea­ tures, the organization and method of operation of the invention will become more apparent, and the invention will best be understood, by referring to the following detailed description in conjunction with the accompanying drawings in which: FIG. l is a logic diagram of a free-running clock signal generator of the type utilized in the present invention; FIG. 2 is a logic diagram of a free-running clock signal generator in accordance with the instant invention; and FIGS. 3 and 4 are timing diagrams useful in explaining the operation of the present invention. Description of the Preferred Embodiment Referring now to the drawings for a more detailed description of the construction, operation and other features of the instant invention by characters of reference, FIG. l shows a free-­ running oscillator circuit comprising a NAND logic element or gate l0 having an input terminal l2 receivng an asynchronous control signal CS. An output terminal l4 of the NAND gate l0 is connected to an input terminal of a delay element l6 having a time delay of T2, while an output l8 of the delay element l6 is connected as a second input of the NAND gate l0. When the CS control signal is enabled or high, the oscillator runs but when the CS signal is disabled or low, the oscillator output signal KS at the output terminal l4 of the NAND gate l0 remains high. The CS control signal there­ fore turns the oscillator on and off or synchro­ nizes the oscillator output with the rising edge of the CS signal. A first transition from high to low of the KS output signal follows the transition of the CS control signal from low to high by one gate delay, the delay of the NAND logic element l0. Subsequent transitions of the KS clock signal occur after delay T2 plus the gate delay of the NAND gate l0. The oscillator circuit of FIG. l represents the prior art which has a disadvantage of possible unstable operation due to a logic race condition when the CS control signal changes state at the same time the KS output of the NAND gate l0 changes. At such time a KS clock signal having less than acceptable pulse width can occur. Referring now to FIG. 2 in conjunction with the timing diagrams of FIGS. 3 and 4, a free-­ running clock synchronizer circuit in accordance with the present invention includes a transparent latch circuit 30 receiving a CSl control signal on an input terminal 32. The latch circuit 30 may comprise a conventional storage element such as a D bistable or flip-flop. An output terminal 34 of the latch circuit 30, signal CSL, is connected as one input of a NAND logic element or gate 36, the output of which is the clock signal KSl. Clock signal KSl is coupled to input terminal 37 of delay line 38 adapted to produce a first output at tap 40 after a time delay Tl. This first output at tap 40 of the delay line 38, signal DLl, is connected as a first input of an AND gate 4l while a second output tap 44 of the delay line 38, signal DL2, is con­ nected as a second input of the NAND gate 36. The delay at the second output tap 44 of the delay line 38 is T2, T2 being greater than Tl. The CSL signal is connected as a second input of the AND gate 4l, an output of which is supplied as a first input of a NAND gate 42. Tap 44 of the delay line 38 further provides a second input to NAND gate 42 via an inverter 46, and NAND gate 42 supplies a signal DLE at terminal 48 which is connected to a latch-enable input 50 of the latch circuit 30. The KSl signal output of the NAND gate 36 is a square wave clock signal with a period 2(T2 + Dl) where Dl is the gate delay of the NAND gate 36. The latch circuit 30 functions normally as a trans­ parent logic element that passes the CSl control signal directly, with only one gate delay, to the NAND gate 36 as control signal CSL on terminal 34. The CSL signal gates the KSl oscillator output signal on and off so the oscillator output signal is synchronized with the CSl control signal. The enabling (high) output at terminal 48 of the NAND gate 42 normally enables the latch circuit to pass the CSl control signal as the CSL sig­ nal. But after the positive portion of the KSl signal has traversed the delay line 38 and appears at the Tl output tap 40 as the DLl signal, the CSL and DLl signals enable the AND gate 4l which causes the DLE signal output at terminal 48 of the NAND gate 44 to go low for a period T2-Tl and disable latch 30, i.e. when a positive-to-negative transi­ tion of the KSl signal may occur. As shown in FIG. 3, the CSl signal might also be in transition from high to low during such period. The disabled latch circuit 30 then protects the oscillator cir­ cuit from an undefined or unstable logic condition of NAND gate 36 caused by the CSl control signal changing state at or near the same time the KSl output of the NAND gate 36 changes. In particular, the latch circuit 30 prevents KSl from returning immediately to a positive level until after the DLE signal concludes and latch 30 generates the falling edge of CSL. The circuit output is then synchro­ nized with the rising edge of the CSL signal so that delaying the generation of the falling edge of the CSL signal does not adversely affect circuit operation. It is seen, however, that the circuit is prevented from generating pulses shorter than a given duration. FIG. 4 is an expansion of the potentially unstable period when the DLE signal goes low, and shows the gate delays associated with the genera­ tion of a minimum-width negative transition of the KSl clock signal. The protection circuit com­ prising the gates 4l, 42, 46 and the latch circuit 30 ensures that a negative portion of the KSl signal will have a length of at least three gate delays. Referring to FIG. 5, if the CSL signal is in the logic low state when the DLl signal goes true, the AND gate 4l is disabled and the DLE signal output on terminal 48 is inhibited from going low thereby preventing the latch circuit 30 from clos­ ing when the CSl signal is in transition from low to high. While the principles of the invention are clear in reference to the foregoing illustrative embodiment, there will be immediately obvious to those skilled in the art many modifications of structure, arrangement, and components used in the practice of the invention, and otherwise, which are particularly adapted for specific environments and operating requirements without departing from those principles. The appended claims are, therefore, intended to cover and embrace any such modifica­ tions, within the limits only of the true spirit and scope of the invention.

Claims: 1. A clock synchronizer, comprising: means for generating a clock signal; means for enabling the operation of said clock signal generating means, said enabling means including means responsive to a timing signal from an external source for disabling and then restarting said clock signal generating means in synchronism with the timing signal; and means for delaying said disabling means during a period of coincident transition of the clock signal and the timing signal. 2. The clock synchronizer of claim l, wherein said clock signal generating means comprises an inverting logic element having an output in series with a delay line and an input connected to an output of the delay line; and wherein said enabling means includes a second input of the inverting logic element receiving the timing signal. 3. The clock synchronizer of claim 2, wherein said delaying means includes means responsive to said delay line for detecting the transition period of the clock signal. 4. The clock synchronizer of claim 3, wherein said delaying means includes a latch circuit con­ nected to said logic element, said latch circuit receiving the timing signal from the external source, the latch circuit being responsive to said detecting means to regenerate the timing signal, the regeneration being inhibited during the transi­ tion period. 5. An oscillator circuit, comprising: a coincidence logic element having a first input receiving a control signal, a second input, and an output, the control signal having first and second logic states: a delay element having an input terminal connected to the output of said coincidence logic element, and a first output terminal connected to the second input of said coincidence logic element, said delay element passing a signal on the input terminal to the first output terminal after a delay T2, the control signal in the first logic state enabling said coincidence logic element to re­ generate on the output a complement of the signal on the second input, thereby generating on the output of said coincidence logic element a square wave output signal, the control signal in the second logic state disabling said coincidence logic element to stop the operation of said oscillator, said delay element having a second output terminal passing the signal on the input terminal after a delay Tl where Tl < T2; a latch circuit receiving an asynchronous timing signal from an external source and having an enabling input, said latch circuit, when enabled, regenerating the asynchronous timing signal as the control signal; and a logic element having an output connected to the enabling input of said latch circuit, said logic element receiving the first and second out­ puts of said delay element and generating an enabling output signal, the enabling output signal being disabled during a period of positive to nega­ tive transition of the square wave output signal. 6. The oscillator circuit of claim 5, wherein said coincidence logic element comprises a NAND gate and the square wave output signal has a period 2(T2 + D) where D is a gate delay of the NAND gate. 7. A clock synchronizer circuit, comprising: a free-running clock signal generator; a source of an asynchronous timing signal; and enabling means connected to said clock signal generator, said enabling means being respon­ sive to the asynchronous timing signal to provide an enabling signal for stopping and restarting said clock signal generator in synchronism with the timing signal; said enabling means responding to said timing signal with delay during a transition period of the clock signal to inhibit said enabling means from stopping said clock signal generator during a transition period of the clock signal. 8. The clock synchronizer circuit of claim 7, wherein said clock signal generator comprises an inverting logic element and a delay line having an input terminal and an output terminal, a signal at the output terminal of said delay line being recir­ culated through said inverting logic element to the input terminal of said delay line. 9. The clock synchronizer circuit of claim 8, wherein said said inverting logic element comprises a NAND gate. l0. The clock synchronizer circuit of claim 7, wherein said enabling means includes a coincidence circuit detecting the transition period of the clock signal, and a latch circuit receiving and regenerating the timing signal, the regenerating of the timing signal being inhibited by the output of the coincidence circuit during a clock signal tran­ sition period.

Inventors: THEUS, JOHN G.
Applicants: TEKTRONIX, INC.
Application year: 1986
Main classification: H03L7
IPC codes: H03L7, H03K3