A circuit and method for generating clock signals

Abstract: A method is provided for forming non-overlapping clock signals 402 and 404 for an integrated circuit. A reference clock 300 whose frequency is twice that of a desired operating frequency for the integrated circuit is used. A master clock signal is formed which has a high pulse width T9a which is approximately the same as the high pulse width of the reference clock, but the frequency of the master clock is one half the frequency of the reference clock. Likewise, a slave clock signal is formed which has a high pulse width T10a which is approximately the same as the high pulse width of the reference clock, but the frequency of the slave clock is also one half the frequency of the reference clock. The high pulse width of either or both the master clock signal and slave clock signal is then widened by an analog delay means, but by an amount T13 and T14 which is less than the low pulse width of the reference clock, so that the master clock signal and the slave clock signal do not overlap each other. As the propagation time of circuit elements within the integrated circuit varies due to changes in operating temperature or voltage, the analog delay T13 and T14 is changed proportionally to compensate for the changes in propagation time of the circuit elements.

Description: FIELD OF THE INVENTION This invention relates to circuits for generating clock output signals for electronic processing systems and devices. BACKGROUND OF THE INVENTION In an integrated circuit device, it is often desired to utilize a plurality of clocking signals for various portions of the circuit. In order to maximize efficiency, it is preferred to generate all clocking signals used on the integrated circuit from a single clock input. Many circuit designs today employ a two-phase clocking scheme for clocking latches and registers. Typically the two phases consist of a master and slave which must be non-overlapping in order to avoid data run-through or other timing hazards. However, this non-overlap timing can be difficult to maintain across various processes, temperatures, voltages and loading. In a typical circuit for generating non-overlap clock signals shown in prior art Figure 1, a single reference clock 100 is used to derive two non-overlapping clocks, a master 102 and a slave 104. The non-overlap timing is usually implemented by having an analog delay path connected to reference clock 100 to delay each rising edge of master clock 102 by a delay time T1 and by having an analog delay path connected to reference clock 100 to delay each rising edge of slave clock 104 by a delay time T2. A disadvantage of the prior art scheme is that the analog delays are very dependent on process parameters, operating temperature, and operating voltage. Thus, for parts which are produced with operating characteristics which fall in a weak corner of a process schmoo plot of frequency vs. voltage or temperature, the analog delay will increase as indicated by additional delay time T3 in prior art Figure 2, resulting in a shorter HIGH pulse width T4 on clock signals 102 and/or 104. This can cause problems since the device in general will operate slower at weak corners and generally will require longer pulse widths to accommodate the longer delays through the various layers of logic circuitry. Another disadvantage of this scheme is that the analog delay is very difficult to control across all Process/temperature/voltage corners. So if a minimum amount of non-overlap time is required for the device to function properly, the circuit must be designed to provide enough delay to ensure this non-overlap time, even at a strong corner of the process schmoo. Then at the weak corner, the non-overlap time can become excessively large and impact the maximum operating frequency of the device, as mentioned above. A circuit arrangement for the generation of a reference clock (Phi), a master clock (Phi1a), and a slave clock (Phi2a) is disclosed in Patent Abstracts of Japan, vol. 014, no. 12(E-871) published 11 January 1989. This circuit arrangement reduces and/or eliminates the occurrence of overlapping clocking signals. Non-overlapping master/slave signals are generated from a reference signal having twice the frequency in a digital manner using combinational logic circuitry to form two non-overlapping clock signals without the need for analog delay elements. Therefore, it is an object of the present invention to provide a driver circuit which results in the output of non-overlapping clock signals. It is further an object of the present invention to provide a driver circuit which is process insensitive. Other objects and advantages will be apparent to those of ordinary skill in the art having reference to the following figures and specification. SUMMARY OF THE INVENTION In accordance with the present invention, a method is provided for forming non-overlapping clock signals for an integrated circuit. A reference clock is provided that has a series of pulses at a first frequency, each pulse comprising a first transition and a second transition. A master clock signal is then formed at a second frequency in response to the first transition and the second transition of every other pulse of the reference clock. The pulses of the master clock are lengthened by a first analog delay that is associated with the second transition of the pulses of the reference clock. The second frequency is one-half that of the first frequency. A slave clock of the second frequency is formed in response to the first transition and the second transition of every other pulse of the reference clock. Thus, the master clock and the slave clock are out-of-phase and non-overlapping. As the propagation time of circuit elements within the integrated circuit varies due to changes in operating temperature or voltage, the analog delay is changed proportionally to compensate for the changes in propogation time of the circuit elements. BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the present invention will become apparent upon reference to the following description when considered in conjunction with the accompanying drawings, in which: Figure 1 is a timing diagram of prior art non-overlapping clock signals produced with analog delay; Figure 2 is a timing diagram of prior art non-overlapping clock signals shown timing degradation. Figure 3 is a timing diagram of non-overlapping clock signals produced by digital means according to an illustrative embodiment of the present invention; Figure 4 is a timing diagram of the non-overlapping clock signals of Figure 3 enhanced with an analog delay of trailing edges, according to an illustrative embodiment of the present invention; Figure 5 is a schematic diagram of a clock circuit for forming non-overlapping clock signals according to an illustrative embodiment of the present invention; Figure 6 is a timing diagram of the clock circuit of Figure 5;and Figure 7 is a schematic diagram of an alternate embodiment of a clock circuit for forming non-overlapping clock signals according to an illustrative embodiment of the present invention. Corresponding numerals and symbols in the different figures and tables refer to corresponding parts unless otherwise indicated. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION It has now been discovered that problems which were discussed in reference to prior art non-overlap disclosed herein which uses a digital circuit to form non-overlapping clock signals and then uses supplemental analog delays to lengthen pulse widths of the clock signals. Figure 3 is a timing diagram of non-overlapping clock signals produced by digital means. This scheme requires the reference clock 302 to be twice as fast as the desired output clock frequency. The actual non-overlap timing is performed digitally while the clock pulse width is improved with analog delays. Non overlapping clock signals are created digitally by using every other high pulse of reference clock 300 to form master clock 302 and slave clock 304. Reference clock 300 has a period of time T7 which includes a high pulse width T5 and a low pulse width of T6. Master clock 302 is formed with a period time of T8, which is twice as long as period T7. Master clock 302 has a high pulse width T9a which is approximately equal to high pulse width T5 of reference clock 300. Likewise, slave clock 304 is formed with a period time of T8, which is twice as long as period T7 and has a high pulse with T10a which is approximately equal to high pulse width T5. Thus, there is always a non-overlap time T11a and T12a between each high pulse of clock 302 and 304 that is equal to approximately low pulse width T6 of reference clock 300. A drawback of this fully digital scheme is that the master/slave high pulse widths T9a and T10a may not be wide enough for certain logic circuit designs within an integrated circuit (IC). This is handled by introducing some analog delay on the falling edge of each master clock signal edge, as shown in Figure 4, as delay time T13 and on the falling edge of each slave clock signal edge as delay time T14. Advantageously, unlike the prior art non-overlap circuit, process dependence of the analog delay is not a problem when used to widen pulse width T9a and T10a as described herein. Various process parameters such as doping, oxide thickness, resistivity, etc. can cause variations in the propagation delay of circuit elements on an integrated circuit. For an IC which is produced in a slow corner of a production process schmoo plot, analog delay will increase, therein increasing pulse width T9b and T10b and thereby giving slow timing paths in a logic circuit design more time to propagate. For an IC in a fast corner, pulse width T9b and T10b will decrease, but the timing paths within the IC will also speed up and need less propagation time. Another advantage of the analog delay technique of this embodiment is analog delays T13 and T14 only need to be optimized at the weak corner of the process plot to ensure the desired minimum non-overlap time T11b and T12b. Figure 5 is a schematic diagram of a clock circuit 500 for forming non-overlapping clock signals according to a further embodiment, while Figure 6 is a timing diagram illustrating the operation of this circuit. A single reference clock signal CLK is provided to clock circuit 500. Two outputs of circuit 500 provide a two-phase non-overlapping clock which consists of clock signal MASTER and clock signal SLAVE running at one half the frequency of reference clock signal CLK. Reference signal CLK is received by receiving circuitry 510. Gates 511-513, 514a-n and 515a-n form two preliminary non-overlapping clock signals S and M using a typical circuit which delays the rising edge of signals S and M, as shown in Figure 6. These preliminary non-overlapping clock signals are needed to ensure that no glitches occur during the formation of clock signals MASTER and SLAVE. Note that the preliminary non-overlap circuitry is the traditional type which delays leading edges. This is acceptable in this case since the local S/M clocks go to minimal logic and can be well controlled. Local clock signal S clocks a D flip-flop 516 to produce a signal SET_CLK. Signal SET-CLK is simply signal S divided by two and is used by slave circuitry 530 to form clock signal SLAVE and by master circuitry 540 to form clock signal MASTER. Slave circuitry 530 has an asynchronous latch formed by and-or gate 531 which produces signal SLAVE. Signal SET_CLK gates signal M to set this latch to form a rising edge of clock signal SLAVE, as shown in Figure 6 at time T600. Similarly, inverted SET_CLK gates signal M to set another asynchronous latch 541 in master circuitry 540 which produces a rising edge of clock signal MASTER at time T601. Note that signal M is asserted only when SET_CLK is stable, thus ensuring that there will be no glitches that could accidentally set asynchronous latch 541. This is the reason why local non-overlapping clocks M and S were generated. Signal M clocks a flip-flop 521 in delay circuitry 520 to produce a signal SET_CLKD, which is a delayed version of signal SET_CLK. SET_CLK and SET_CLKD are XNORed with gate 522 to produce a signal CLR_CLK. CLR_CLK is then delayed by delay block 523 to produce signal CLK_CLKD. When CLR_CLKD goes LOW, asynchronous latches 531 and 541 for SLAVE and MASTER are cleared at times T602 and T603, respectively. The delay time of delay block 523 which produces CLR_CLKD can be "fine-tuned" to achieve the desired minimum non-overlap time and minimum pulse width by varying the number of inverters 523a-n, for example. Note that the same fall time delay will apply to both MASTER and SLAVE. This is why signal SET_CLKD and XNOR gate 522 were used. Figure 7 is a schematic diagram of an alternate embodiment of a clock circuit 700 for forming non-overlapping clock signals. Clock circuit 700 is similar to clock circuit 500, except that two delay blocks 723 and 724 are used to clear asynchronous latches 531 and 541 for SLAVE and MASTER. Delay blocks 723 and 724 may be fine-tuned to have a different delays so that clock signals SLAVE and MASTER have different pulse widths. As used herein, the terms "applied", "connected," and "connection" mean electrically connected, including where additional elements may be in the electrical connection path. While the invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various other embodiments will be apparent to persons skilled in the art upon reference to this description. For example, falling edges of reference clock CLK could be delayed with a delay block to form a non-symmetrical reference clock from which signals MASTER and SLAVE are formed.

Claims: A method for forming non-overlapping clock signals (300;302;304) for an integrated circuit, which method comprising: providing a reference clock (300) having a series of pulses at a first frequency, said reference clock (300) having pulses each having a first transition and a second transition; forming a master clock (302) having a second frequency that is one half the first frequency and having pulses formed in response to said first transition and said second transition of every other pulse of said reference clock (300); forming a slave clock (304) having a second frequency that is one half said first frequency and having pulses formed in response to said first transition and said second transition of every other pulse of the reference clock (300), such that said master clock (302) and said slave clock (304) are out of phase and non-overlapping; and characterized in that the pulses of said master clock (302;402) are lengthened by a first analog delay that is associated with the second transition of the pulses of said reference clock (300). The method of Claim 1, wherein the step of forming the slave clock (304;404) comprises: forming a slave clock (304;404) having pulses that are lengthened by a second analog delay that is associated with the second transition of said reference clock (300). The method of Claim 2, further comprising: providing the first analog delay and the second analog delay with one analog delay element. The method of any of Claims 2-3, further comprising: proportionally changing the first analog delay and the second analog delay to a change in signal propagation time of a logic element within said integrated circuit in response to a change in operating temperature of said integrated circuit. The method of any of Claims 2-4, further comprising: proportionally changing the first analog delay and the second analog delay to a change in signal propagation time of a logic element within said integrated circuit in response to a change in operating voltage of said integrated circuit. The method of any of Claims 2-5, further comprising: proportionally changing the first analog delay and the second analog delay to a change in signal propagation time of a logic element within said integrated circuit in response to a change in process parameters of said integrated circuit. A clock circuit for generating non-overlapping clock signals for the integrated circuit, which clock circuit comprising: receiving circuitry (510) for receiving a reference clock (300) with a series of pulses at a first frequency, the reference clock (300) having pulses each having a first transition and a second transition; master circuitry (540) connected to the receiving circuitry (510) for forming a master clock (302;402) having a second frequency that is one half the first frequency and having pulses formed in response to the first transition and the second transition of every other pulse of the reference clock (300); and slave circuitry (530) connected to the receiving circuitry (510) for forming a slave clock (304;404) having the second frequency that is one half the first frequency and having pulses formed in response to the first transition and the second transition of every other pulse of the reference clock (300), such that the master clock (302;402) and slave clock (304;404) are out of phase and non-overlapping; characterized in that a first delay circuit (520) is connected to the master circuitry (540) for lengthening the pulses of the master clock (302;402) by a first analog delay associated with the second transition of the pulses of the reference clock (300). The integrated circuit of Claim 7, wherein a second delay circuit (520) is connected to the slave circuitry (530) for lengthening the pulses of the slave clock (304;404) by a second analog delay that is associated with the second transition of the reference clock (300). The integrated circuit of Claim 8, wherein the first analog delay and the second analog delay are both produced by one delay circuit (530). The integrated circuit of any of Claims 8-9, wherein the or each delay circuit (530) is arranged to provide a first analog delay and a second analog delay that change proportionally to a change in signal propagation time of a logic element within the integrated circuit in response to a change in operating temperature of the integrated circuit. The integrated circuit of any of Claims 8-10, wherein the or each delay circuit is arranged to provide a first analog delay and a second analog delay that change proportionally to a change in signal propagation time of a logic element within the integrated circuit in response to a change in operating voltage of the integrated circuit. The integrated circuit of any of Claims 8-11, wherein the or each delay circuit is arranged to provide a first analog -delay and a second analog delay that change proportionally to a change in signal propagation time of a logic element within the integrated circuit in response to a change in process parameters of the integrated circuit.

Inventors: JONES JASON A T, SWOBODA GARY L, JONES, JASON A. T., SWOBODA, GARY L.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1997
Main classification: H03K5
IPC codes: G06F1, H03K5