Flip-flop circuit

Abstract: A flip-flop circuit comprises a first transfer gate (11) receiving input data (Di) and being conductive responsive to a clock signal (CL), a second transfer gate (12) receiving an inverted input data ( D i) and being conductive responsive to said clock signal (CL), a third transfer gate (21) receiving said input data (Di) and being conductive responsive to an inverted clock signal ( CL ), and a fourth transfer gate (22) receiving said inverted input data ( D i) and being conductive responsive to said inverted clock signal ( CL ). First and second inverters (13,14) are cross-coupled across output nodes (N11,N12) of said first and second transfer gates (11,12) to serve as a latch circuit (10). Third and fourth inverters (23,24) are cross-coupled across output nodes (N21,N22) of said third and fourth transfer gates (21,22) to serve as a latch circuit (11). A selector (30) selects either the outputs (N11,N12) of the first and second transfer gates (11,12), or the outputs (N21,N22) of the third and forth transfer gates (23,24), as output data (Do) and inverted output ( D o) data of the flip-flop circuit, in accordance with said clock (CL) and inverted clock signals ( CL ).

Description: BACKGROUND OF THE INVENTION This invention relates to a high-speed flip-flop circuit possessing functions substantially equivalent to those of a delay flip-flop. A delay flip-flop (hereinafter referred to as a D-FF) is in general a circuit that takes in data in synchronization with a clock signal and outputs it at a certain time. At high clock frequencies, this type of D-FF is liable to malfunction in taking in the data. Various flip-flop circuits (hereinafter referred to as FF circuits) suitable for high-speed operation have therefore been proposed. A prior-art FF circuit of this type has been described in the article "5-Gbit/s Si Integrated Regenerative Demultiplexer and Decision Circuit" by Detlef Clawin, Ulrich Langmann, and Hans-Ulrich Schreiber, published on pages 385 to 389 of the (American) IEEE Journal of Solid-State Circuits , Vol. SC-22, No. 3, in June 1987. A description thereof will be given with reference to the drawings. Fig. 1 is a block diagram showing an example of the configuration of a prior-art FF circuit, known e.g. from IEEE Journal of Solid-State Circuits, vol. SC-22, no. 6, December 1987, pages 1048-1054; K.C. HSIEH et al: "A 12-bit 1-Gword/s GaAs digital - to - analog converter system". This FF circuit comprises a pair of master-slave flip-flops (hereinafter referred to as MS-FFs) 1 and 2 for latching data Di and the inverted data D i in synchronization with a clock signal CL and its inverted clock signal CL , and a selector 3 for selecting the outputs of the MS-FFs 1 and 2 responsive to the clock signal CL and the inverted clock signal CL and producing them in the form of data Do and the inverted data D o. The MS-FF 1 is a circuit that latches the data Di and the inverted data D i on the rise of the clock signal CL and outputs them on the fall of the clock signal CL, and has an input terminal D1 for input of the data Di, an inverted input terminal D 1 for input of the inverted data D i, a clock input terminal C1 for input of the clock signal CL, an inverted clock input terminal C 1 for input of the inverted clock signal, an output terminal Q1, and an inverted output terminal Q 1. The MS-FF 2 is a circuit that latches the data Di and the inverted data D i on the rise of inverted clock signal CL and outputs them on the fall of the inverted clock signal CL , and has an input terminal D2 for input of the data Di, an inverted input terminal D 2 for input of the inverted data D i, a clock input terminal C2 for input of the inverted clock signal CL , an inverted clock input terminal C 2 for input of the clock signal CL, an output terminal Q2, and an inverted output terminal Q 2. The selector 3 has an input terminal I1 connected to the output terminal Q1, an inverted input terminal I 1 connected to the inverted output terminal Q 1, an input terminal I2 connected to the output terminal Q2, an inverted input terminal I 2 connected to the inverted output terminal Q 2, a control terminal CS for input of the clock signal CL, an inverted control terminal CS for input of the inverted clock signal CL , an output terminal O for output of the data Do, and an inverted output terminal O for output of the inverted data D o. When the logic level of the control terminal CS is high (hereinafter denoted "H",) and the logic level of the inverted control terminal CS is low (hereinafter denoted "L"), the selector 3 produces the logic level of the input terminal I1 at the output terminal O, and produces the logic level of the inverted input terminal I 1 at the inverted output terminal O ; when the control terminal CS is "L", and the inverted control terminal CS is "H," it produces the logic level of the input terminal I2 at the output terminal O and the logic level of the inverted input terminal I 2 at the inverted output terminal O . Fig. 2 is a timing chart for Fig. 1, with times t0 to t4 indicated on the horizontal axis. The operation of Fig. 1 will be explained with reference to this chart. At time t0, the clock signal CL is "H," the inverted clock signal is "L", the data Di is L,", and the inverted data D i is "H," so the output terminal Q1 of the MS-FF 1 is held at "L," the inverted output terminal Q 1 is held at "H," the output terminal Q2 of the MS-FF 2 goes to "L," and the inverted output terminal Q 2 goes to "H," making the data Do at the output terminal O of the selector 3 "L" and the inverted data D o at the inverted output terminal O H. At time t1, when the clock signal CL goes to "L" and the inverted clock signal CL goes to "H," since the data Di is "H" and the inverted data D i is "L," the output terminal Q1 of the MS-FF 1 rises to "H" and the inverted output terminal Q 1 falls to "L." The output terminals Q2 and Q 2 are not changed. At time t2, when the clock signal CL goes to "H" and the inverted clock signal CL goes to "L," since the data Di is "L" and the inverted data D i is "H," the output terminal Q2 of the MS-FF 2 is "L" and the inverted output terminal Q 2 is "H," the output terminal Q1 of the MS-FF 1 remains "H" and the inverted output terminal Q 1 remains "L." The data Do at the output terminal O of the selector 3 is therefore "H," and the inverted data D o at the inverted output terminal O is "L." At time t3, when the clock signal CL goes to "L", and the inverted clock signal CL goes to "H," since the data Di is "H" and the inverted data D i is "L," the output terminal Q1 of the MS-FF 1 is "H" and the inverted output terminal Q 1 is "L," the output terminal Q2 of the MS-FF 2 remains "L" and the inverted output terminal Q 2 remains "H," the data Do goes to "L," and the inverted data D o goes to "H." At time t4, when the clock signal CL goes to "H" and the inverted clock signal CL goes to "L,", since the data Di is "H" and the inverted data D i is "L," the output terminal Q1 of the MS-FF 1 remains "H" and the inverted output terminal Q 1 remains "L," the output terminal Q2 of the MS-FF 2 goes to "H" and the inverted output terminal Q 2 goes to "L," and the data Do goes to "H" while the inverted data D o goes to "L." The FF circuit in Fig. 1 thus operates as a D-FF by outputting the logic levels of the data Di and the inverted data D i when the clock signal CL and the inverted clock signal CL change as the data Do and inverted data D o at the next change of the clock signal CL and the inverted clock signal CL . When the clock signal CL is "H" and the inverted clock signal CL is "L," the output signals of the MS-FF 1 are output by the selector 3, while when the clock signal CL is "L" and the inverted clock signal CL is "H," the output signals of the MS-FF 2 are output by the selector 3, so stable, high-speed operation is possible even at high frequencies of the clock signal CL. The reason that the FF circuit in Fig. 1 is capable of high-speed operation can be stated as follows. The FF circuit in Fig. 1 is constructed so that the MS-FF 1 and the MS-FF 2 operate at only half the clock frequency at which they would operate if used as stand-alone D-FFs. The FF circuit in Fig. 1 can therefore operate at a bit rate equal to twice the limiting clock frequency of a stand-alone D-FF. From EP-A-0 110 060 is known a flip-flop structure having crossed-coupled inverters of the dynamic type which are driven by a clock pulse (csi). A problem with the FF circuit described above is that, since it employs two MS-FFs 1 and 2 and a selector 3, if these comprise NOR gates, for example, eighteen to twenty-two NOR gates are required; the large number of elements required and the complex circuit structure present an obstacle to high levels of circuit integration, and consume more power than the usual D-FF. A further problem is that the output is delayed by one bit as compared with the usual D-FF, which restricts the applicability of the circuit. SUMMARY OF THE INVENTION An object of the invention is to provide an FF circuit that solves the problem of the large number of elements and complex circuit structure, the problem of large power consumption, and the problem of output delay found in the prior art described above. A flip-flop circuit according to the invention is defined in claim 1. In an FF circuit configured according to this invention as described above, the first and second transfer gates and the third and fourth transfer gates switch alternately on and off in synchronization with the clock signal and the inverted clock signal and alternately take in the data and the inverted data. The first and second inverters and the third and fourth inverters temporarily store, then output, the data and inverted data taken in by the transfer gates. The selector alternately outputs the first and second FF outputs in synchronization with the clock signal and inverted clock signal. The circuit thus operates, for example, at high speed in the manner of a D-FF. Since the first and second FFs have few elements, the circuit configuration is simplified and can yield improved scales of integration while consuming less power. There is furthermore no one-bit output delay in the configuration of the first and second FFs. The above-mentioned problems can therefore be eliminated. The selector circuit can also be simplified by employment of a configuration comprising, for example, six NOR gates. Further embodiments of the invention are defined in the sub-claims BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a prior-art FF circuit. Fig. 2 is a timing chart for Fig. 1. Fig. 3 is a schematic diagram of an FF circuit illustrating an embodiment of this invention. Fig. 4 is a timing chart for Fig. 3. DETAILED DESCRIPTION OF THE EMBODIMENTS Fig. 3 is a schematic diagram of an FF circuit illustrating an embodiment of the present invention. This FF circuit comprises first and second FFs 10 and 20 for latching input data Di and inverted input data D i alternately in synchronization with a clock signal CL and an inverted clock signal CL , and a selector 30 for selecting alternately, responsive to the clock signal CL and the inverted clock signal CL , the data Di and the inverted data D i latched by the first and second FFs 10 and 20 and producing them as data Do and inverted data D o; the overall function of this FF circuit is to operate as a D-FF. The first FF 10 comprises a first transfer gate 11 which is in the form of a field effect transistor, which has one of its source/drain electrodes connected to an input data terminal, which is also denoted by Di, to receive the data Di, whose gate electrode is connected to a clock signal input terminal CL so that it turns on and off responsive to the clock signal CL and which, when it is on, takes in the data Di. The first FF 10 also comprises a second transfer gate 12 which is in the form of a field effect transistor, which has one of its source/drain electrodes connected to an inverted input data terminal D i to receive the inverted data D i, which turns on and off responsive to the clock signal CL and which, when it is on, takes in the inverted data D i. The other of the source/drain electrodes of the transfer gate 11 is connected to an output node N11 of the flip-flop circuit 10. The other of the source/drain electrodes of the transfer gate 12 is connected to an output node N12 of the flip-flop circuit 10. The first FF 10 further comprises a latch circuit 15 comprising first and second inverters 13 and 14 which are cross-coupled across the output nodes N11 and N12 of the first and second transfer gates 11 and 12, as illustrated in Fig. 3. More specifically, the first inverter 13 has input and output nodes connected to the outputs of the first and second transfer gates 11 and 12, respectively. The second inverter 14 has input and output nodes connected to the outputs of the second and first transfer gates 12 and 11, respectively. The second FF 20 comprises a third transfer gate 21 which is in the form of a field effect transistor, which has one of its source/drain electrodes connected to the input data terminal Di to receive the data Di, whose gate electrode is connected to an inverted clock signal input terminal CL so that it turns on and off responsive to the inverted clock signal CL and which, when it is on, takes in the data Di. The second FF 20 also comprises a fourth transfer gate 22 which is in the form of a field effect transistor, which has one of its source/drain electrodes connected to the inverted input data terminal D i to receive the data D i, whose gate electrode is connected to the inverted clock signal input terminal CL so that it turns on and off responsive to the inverted clock signal CL and which, when it is on, takes in the data Di . The second FF 20 further comprises a latch circuit 25 comprising third and fourth inverters 23 and 24 which are cross-coupled across the output nodes N21 and N22 of the third and fourth transfer gates 21 and 22. More specifically, the third inverter 23 has input and output nodes connected to the outputs of the third and fourth transfer gates 21 and 22, respectively. The fourth inverter 24 has input and output nodes connected to the outputs of the fourth and third transfer gates 24 and 23, respectively. The first to fourth transfer gates 11, 12, 21, and 22 which comprise field-effect transistors (hereinafter referred to as FETs) turn on when the clock signal CL and inverted clock signal CL applied to their gate electrodes are "H," and turn off when they are "L." The selector 30 comprises first to sixth two-input NOR gates 31, 32, 33, 34, 35, and 36; the clock signal CL and the node N11 are connected to the inputs of the first NOR gate 31, the clock signal CL and the node N12 to the inputs of the second NOR gate 32, the inverted clock signal CL and the node N21 to the inputs of the third NOR gate 33, and the inverted clock signal CL and the node N22 to the inputs of the fourth NOR gate 34. The output nodes N31 and N33 of the first and third NOR gates 31 and 33 are connected to the inputs of the fifth NOR gate 35, the output nodes N32 and N34 of the second and fourth NOR gates 32 and 34 are connected to the inputs of the sixth NOR gate 36, and the data Do and inverted data D o are output from the outputs of the fifth and sixth NOR gates 35 and 36. Fig. 4 is a timing chart for Fig. 3, with times t0 to t4 indicated on the horizontal axis. The operation of Fig. 3 will be explained with reference to this chart. At time t0, the data Di is "L," the inverted data D i is "H," the clock signal CL rises to "H," and the inverted clock signal CL falls to "L," so the first and second transfer gates 11 and 12 turn on, the output node N11 goes to "L," the output node N12 goes to "H," the third and fourth transfer gates 21 and 22 turn off, and the output nodes N21 and N22 retain their previous "L" and "H" states, respectively. The "L" of the node N11 and the "H" of the clock signal CL are NORed by the NOR gate 31, the output node N31 of which goes to "L." Similarly, the "H" of the node N12 and the "H" of the clock signal CL pass through the NOR gate 32, the output node N32 of which is kept at "L," the "L" of the node N21 and the "L" of the inverted clock signal CL pass through the NOR gate 33, the output node N33 of which goes to "H", and the "H" of the node N22 and the "L" of the inverted clock signal CL pass through the NOR gate 34, the output node N34 of which is kept at "L." The "L" of the node 31 and the "H" of the node 33 are NORed by the NOR gate 35 causing the output data Do to be at "L", and the "L" of the node 32 and the "L" of the node 34 are NORed by the NOR gate 36 causing the inverted output data D o to be at "H." At time t01, the data Di rises to "H," and the inverted data D i falls to "L." Accordingly, the node N11 rises to "H," and the node N12 falls to "L." Because the clock signal CL is "H," the nodes N31 and N32 are kept at "L." At time t1, when the clock signal CL goes to "L" and the inverted clock signal CL goes to "H," since the data Di is "H" and the inverted data D i is "L," the node N11 is kept at "H," the node N12 is kept at "L," the node 21 goes to "H," the node 22 goes to "L", the node 31 is kept at "L," the node 32 goes to "H," the node 33 goes to "L," the node 34 is kept at "L," the data Do goes to" H," and the inverted data D o goes to "L." At time t11, the data Di falls to "L," and the inverted data D i rises to "H." Accordingly, the node N21 falls to "L," and the node N22 rises to "H." Because the inverted clock signal CL is "H," the nodes N33 and N34 are kept at "L." At time t2, when the clock signal CL goes to "H" and the inverted clock signal CL goes to "L," since the data Di is "L" and the inverted data D i is "H" the nodes N11, N12, N21, N22, N31, N32, N33 and N34 will be at "L," "H," "L," "H," "L," "L," "H," and "L," respectively, the data Do goes to "L," and the inverted data D o goes to "H." At time t21, the data Di rises to "H," and the inverted data D i falls to "L." Accordingly, the node N11 rises to "H," and the node N12 falls to "L." Because the clock signal CL is "H," the nodes N31 and N32 are kept at "L." At time t3, when the clock signal CL goes to "L," and the inverted clock signal CL goes to "H," since the data Di is "H" and the inverted data D i is "L," the nodes N11, N12, N21, N22, N31, N32, N33, and N34 will be at "H," "L," "H," "L," "L," "H," "L," and "L," respectively, the data Do goes to "H," and the inverted data D o goes to "L." It is assumed that there is no change in the data Di and the inverted data D i between time t3 and time t4. At time t4, when the clock signal CL goes to "H" and the inverted clock signal CL goes to "L," since the data Di is "H" and the inverted data D i is "L," the nodes N11, N12, N21, N22, N31, N32, N33, and N34 will be at "H," "L,", "H," "L," "L," "L," "L,", and "H," respectively, the data Do goes to "H," and the inverted data D o goes to "L." Thus when the clock signal CL goes to "H" and the inverted clock signal CL goes to "L," the selector 30 produces the outputs of the second FF 20, namely the signals at the nodes N21 and N22, as the data Do and the inverted data D o; when the clock signal CL goes to "L" and the inverted clock signal CL goes to "H," it produces the outputs of the first FF 10, namely the signals at the nodes N11 and N12, as the data Do and the inverted data D o. The FF circuit in Fig. 3 therefore operates as a D-FF by outputting, as the data Do and inverted data D o, the logic levels of the data Di and the inverted data D i when the logic levels of the clock signal CL and the inverted clock signal CL change. This embodiment possesses the following advantages: (a) Whereas the prior-art FF circuit required eighteen to twenty-two NOR gates, the FF circuit of this embodiment comprises four transfer gates 11, 12, 21, and 22, four inverters 13, 14, 23, and 24, and six NOR gates 31 to 36; thus it has fewer elements, hence consumes less power, and enables higher-scale integration to be achieved due to its simpler circuit structure. (b) The data Do and inverted data D o are output with the same timing as in the usual D-FF, so this embodiment can be widely used in the same applications as the usual D-FF, where high processing speeds are required. The invention is not limited to the embodiment shown in the drawings, but permits various modifications, some examples of which are given next. (i) The transfer gates 11, 12, 21, and 22 may comprise transistors other than FETs, or may comprise devices such as analog switches having a pair of transistors connected in parallel. (ii) The selector 30 may comprise a combination of NOR gates and gates other than NOR gates, such as NAND gates, or may comprise only gates other than NOR gates. In place of the combination of the NOR gates, the selector 30 can comprise other logic circuit of the same function. Moreover, it can comprises two pairs of transfer gates controlled by the clock signal CL and the inverted clock signal CL , and selectively transfer the outputs of the nodes N11 and N12, or the nodes N21 and N22, to the output nodes Do and D o. As has been explained in detail, since the first and second FFs according to this invention comprise two transfer gates and two inverters apiece, the number of elements is small, power consumption can be accordingly reduced, and the circuit structure can be simplified, permitting a higher scale of integration. There is furthermore no one-bit output delay as in the prior art; the removal of this restriction enables many applications.

Claims: A flip-flop circuit comprising first and second flip-flops (10, 20) each having respective first and second output nodes (N11, N12; N21, N22): said first flip-flop (10) being coupled for latching input data (Di) and inverted input data ( Di ) in synchronization with a clock signal (CL) and for generating a first output signal and a first inverted output signal at said first output node (N11) and said second output node (N12) of said first flip-flop (10); said second flip-flop (20) being coupled for latching said input data (Di) and inverted input data ( Di ) in synchronization with the inverted clock signal ( CL ) and generating a second output signal and a second inverted output signal at said first output node (N21) and said second output node (N22) of said second flip-flop (20); and a selector circuit (30) coupled to said first and second flip-flops (10; 20) for selecting and outputting said first output signal and first inverted output signal responsive to said clock signal (CL) and selecting and outputting said second output signal and second inverted output signal responsive to said inverted clock signal ( CL ); characterized in that said first flip-flop (10) comprises first and second transfer gates (11, 12) connected for directly receiving said input data (Di) and inverted input data ( Di ), respectively, responsive to said clock signal (CL), and first and second inverters (13, 14) cross-coupled across the output nodes of said first and second transfer gates (11, 12) forming said first and second output nodes (N11, N12) of said first flip-flop (10); and wherein said second flip-flop (20) comprises third and fourth transfer gates (21, 22) connected for directly receiving said data (Di) and inverted input data ( Di ), respectively, responsive to said inverted clock signal ( CL ), and third and fourth inverters (23, 24) cross-coupled across the output nodes of said third and fourth transfer gates (21, 22), said output nodes of said first and second transfer gates forming said first and second output nodes (N21, N22) of said second flip-flop (20). A flip-flop circuit according to claim 1, wherein said selector circuit is coupled to select said first output signal and said first inverted output signal when said clock signal is Low; and said selector circuit (30) is coupled to select said second output signal and second inverted output signal when said inverted clock signal is Low. A flip-flop circuit according to claim 1, wherein said selector circuit comprises first and second NOR gates (31, 32) for NORing the output signals of said first and second transfer gates (11, 12) with said clock signal, third and fourth NOR gates (33, 34) for NORing the output signals of said third and fourth transfer gates (21, 22) with said inverted clock signal, a fifth NOR gate (35) for NORing the outputs of said first and third NOR gates, and a sixth NOR gate (36) for NORing the outputs of said second and fourth NOR gates. A flip-flop circuit according to claim 1, wherein said first output signal and first inverted output signal of said first flip-flop are the outputs of said first and second transfer gates (11, 12); and said second output signal and second inverted output signal of said second flip-flop are the outputs of said third and fourth transfer gates (21, 22). A flip-flop circuit according to claim 1, wherein said first inverter (13) has input and output nodes connected to the outputs of said first and second transfer gates (11, 12), respectively; said second inverter (14) has input and output nodes connected to the outputs of said second and first transfer gates (12, 11), respectively; said third inverter (23) has input and output nodes connected to the outputs of said third and fourth transfer gates (21, 22), respectively; and said fourth inverter (24) has input and output nodes connected to the outputs of said fourth and third transfer gates (22, 21), respectively. A flip-flop circuit according to claim 1, wherein said first transfer gate (11) comprises a first field-effect transistor having one of its source/drain electrodes connected to receive said input data (Di) and having the other of its source/drain electrodes constituting said output node (N11) of said first transfer gate (11), and said second transfer gate (12) comprises a second field-effect transistor having one of its source/drain electrodes connected to receive said inverted input data ( Di ) and having the other of its source/drain electrodes constituting said output node (N12) of said second transfer gate (12), the gate electrodes of said first and second field-effect transistors are connected to receive said clock signal (CL); said third transfer gate (21) comprises a field-effect transistor having one of its source/drain electrodes connected to receive said input data (Di) and having the other of its source/drain electrodes constituting said output node (N21) of said third transfer gate (21), and said fourth transfer gate (22) comprises a fourth field-effect transistor having one of its source/drain electrodes connected to receive said inverted input data ( Di ) and having the other of its source/drain electrodes constituting said output node (N22) of said fourth transfer gate (22); and the gate electrodes of said third and fourth field-effect transistors are connected to receive said inverted clock signal ( CL ).

Inventors: SHIKATA MAKOTO, SHIKATA, MAKOTO, SHIKATA, MAKOTO C/O OKI ELECTRIC INDUSTRY CO., LTD
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1989
Main classification: H03K3
IPC codes: H03K3