Abstract:
Described are high-speed differential flip-flops. A flip-flop in accordance with one embodiment incorporates some combinational logic, eliminating the need for separate combinational logic when the flip-flop is employed in certain circuit configurations. A flip-flop in accordance with another embodiment includes differential input and output stages, each of which includes a transistor connected across its differential output terminals. The transistors are clocked to short the differential output terminals between expressions of logic levels, thereby limiting the maximum amount of voltage swing required to express subsequent logic levels.

Description:
BACKGROUND 
     Logic circuits can be classified into two broad categories, combinational logic circuits and sequential logic circuits. The basic building block of sequential logic circuits is the flip-flop, also called a bi-stable multi-vibrator or latch. In most cases, logic circuits employ both sequential and combinational logic. 
     FIG. 1 (prior art) depicts an exemplary logic circuit  100  that includes both combinational and sequential logic elements. Logic circuit  100  is a divide-by-five counter  100  with a pair of NOR gates  105  in a feedback path of a series of differential-input flip-flops  110 . Circuit  100  receives a pair of complementary clock signals C and Cb, which extend to clock input terminals of each of the flip-flops  110 . Circuit  100  produces a pair of complementary clock signals C/ 5  and Cb/ 5  with a frequency one fifth that of the input clock signals. The differential nature of circuit  100  allows for higher clock frequencies than would a similar divide-by-five circuit using single-ended sequential logic elements. 
     FIG. 2 (prior art) depicts an embodiment of a differential-input flip-flop  110  for use in circuit  100  of FIG.  1 . The operation of flip-flop  110  is commonly understood by those of skill in the art, so a detailed description of flip-flop  110  is omitted here for brevity. 
     If manufactured using commonly available CMOS processes, flip-flop  110  can perform with clock frequencies as high as about 2 GHz. Unfortunately, modern high-speed digital communication systems employ clock and data recovery circuits operating in the 10 Gb/s range. The frequency response of flip-flop  110  is therefore insufficient to meet the needs of some modern systems. 
     SUMMARY 
     The present invention is directed to high-speed flip-flops. A flip-flop in accordance with one embodiment of the invention has a differential input stage that incorporates some combinational logic. This embodiment improves speed performance by reducing or eliminating the need for separate combinational logic circuits when the flip-flop is employed in certain circuit configurations. In one example, a flip-flop incorporating combinational logic is used in conjunction with other flip-flops to create a counter circuit that would otherwise require separate combinational logic. 
     A flip-flop in accordance with another embodiment of the invention includes differential input and output stages, each of which includes a transistor connected across its differential output terminals. The transistors are clocked to short the differential output terminals between expressions of logic levels, thereby limiting the maximum amount of voltage swing required to express subsequent logic levels. 
     This summary does not define the scope of the invention, which is instead defined by the appended claims. 
    
    
     A BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 (prior art) depicts an exemplary logic circuit  100  that includes both combinational and sequential logic elements. 
     FIG. 2 (prior art) depicts an embodiment of a differential-input flip-flop  110  for use in circuit  100  of FIG.  1 . 
     FIG. 3 depicts a divide-by-five circuit  300  that divides a pair of complimentary clock signals C and Cb by five. 
     FIG. 4 depicts a flip-flop  400  that is an embodiment of flip-flop  310  of FIG.  3 . 
     FIG. 5 is a waveform diagram  500  depicting exemplary signals associated with the operation of flip-flop  400  of FIG.  4 . 
     FIG. 6 depicts a flip-flop  600  that may be used in place of flip-flop  305  (FIG. 3) in one embodiment of the invention. 
     FIG. 7 depicts a differential circuit that can be used as circuit  315  of FIG. 3 to convert the differential signal on terminals Q and Qb of the last flip-flop  310  in circuit  300  into full-swing, stable differential output signals. 
    
    
     DETAILED DESCRIPTION 
     FIG. 3 depicts a divide-by-five circuit  300  that divides a pair of complimentary clock signals C and Cb by five. Like circuit  100  of FIG. 1, circuit  300  employs differential signaling to improve performance. Circuit  300  differs from circuit  100 , however, in that the logic associated with NOR gates  105  of FIG. 1 is incorporated into a single flip-flop  305  adapted to receive two pairs of complimentary inputs D 0 , D 0   b  and D 1 , D 1   b . The operation of an embodiment of flip-flop  305  is detailed below in connection with FIG.  6 . 
     Circuit  300  also includes a number of flip-flops  310  that are modified in accordance with the invention to improve speed performance. Finally, circuit  300  includes a differential to single-ended converter  315 . Flip-flops  310  and converter  315  are described below in connection with FIGS. 4,  5 , and  7 . 
     FIG. 4 depicts a flip-flop  400  that is an embodiment of flip-flop  310  of FIG.  3 . Flip-flop  400  includes an input stage (left) and an output stage (right). The input stage includes a pair of differential transistors  400  and  405 , the control inputs of which are connected to respective complementary data inputs D and Db. The output terminals X 1  and X 2  of the input stage connect to respective control terminals of a pair of differential transistors  410  and  415  in the output stage. The input and output stages include respective cross-coupled PMOS transistor loads  420  and  425 . 
     The input stage includes an NMOS transistor  430  X having one current handling terminal connected to output terminal X 1  and the other connected to output terminal X 2 . The control input (gate) of transistor  430  is connected to the clock signal Cb. The output stage likewise includes a transistor  435 , one current-handling terminal of which is connected to output terminal Q, the other current-handling terminal to output terminal Qb. The control terminal of transistor  435  is connected to clock signal C. In another embodiment, one of transistors  430  and  435  is substituted with a PMOS transistor, allowing both control terminals associated with transistors  430  and  435  to be controlled by the same clock signal. 
     FIG. 5 is a waveform diagram  500  depicting exemplary signals associated with the operation of flip-flop  400  of FIG.  4 . Diagram  500  depicts complimentary clock signals C and Cb, data signals D and Db, input-stage output signals X 1  and X 2 , and output terminals Q and Qb. The various node labels refer to both the signal and the corresponding circuit node. Whether a given designation refers to a node or a signal will be clear from the context. 
     Prior to time T0, clock signal Cb is high, so transistor  430  connects output terminals X 1  and X 2  of the input stage of flip-flop  400 . The logic 0 input on the differential terminals D and Db consequently produces only a relatively small voltage difference across terminals X 1  and X 2 . Though limited by the on resistance of transistor  430 , the voltage across terminals X 1  and X 2  does reflect a logic 0 (i.e., X 1 &gt;X 2 ). 
     At time T0, clock signal Cb goes low, turning off transistor  430  to disconnect terminals X 1  and X 2 . The voltage between terminals X 1  and X 2  thus increases, better representing the difference between input signals on terminals D and Db. Also at time T0, clock signal C goes high, causing transistor  435  to connect output terminals Q and Qb. The voltage difference between signals Q and Qb therefore diminishes. Though limited by the on resistance of transistor  435 , the voltage across terminals Q and Qb continues to reflect a logic 0 (i.e., Q&lt;Qb). 
     Next, at time T1, clock signal C returns low and complimentary clock signal Cb returns high. Respective transistors  430  and  435  consequently change states, so that terminals X 1  and X 2  are once again connected and terminals Q and Qb are disconnected. In this new state, terminals X 1  and X 2  begin to approach one another and output terminals Q and Qb swing away from one another to reflect the differential input signals to transistors  410  and  415 . 
     Before the receipt of a new data bit on differential input terminals D and Db, the pairs of output terminals X 1 ,X 2  and Q,Qb approach one another to limit the maximum amount of voltage swing required to move the differential output signal to the next logic bit. For example, the logic level expressed on output terminals Q and Qb from time T2 to time T4 switches from a logic 0 to a logic 1, and therefore requires a maximum voltage swing for each of output terminals Q and Qb. The present invention expedites the time required to make this transition by beginning to bring Qb low and Q high prior to receipt of the data signal indicating the logic transition. The resulting reduction in the maximum voltage swing required to change the logic level expressed on terminals Q and Qb reduces the maximum amount of time required to make logic transitions on terminals Q and Qb. This embodiment of the invention thus speeds the logic transitions on the outputs of flip-flop  310 . 
     As illustrated between times T4 and T6, the voltage difference between terminals Q and Qb is reduced even if the next data bit turns out to be the same logic level as the one presently represented. This is because flip-flop  400  cannot anticipate the next logic level, and consequently must prepare for either of the two alternatives. Flip-flop  400  therefore requires some amount of time to “transition” between two logic zeroes or two logic ones. The overall speed of flip-flop  400  increases because the time required to transition between different logic levels is reduced. 
     Reducing the time required for flip-flop  400  to transition between different logic levels translates directly into improved speed performance. Moreover, as compared with flip-flop  110  (FIG.  2 ), flip-flop  400  has far fewer transistors, and can therefore be implemented using less die area. These changes also result in significantly reduced power consumption for a given level of speed performance. 
     FIG. 6 depicts a flip-flop  600  that may be used in place of flip-flop  305  (FIG. 3) in one embodiment of the invention. Flip-flop  600  is largely similar to flip-flop  310  of FIG. 4, like numbered features being identical. Due to the similarities of flip-flop  600  and flip-flop  310 , a detailed description of flip-flop  600  is omitted for brevity. 
     Flip-flop  600  is modified in accordance with an embodiment of the invention to receive two pairs of differential inputs D 0 ,D 0   b  and D 1 ,D 1   b . As noted above in connection with FIG. 1, NOR gates  105  impose some delay that slows the maximum operating speed of circuit  100  of FIG.  1 . Flip-flop  600  incorporates a pair of logic gates  605  and  610  that eliminate the need for NOR gates  105 , and therefore speed up divide-by-five circuit  300  of FIG.  3 . Gate  605 , composed of a pair of transistors  615  and  620 , performs a NOR function of inputs D 0  and D 1 ; a pair of serial-connected transistors  625  and  630  within gate  610  performs a NAND function of input signals D 0   b  and D 1   b.    
     As shown in FIG. 5, the data output on complimentary output terminals Q and Qb fluctuates even when the data signal does not. FIG. 7 depicts a differential circuit that can be used as circuit  315  of FIG. 3 to convert the differential signal on terminals Q and Qb of the last flip-flop  310  in circuit  300  into full-swing, stable differential output signals. 
     While the present invention has been described in connection with specific embodiments, variations of these embodiments will be obvious to those of ordinary skill in the art. For example, the invention can be adapted for use with other types of sequential logic elements, such as single-stage latches, toggle flip-flops, a J-K flip-flops, AND-input flip-flops, or XOR-input flip-flops. Moreover, some components are shown directly connected to one another while others are shown connected via intermediate components. In each instance, the method of interconnection establishes some desired electrical communication between two or more circuit nodes, or terminals. Such communication may often be accomplished using a number of circuit configurations, as will be understood by those of skill in the art. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.