Abstract:
A toggle flip-flop with reduced integration area, comprising a flip-flop of the D-type with an inverting input stage and a master-slave portion. Three transistors connected to the inverting stage form a logic gate of the XOR type whereto the output terminal of the master-slave portion is fed back.

Description:
TECHNICAL FIELD 
     This invention relates to logic networks of the sequential type, and in particular to bistable circuits of the T-type, also referred to in the literature as toggle flip-flops (FFTs). 
     BACKGROUND OF THE INVENTION 
     Exemplary applications thereof include binary counters and synchronous frequency dividers. Considering binary counters, a one-bit counter has an input and n outputs which are the encoded form of the number of pulses that have been applied to the input from the initial time when the counter is reset. Therefore, the output states are 0, 1, 2, . . . , m, and 0, 1, 2, . . . , m. The n outputs allow 2 n  different numbers to be encoded where m&lt;2 n  -1. 
     For a synchronous binary counter, the outputs are the numbers 0, 1 , . . . , 2 n  -1 expressed in the binary system. The transfer function is [C]=[C]+1 (mod 2 n ). This function has the following equations: 
     
         y.sub.0 =y.sub.0 XOR1 
    
     
         y.sub.i =y.sub.i XOR y.sub.i-1 XOR y.sub.i-2. . . XOR with y.sub.0 i=1, 2, . . . , n-1, 
    
     and where XOR is the exclusive OR logic operation. 
     These equations suggest using a toggle flip-flop cell. The toggle flip-flop changes over when T=1, otherwise the FFT cell would store the previous output value. 
     As shown in FIG. 1 of the drawings, and as well known to skilled persons in the art, a toggle flip-flop for use in monolithically integrated circuit devices generally comprises a flip-flop of the D-type (FFD) 2 and an XOR logic gate circuit 4. The XOR circuit has its output terminal connected to the input terminal of FFD and an input terminal connected to the output terminal of the flip-flop. The other input terminal is the T input of the resultant flip-flop of the T-type. Let D be the input data of FFD 2 and Q the output data, the equation at the input becomes: 
     
         D=(T) XOR (Q), 
    
     which is of the same type as the equations for the binary counter. 
     The most commonly adopted constructions for a flip-flop of the T-type are as detailed herein below. 
     One construction provides standard logic gates 6, 8, 10 for the XOR input gate 4, and a cascade-connected flip-flop 2 of the D-type. This solution has an advantage in that it has good driving capability. But where NAND gates are used, for example, a large number of transistors (16=3 NANDs+2 NOTs) and complementary input signals (T, NT, Q, NQ) become necessary, as shown in FIG. 2 of the drawings. 
     A similar construction, retaining the valuable driving features of the former, may be implemented using a complex gate of the type shown in FIG. 3 for the XOR gate in order to use fewer transistors (10=complex gate) and avoid the need for complementary signals. 
     Using transfer gates as shown in FIG. 4 of the drawings may further reduce the number of transistors used (8=2 transfer gates+NOTs), but at the cost of a lower driving capability than that of the previous solutions, and requiring complementary input signals. The construction shown in FIG. 4 is preferred where savings in integration area are a condition. 
     SUMMARY OF THE INVENTION 
     An embodiment of this invention provides a toggle flip-flop which does not require any complementary input signals and has good driving capability, while requiring a significantly smaller number of transistors for implementing the XOR input gate. 
     A sequential logic network includes a D-type flip-flop that has a bistable output circuit and an input gate that transmits signals to the bistable output circuit. Timing signals applied to control terminals of the flip-flop control both the input gate and the bistable output circuit. The input gate has three electronic switches arranged such that two of the switches are connected in series between the input and output of the input gate, and the third switch is connected between a reference voltage and the output of the input gate. The first and third switches are controlled by the output of the bistable output circuit, while the second switch is controlled by one of the timing signals applied to the flip-flop. The features and advantages of a flip-flop of the T-type according to the invention can be appreciated from the following description of an embodiment thereof given by way of non-limitative example in relation to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a schematic diagram of a flip-flop of the T-type as implemented by a flip-flop of the D-type and a XOR logic gate. 
     FIGS. 2, 3 and 4 show prior art embodiments of the XOR logic gate employed for a flip-flop of the T-type. 
     FIG. 5 shows a prior art embodiment of a flip-flop of the D-type having an inverting input stage. 
     FIG. 6 shows the modified input stage that characterizes a flip-flop of the T-type according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Shown in FIG. 5 is a bistable output circuit 11 implemented by a flip-flop of the D-type, having a master-slave configuration and an input stage 12 of the inverting type to regenerate the signal applied to the input terminal D, as generally implemented in the state of the art by transistors of the complementary type. The master-slave configuration can be implemented in different circuit forms, but a tristate buffer with a high-impedance isolating and decoupling function is generally employed for the input stage. 
     According to an embodiment of the invention, a single input stage 14 is produced for the D-type flip-flop 11, thereby providing a novel toggle flip-flop (FFT) structure having the aforementioned features. 
     Assume a static master-slave FFD of the type shown in FIG. 5, having an inverting input stage 14 shown in FIG. 6. The novel flip-flop input stage 14 in FIG. 6 performs the XNOR function, that is, the equivalent function to the XOR gate plus the inverting input stage of the flip-flop. Compared to the previous constructions, the configuration shown in FIG. 6 has a smaller number of transistors, requires no complementary signals, and has good driving capability. 
     As shown in FIG. 6, only 3 transistors are used in addition to those in the standard inverting input stage 12 of FIG. 5, which stage is driven by standard timing signals fi and Nfi (with non-overlapping phases). 
     According to an embodiment of the invention, the novel XNOR input stage 14 comprises first and second transistors, M 1  and M 2 , which are connected in series with each other, through respective source and drain terminals, between an input terminal T and an output circuit node A. A gate terminal of the first transistor M 1  is connected to a node C, while a gate terminal of the second transistor M 2  is applied the timing signal Nfi. A third transistor M 3  is connected between a circuit power supply line, Vdd, and a circuit node B. The output terminal of the flip-flop, corresponding to the output terminal Q of the D-type flip-flop shown in FIG. 5, is feedback connected to a gate terminal of the transistor M 3  and the circuit node C. In the embodiment shown in FIG. 6, the first and second transistors M1, M2 are N-channel MOS transistors and the third transistor M3 is a P-channel MOS transistor. 
     In FIG. 6, the feedback connections are only symbolically depicted for simplicity. 
     The input stage 14 also includes a fourth and fifth transistors M4, M5 series connected between the nodes B and A and sixth and seventh transistors M6, M7 series connected between the nodes A and C. The fourth and fifth transistor M4, M5 are P-channel transistors and the sixth and seventh transistors M6, M7 are N-channel transistors. The fourth and seventh transistors M4, M7 each have a gate terminal connected to the input terminal T. The fifth transistor M5 has a gate driven by the timing signal fi and the sixth transistor M6 has a gate driven by the timing signal Nfi. 
     Taking the input stage alone, the truth table that ties together the three inputs (Q, T, fi) and the output node A, is the following: 
     
                       TABLE______________________________________T       Q               fi    A______________________________________0       0               0     10         0              1        X0         1              0        00         1              1        X1         0              0        01         0              1        X1         1              0        11         1              1        X______________________________________ 
    
     The Table read as follows: 
     0=ground potential, GND; 
     1=vdd; 
     X=tristate condition; 
     fi=clock signal, Nfi being equivalent to negated fi. 
     The timing signals fi, Nfi are synchronized in a predetermined manner such that the FFT structure 14 operates in a toggle flip-flop mode. In conclusion, the performance of this flip-flop of the T-type is comparable to that of the previous ones, but is obtained with minimum are. Where a BCD or mixed technology is used, area optimization is a condition. 
     The above advantages are specially usefuil with counter implementations where n is large, the saving in area as n is increased being self-evident. 
     Modifications or substitutions may be made unto the embodiment described hereinabove, in manners known to those skilled in the art. In fact, the invention applicability need not be limited to an inverting input buffer, or to one of the tristate type. 
     From the forgoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly,the invention is not limited except as by the appended claims.