Abstract:
The present invention relates to a latch including two first N-channel transistors connected to a low supply potential and controlled by a clock signal; two second transistors respectively connecting the two first transistors to an inverted output terminal and to a non-inverted output terminal and respectively controlled by a data line and a complementary data line; two third P-channel transistors respectively coupling the two second transistors to a high supply potential and cross-controlled by the output terminals; and circuitry for maintaining the states of the output terminals when the first transistors are non-conductive.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a latch, i.e., a circuit which, for a first state of a clock signal, transmits without modification a datum present at its input and, for a second state of the clock signal, stores or locks the last state of the datum. The present invention more specifically relates to such a latch which operates with a low swing clock signal for the purpose of reducing the power consumed by the distribution of the clock signal.  
           [0003]    2. Discussion of the Related Art  
           [0004]    [0004]FIG. 1 shows a conventional latch. Datum D is applied on the input of an inverter  10 , the output of which is coupled to the input of an inverter  12  via a switch formed of the parallel association of an N-channel MOS transistor MN 1  with a P-channel MOS transistor MP 1 . Transistor MN 1  is controlled by a clock signal CK, while transistor MP 1  is controlled by the complement CK/ of this clock signal. The output of inverter  12  forms the output Q of the latch. This output Q is further connected to the input of an inverter  14 , the output of which forms a complementary output Q/ of the latch. This output Q/ is connected to the input of inverter  12  via a switch formed of the parallel association of an N-channel MOS transistor MN 2  with a P-channel MOS transistor MP 2 . Transistor MN 2  is controlled by complementary clock signal CK/ while transistor MN 2  is controlled by clock signal CK.  
           [0005]    Two types of transistors are provided in each switch, since only one will be properly conductive according to the level of the signal to be transmitted. More specifically, the P-channel transistor will transmit the high levels of the signal, and the N-channel will transmit the low levels.  
           [0006]    When clock signal CK is high, switch MN 1 -MP 1  is on whereas switch MN 2 -MP 2  is off. The state of datum D is then transmitted as is to output Q via inverters  10  and  12 .  
           [0007]    When clock signal CK is low, switch MN 1 -MP 1  is off whereas switch MN 2 -MP 2  is on. Inverter  10  is disconnected from the input of inverter  12  whereas inverters  12  and  14  are connected head-to-tail to form a memory node which stores the state of signal D at the time when clock CK switched to the low state.  
           [0008]    The power consumed to distribute the clock signal is proportional to CFV 2 , where C is the capacitance of the clock line, F is the frequency of the clock signal, and V is the peak-to-peak voltage, or swing, of the clock signal. Since integrated circuits become larger and larger and operate at higher and higher frequencies, the power consumed for the clock signal distribution becomes significant.  
           [0009]    In recent circuits, it has been attempted to use a clock signal having a much lower swing than the nominal supply voltage of the circuit, which reduces considerably the consumed power, since the power increases with the square of the clock signal swing. For example, for circuits operating at a nominal voltage of 3.3 or 2.5 volts, one uses a clock signal varying between 0 and 1 volt, which reduces the consumed power by a factor 10 or 6, respectively.  
           [0010]    However, some circuits, especially latches of the type of FIG. 1, cannot operate with a clock signal varying between 0 and 1 volt. Indeed, the 1-volt maximum value of clock signal CK or CK/ is too far from the nominal supply voltage to turn off P-channel MOS transistors MP 1  and MP 2 . Accordingly, when clock signal CK is low, and switch MN 1 -MP 1  is supposed to be off, transistor MP 1  is however capable of passing a high level which may cause a switching of the memory node formed by inverters  12  and  14 . Further, N-channel transistors MN 1  and MN 2 , controlled by a voltage close to their threshold voltage, are only slightly conductive and may even not be conductive at all if their sources are at a residual potential above the low supply potential. This may especially happen in the case of FIG. 1, where the sources of transistors MN 1  and MN 2  are connected to the low potential via transistors forming inverters  10  and  14 .  
         SUMMARY OF THE INVENTION  
         [0011]    An object of the present invention is to provide a latch which can operate with a low swing clock.  
           [0012]    This and other objects are achieved by means of a latch including two first transistors of a first conductivity type connected to a first supply potential and controlled by a clock signal; two second transistors respectively connecting the two first transistors to an inverted output terminal and to a non-inverted output terminal and respectively controlled by a data line and a complementary data line; two third transistors of a second conductivity type respectively coupling the two second transistors to a second supply potential and cross-controlled by the output terminals; and means for maintaining the states of the output terminals when the first transistors are off.  
           [0013]    According to an embodiment of the present invention, said means include two fourth transistors of the first conductivity type respectively connecting the output terminals to the first supply potential via a fifth transistor of the first conductivity type, and cross-controlled by the output terminals, the fifth transistor being controlled by an inverted clock signal.  
           [0014]    According to an embodiment of the present invention, the on-state conductivity of the third transistors is lower than that of the series connection of one of the first transistors with one of the second transistors.  
           [0015]    According to an embodiment of the present invention, the first conductivity type is type N, the first supply potential is a low potential, the second supply potential is a high potential, and the level of the clock signal varies between the low supply potential and a level lower than the high supply potential.  
           [0016]    The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments made in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1, previously described, shows a conventional latch;  
         [0018]    [0018]FIG. 2 shows an embodiment of a latch according to the present invention; and  
         [0019]    [0019]FIG. 3 shows an equivalent diagram of a portion of the latch of FIG. 2. 
     
    
     DETAILED DESCRIPTION  
       [0020]    In FIG. 2, a latch according to the present invention includes two N-channel MOS transistors MN 3  and MN 4 , the sources of which are directly connected to low supply potential Vss. Both transistors are controlled by a clock signal CK. Two N-channel MOS transistors MN 5  and MN 6  connect transistors MN 3  and MN 4  respectively to an inverted output terminal Q/ and to a non-inverted output terminal Q of the latch. Transistors MN 5  and MN 6  are respectively controlled by a datum D and its complement D/. Output terminals Q/ and Q are connected to high supply potential Vdd via two respective P-channel MOS transistors MP 3  and MP 4 . Transistors MP 3  and MP 4  are cross-controlled by outputs Q/ and Q, that is, the gate of transistor MP 4  is connected to output Q/ while the gate of transistor MP 3  is connected to output Q.  
         [0021]    The operation of the latch can already be understood without considering the elements of the upper portion of FIG. 2 which will be described hereafter.  
         [0022]    When clock signal CK is low, transistors MN 3  and MN 4  are off. Then, that of outputs Q and Q/ which is at  0  floats and maintains its state by capacitive effect. For example, output Q/ floats and its  0  state is maintained by the gate capacitance of transistor MP 4 . The gate of transistor MP 4 , thus being at  0 , transistor MP 4  is on and transmits high supply potential Vdd, i.e., state  1 , to output Q. As long as transistors MN 3  and MN 4  are off, the state of the latch cannot be altered neither by datum D, nor by its complement D/, that is, the latch is in the locked state.  
         [0023]    When signal CK is at a value slightly above the threshold voltage of transistors MN 3  and MN 4 , these transistors turn on.  
         [0024]    In this case, if datum D is at state  1 , transistor MN 5  is on and draws output Q/ towards potential Vss, that is, towards state  0 . In the example, this has no effect upon the state of the latch. Transistor MN 6 , receiving state  0  on its gate, is off and cannot alter the state of output Q.  
         [0025]    If datum D is at state  0 , transistor MN 5  is off and transistor MN 6  turns on and draws output Q towards potential Vss, which alters state  1  of output Q in the example. For the state of output Q to be drawn to  0 , transistor MP 4  is sized to have a lower conductivity than that of the series connection of transistors MN 6  and MN 4 . In this case, the potential of output Q ends up decreasing to the threshold voltage of transistor MP 3  which then turns on. Transistor MP 3  rapidly increases the potential of output Q/ towards potential Vdd, since output Q/ is floating. The level of output Q/ rapidly exceeds the threshold of transistor MP 4  which turns off and enables output Q to be drawn to potential Vss unimpeded.  
         [0026]    If the state of datum D changes again while signal CK still is high, the state of the latch also changes, and this, symmetrically to what has just been described. In other words, as long as signal CK is high, the latch is transparent.  
         [0027]    The latch can properly operate with a clock signal of reduced swing, varying, for example, between 0 and 1 volt, since it uses circuitry that enables this such as N-channel transistors, the sources of which are directly connected to low supply potential Vss.  
         [0028]    With the elements just described, when the latch is locked on a state, this state is only maintained by capacitive effect. Leakage currents could thus cause a modification of the state of the latch when it remains locked for too long.  
         [0029]    The circuit of the upper portion of FIG. 2 is provided to avoid this. This circuit includes two N-channel MOS transistors MN 7  and MN 8  connected in series between outputs Q/ and Q. Transistors MN 7  and MN 8  are cross-controlled, that is, the gate of transistor MN 8  is connected to output Q/ and the gate of transistor MN 7  is connected to output Q. The connection node between transistors MN 7  and MN 8  is connected to low potential Vss by an N-channel MOS transistor MN 9  which is controlled by the complement CK/ of clock signal CK.  
         [0030]    When the latch is in the transparent state, that is, when clock signal CK is high, transistor MN 9  is off. As a result, transistors MN 7  and MN 8  do not interfere in the operation just described.  
         [0031]    When the latch is in the locked state, that is, when signal CK is low, transistor MN 9  turns on. Considering again the example where output Q is at  1  and output Q/ is at  0 , transistor MN 7  is on while transistor MN 8  is off. Thus, output Q/ is connected to potential Vss via transistors MN 7  and MN 9 , which confirms the  0  state of this output Q/. State  1  of output Q is still confirmed by transistor MP 4 . The two latch outputs are thus confirmed and no drift due to a leakage current is possible.  
         [0032]    Signal CK/ may be of low swing, like signal CK, since the type and the connection mode of MOS transistor MN 9  are the most appropriate for this purpose.  
         [0033]    [0033]FIG. 3 shows an equivalent diagram of the upper portion of the latch of FIG. 2. Transistors MP 3 , MP 4 , MN 7 , and MN 8  form two inverters connected head-to-tail. These inverters are directly supplied by high potential Vdd and supplied from low potential Vss via transistor MN 9 .  
         [0034]    Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The invention is limited only as defined in the following claims and the equivalent thereto.