Abstract:
To establish whether a precharged node remains isolated or alternatively is subject to discharge, the conventional circuit allows uncertainty. For a period after evaluation starts, the conventional circuit will give a tentative result that may subsequently turn out to be wrong. During evaluation power is dissipated. A differential offset dynamic comparator and timing circuit are used to evaluate whether the node is being discharged. Because the comparator has an offset, much smaller deviations from the precharge potential can be sensed: because it is dynamic, it does not consume steady state current. The timing circuit permits precise knowledge of when to look at the output: before the timing period has elapsed, the result is known to be invalid.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is concerned with determining whether a node which has been charged to a given potential remains at that potential or whether the node is being discharged, and thus exhibits a falling potential.  
           [0003]    2. Description of the Related Art  
           [0004]    There are a number of applications in which a node is charged or precharged by connection to a potential, typically a power supply potential, and a determination is needed as to whether any path exists that is operative to discharge the node once the connection to the potential is removed. In many situations, a circuit has a first logic value when the node remains isolated and a second logic value when a discharge path exists. Typical applications include so-called “precharge-evaluate logic”.  
           [0005]    Conventional circuits may give rise to a tentative result that may or may not be correct. For example, if a node is falling slowly in potential from its precharged level, and the node potential is being fed as an input to an inverter, the potential fall may need to be quite a substantial fraction of the precharge potential before the inverter is able to respond to it. Hence for a relatively long period of time—for a slow potential variation—the inverter output remains at its original precharge value and only after its input potential has fallen sufficiently will the output change.  
           [0006]    During that time conventional circuits consume power. The tentative result also makes such circuits unsuitable for applications where the output is used asynchronously to enable or otherwise control a later process. To do this, the “disabled state must have an output the same as detecting “discharge” in the active state.  
           [0007]    It is possible to avoid effects due to the tentative result either by using a timing circuit to mask the output from later circuitry until the output is known to have a valid result, or by using a clocked comparator with a pseudo-differential reference voltage. The latter is advantageous because it is faster, and so it draws power for a shorter time than does the conventional circuit. However it is also disadvantageous because it needs a reference voltage to be created. A reference circuit is likely to create a quiescent power drain, and also consumes chip area and components.  
           [0008]    It is an object of the present invention to at least partly mitigate difficulties of the prior art.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    According to one aspect the invention provides a method of determining whether the potential on a node remains constant, the method comprising: providing a differential comparator having a first input connected to said node, a second input connected to a constant potential and an output, wherein said first and second inputs become active at respectively lower and higher potentials, the comparator having a first enabled state in which said output represents the difference between the first and second input potentials and a second disabled state, wherein the comparator has no path for dc currents in said enabled and disabled states; connecting the node to said constant potential whereby said node is precharged, while maintaining the disabled state; disconnecting the node from the precharge potential; and after a time delay, causing said comparator to switch to the enabled state, whereby said output indicates whether the node potential remains constant.  
           [0010]    According to a second aspect of the invention there is provided a circuit responsive to deviation of a node from a precharge potential after a precharge supply is disconnected from said node, the circuit comprising: a dynamic comparator having first and second first conductivity differentially connected transistors, the first transistor having a gate connected to the said node and the second having a gate connected to the precharge potential, and wherein the first transistor has a lower threshold voltage than the second; the first and second transistors second and third electrodes, a load circuit being connected between the second electrodes and a positive supply node, the load circuit comprising a pair of cross-coupled second conductivity load transistors, said second conductivity being complementary to the first conductivity; the third electrodes of the first and second transistors being connected to a reference potential via a current source transistor of the first conductivity type.  
           [0011]    A preferred but exemplary embodiment of the invention will now be described with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 shows a conventional precharge-evaluate NOR gate using N-type transistors; and  
         [0013]    [0013]FIG. 2 shows an evaluation circuit responsive deviation of a precharge node in accordance with the present invention. 
     
    
       [0014]    In the various figures, like reference numerals indicate like parts.  
       DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    Referring to FIG. 1, a precharge-evaluate logic circuit has 3 N-type gate transistors  1 ,  2 ,  3  having their drain electrodes connected in common to a precharge node  4  and their source electrode connected in common to a common source node  5 .  
         [0016]    Each of the gate transistors  1 ,  2 ,  3  has a respective gate connection to a logic input connection  11 ,  12 ,  13 .  
         [0017]    The precharge node  4  is connected via a P-type precharge transistor  20  to a positive power supply Vcc and the common source node  5  is connected to a reference potential Vss via an N-type transistor  30 . The gate electrodes of the P-type transistor  20  and the N-type transistor  30  are connected in common to a clock node  21 . The precharge node  4  is connected to an output terminal  6  via an inverter  7  and the output terminal  6  forms the gate connection of a weak pull-up P-type transistor  8  which is connected between the precharge node  4  and the positive supply Vcc.  
         [0018]    In use a clock pulse is provided at the clock terminal  21  and the clock cycle is split into precharge and evaluate phases. When the clock pulse is at logic 0, the P-type transistor  20  conducts and charges the precharge node  4  to substantially Vcc. The N-type transistor  30  is maintained non-conductive. In the logic 1 state of the clock pulse at node  21  the P-type transistor  20  becomes non-conducting and the N-type transistor  30  conducts. If all of the logic inputs  11 ,  12 , and  13  are at logic 0 then all of the logic transistors  1 - 3  remain non-conducting and the precharge node remains at Vcc. If one or more of the logic inputs  11 - 13  is at logic 1 then a corresponding number of logic transistors  1 - 3  conducts and the node  4  discharges via the logic transistors and the N-type transistor  30  to Vss.  
         [0019]    When the precharge node  4  is at Vcc, the inverter  7  provides a logic 0 output, and as a result the weak P-type transistor  8  conducts. Clearly the output node  6  is at logic 0. When the clock pulse changes phase and a logic 1 is provided at clock input  21 , the inverter  7  serves to evaluate whether or not the precharge node is remaining at the precharge state Vcc or is falling in potential. If one or more of the logic transistors  1 - 3  conducts then the potential at node  4  will fall as the conductivity of the weak P-type transistor  8  is insufficient to maintain at the node at its high level. When the potential at node  4  falls to around half Vcc the inverter  7  will switch its output so that the weak P-type transistor  8  is turned off and the output node  6  goes to logic 1.  
         [0020]    It will be appreciated that there is a DC path in this circuit which allows for power to be dissipated whenever there is conduction through one or more of the logic transistors  1 - 3 , and until the evaluation is complete at which time the weak P-type pull-up transistor  8  ceases to conduct.  
         [0021]    It will be seen that the output node  6  remains at the wrong logic state until such time as the precharge node  4  falls to around half Vcc. Therefore, to prevent this tentative logic 0 (which should be a final logic 1) from reaching later components it may be necessary to include a masking arrangement to isolate the node  6  from later components until the latest time at which node  4  could have fallen to half Vcc.  
         [0022]    Referring now to FIG. 2, an embodiment of the invention will now be described. Comparing FIG. 2 with FIG. 1 it will be seen that the inverter and pull-up arrangement of FIG. 1 is replaced by a dynamic differential comparator  100 . It will also be seen that the circuit includes a time delay  150  connected between the clock node  21  and circuitry of the comparator.  
         [0023]    The comparator  100  has a first and a second differentially connected N-type transistor  101 ,  102  with the first of these transistors  101  having its gate connected to the precharge potential Vcc. The second transistor  102  has its gate connected to the precharge node  4 . The sources of the first and second transistors  101 ,  102  are connected in common and via a switched current source N-type transistor  103  to the reference potential Vss. The drain of the first transistor  101  is connected to a positive supply potential, here Vcc via the main current path of a first P-type transistor  110  and the drain of the second transistor  102  is connected to the positive supply via the main current path of a second P-type transistor  111 . The common node between transistor  101  and  110  is connected to the gate of the second P-type transistor  111 . The common node between the second N-type transistor  102  and the second P-type transistor  111  is connected to the gate of the first P transistor  110 . The gate of the first P-type transistor  110  is connected to the positive supply Vcc via a first P-type control transistor  115  and the gate of the second P-type transistor  111  is connected to the positive supply via a second P-type control transistor  116 . The gates of the P-type control transistors  115 ,  116  are connected together, and connected to the output  151  of the time delay circuit  150 , the output  151  of the time delay  150  is also connected to the gate of the switched current source transistor  103 .  
         [0024]    Further reference to FIG. 2 shows that the inverter  7 , having the output node  6  is connected to the common node of the second N transistor  102  and the second P transistor  111 .  
         [0025]    Although the comparator circuit  100  may be otherwise symmetric, the second N-type transistor  102  has a lower threshold voltage than the first N-type transistor  101 .  
         [0026]    In use when the clock pulse is at a logic 0 the precharge transistor  20  is conducting and precharges the node  4  to substantially Vcc. In this state the time delay  150  has a low output which has the effect of turning on the control transistor  115  and  116  and turning off the current source transistor  103 . Thus, the first and second P transistors  110  and  111  are both turned off because their gates are connected to the positive supply and the source nodes of the comparator transistors  101 ,  102  are isolated from the reference supply.  
         [0027]    At a later time, the clock pulse changes to logic 1 which causes the N transistor  30  to conduct and, according to the logic levels at node  11 ,  12 ,  13  the node  4  will either remain at Vcc or will drop in potential.  
         [0028]    The time delay  150  is selected to provide an output transition at a time sufficiently later than the transition from precharge to evaluate that the potential at node  4  would have fallen sufficiently to be evaluated (if any fall occurs). At this later time, when the output  151  goes high, the N-type switch current source  103  starts to conduct and the P-type control transistors  115 ,  116  are turned off. As a result, and due to the second N-type transistor  102  having a lower threshold voltage than the first N-type transistor  101 , provided node  4  has not fallen in potential, the second N-type transistor  102  will conduct more strongly than the first N-type transistor  101  with the result that the first P-type transistor  110  will be turned on faster than the second P-type transistor  111 . Hence the gate of the second P-type transistor  111  is pulled towards the positive supply, thus turning off the second P-type transistor  111 . The input to the inverter  7  will thus be close to Vss which means that the output node  6  will be at logic 1.  
         [0029]    If however the precharge node  4  has fallen substantially then the first N-type transistor  101  will conduct more strongly than the second N-type transistor  102  with the opposite effects to those described above. More specifically, the result of transistor  101  turning on harder than transistor  102  is that the second P transistor  111  will turn on more quickly than the first transistor  110  which means that the gate of transistor  110  will be pulled high. The result is that the inverter  7  will receive a logic 1 at its input and the output  6  will be at logic 0.  
         [0030]    It will be seen that the comparator  100  is a dynamic device in that there is no path for current flow in the steady state. If transistor  102  is on then transistor  111  is off and if transistor  101 , then transistor  110  is off.  
         [0031]    The result of this is that the device of FIG. 2 is power efficient.  
         [0032]    The device of FIG. 2 has been described in the context of precharge-enable logic circuit. It will be understood by those skilled in the art that other precharge-evaluate logic circuitry could be used instead, and more specifically that there are many other types of precharge evaluate logic circuits which could be used including NAND gates.  
         [0033]    The arrangement is primarily suitable for determining whether the node  4  has deviated downwardly from its precharge state. As such, it will be appreciated by those skilled in the art that it could be used to determine the logic state of a transistor in a memory arrangement. If a transistor is provided that is connected to a precharge node such that in one memory state the transistor is off and in the other memory state the transistor is on, then the use of the dynamic differential offset comparator disclosed in FIG. 2, or similar such circuitry, could be used to evaluate the state of the memory cell comprising the transistor. It is therefore possible to dispense with the need for complementary bit lines.  
         [0034]    It will be appreciated by those skilled in the art that when the clock pulse  21  goes from the precharge to the evaluate condition there may be some coupling between the gates of the precharge transistor  20  and the precharge node  4 . This coupling takes the form of a capacitive coupling and it is possible for the coupling to be manipulated by arranging for the line shown as node  4  to run close to the clock line. In this situation the node  4  can be precharged to above Vcc by use of a bootstrap effect. Given the ability to manipulate the dimensions of the circuit sufficiently, it may be possible to dispense with the need for different threshold voltages for the comparator transistor  101 ,  102 , instead relying upon the additional voltage at node  4  due to this bootstrap effect.  
         [0035]    In the described embodiment, the comparator is powered by the positive supply which also provides the precharge voltage. It will be clear to those skilled in the art that this is not in fact essential and that the positive supply to the P-type transistors  110 ,  111 ,  115  and  116  could in fact be another positive supply if such were available. The primary feature of the embodiment of FIG. 2 is the fact that the comparator is capable of adopting one state when its two input levels are equal, and another state when a defined one of those two inputs is lower than the other.  
         [0036]    From the foregoing 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.