Abstract:
Method and apparatus are provided for trapping metastability events to provide a metastable-free output signal. At least three successive values of an input signal are latched successively over a predetermined period which is less than half of a fundamental period of the input signal to provide at least three corresponding latched values. First and second intermediate signals are activated when outputs of all of the at least three corresponding latched values are in respective first and second logic states. An output signal is placed in a first predetermined logic state in response to the second intermediate signal and is changed from the first predetermined logic state to a second predetermined logic state in response to the first intermediate signal.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention generally relates to electrical circuits, and more particularly relates to latch circuits for receiving an input signal using a clock signal that is asynchronous to the input signal. 
     BACKGROUND OF THE INVENTION 
     Digital integrated circuits (ICs) generally operate in a synchronous mode. Data is transmitted synchronously within an IC when a clock signal captures the data output by one stage at the input of another stage. Clock signals are distributed all around the various stages and functional units on an IC and, along with signals used to select the intended stage or unit, cause the capturing of the data. Various circuits are used to capture data, such as flip-flops and latches, and even though each operates somewhat differently, they all utilize clock signals to capture data. 
     Capturing data at the input of a stage within an IC can be easily accomplished as long as a proper relationship between transitions in the clock and data signals is maintained. This proper relationship is usually defined in terms of minimum setup and hold times and can usually be controlled within the IC. However a problem arises when data is transferred between two domains that operate asynchronously with respect to each other. For example an IC may receive an external signal that is asynchronous to the IC&#39;s internal clock signal. Capturing circuits such as flip-flops are unable to capture the external signal when the signal changes during a transition in the clock signal since the signal is in mid-transition. In addition not only is the data not “caught” or captured correctly at that edge, but additionally the capturing circuit suffers from a “confusion” of sorts. It captures a “confused” or intermediate mid-point value which is then output to the next stage requiring data. The time it takes for the capturing circuit to become “unconfused” can be statistically determined, but can be in some rare cases quite long. So the problem is not so much that the data is not captured perfectly at the exact earliest edge possible, but that the capturing device can be forced into this confused state . The confused state is known as metastability. Once a flip-flop becomes metastable, its output can take a significant amount of time to correctly transition to a recognizable logic state, and sometimes this logic state is not the correct one. The output signal can take many forms during metastability, such as assuming an intermediate voltage and oscillating for an extended period. 
     The metastability problem can be avoided within a digital IC or between digital ICs by obeying minimum setup and hold times. Most busses used to communicate data and actions between various ICs are specified so that data is always ready to be input at the input of the next IC or section in time for the next clock, in much the same way that circuits within an IC are designed. 
     However no known circuit can guarantee the correct operation of a capturing device with a completely unknown external data transition. While some precautions can be taken to reduce the effects of metastability, no known circuit can completely remove it. FIG. 1 illustrates a latch circuit  20  that reduces the effects of metastability known in the prior art, including a buffer  22  and two clocked D-type flip-flops  24  and  26 . Flip-flops  24  and  26  are driven by the same clock signal, and the output of the first flip-flop  24 , labeled “Q 1 ”, feeds the D input of the second flip-flop  26 . The output of flip-flop  26 , labeled “Q 2 ”, forms the output of latch circuit  20 . Latch circuit  20  operates under the assumption that a single stage flip-flop will “settle” (end its metastability).within a fixed period of time, and thus be stable before the clock of the next stage transitions. Statistically, the relationship between the inherent settling time and the clock rate of the system determines the likelihood of the metastability working its way through the two flip-flops and into the synchronous system. But it does not eliminate the chance of a metastability-induced error. 
     This phenomenon is better understood with respect to FIG. 2, which illustrates a timing diagram  30  useful in understanding the operation of latch circuit  20  of FIG.  1 . In FIG. 2 the horizontal axis represents time and the vertical axis the amplitude, in volts, of several relevant signals. As shown the INPUT signal makes a transition between a logic low value and a logic high value. In order to avoid metastability, the INPUT signal should be settled for at least a setup time labeled “t SU ” before the rising edge of the CLOCK signal. As shown in FIG. 2 the CLOCK input signal makes a transition just within t SU  and metastability results. Thus signal Q 1  initially assumes an intermediate value. Alternatively instead of assuming an intermediate value, the metastable state may cause signal Q 1 ′ to oscillate between states before finally resolving to a recognizable logic state. As long as the metastability has ended by the next transition of the CLOCK signal, no ultimate problem will result. Even if the metastable condition resolves to a low level, the high level will be recognized at the input of flip-flop  26  at the following CLOCK signal and the operation of the circuit is not affected by the metastability. 
     However the decay time of the metastable event is statistically variable and even in latch circuit  20  there is some probability that the metastable state will last long enough to be seen at the input of flip flop  26  and thus reach the output. The probability is related to the CLOCK rate and increases with increases in the CLOCK rate. The fastest rate at which the two flip-flops can be clocked is set by the known statistical decay of the metastable event in flip-flop  24 , and this value is not guaranteed for all time. This lack of predictability of the circuit and the remote chance that it could pass a metastable event makes it and other similar circuits less than perfect. An additional flip-flop stage could be added to the output of flip-flop  26  but this additional flip-flop would increase the group delay through latch circuit  20  and may not be tolerable. 
     Accordingly, it would be desirable to have a latch circuit which is able to provide an output signal as a correct representation of an input signal regardless of when the input signal changes state in relation to a clock signal. These and other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     BRIEF SUMMARY OF THE INVENTION 
     A latch circuit with a metastability trap is provided. The latch circuit includes at least three input latches, a first logic gate, a second logic gate, and a flip-flop. The at least three input latches capture values of an input signal at corresponding successive points in time distributed over a predetermined period which is less than half of a fundamental period of the input signal. The first logic gate has a plurality of input terminals coupled to corresponding output terminals of each of the at least three input latches, and an output terminal for providing a first intermediate signal. The first logic gate activates the first intermediate signal in response to signals at all of the plurality of input terminals being in a first logic state, and keeps the first intermediate signal inactive otherwise. The second logic gate has a plurality of input terminals coupled to corresponding output terminals of each of the at least three input latches, and an output terminal for providing a second intermediate signal. The second logic gate activates the second intermediate signal in response to signals at all of the plurality of input terminals being in a second logic state. The second logic gate keeps the second intermediate signal inactive otherwise. The flip-flop has a set input terminal coupled to the output terminal of the second logic gate, a reset terminal coupled to the output terminal of the first logic gate, and an output terminal for providing an output signal of the latch circuit. 
     A method is also provided for trapping metastability events to provide a metastable-free output signal. At least three successive values of an input signal are latched over a predetermined period which is less than half of a fundamental period of the input signal to provide at least three corresponding latched values. A first intermediate signals is activated when all of the at least three corresponding latched values are in a first logic state. A second intermediate signal is activated when all of the at least three corresponding latched values are in a second logic state. An output signal is placed in a first predetermined logic state in response to the second intermediate signal and is changed from the first predetermined logic state to a second predetermined logic state in response to the first intermediate signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
     FIG. 1 illustrates in logic diagram form a latch circuit that reduces the effects of metastability known in the prior art; 
     FIG. 2 illustrates a timing diagram useful in understanding the operation of the latch circuit of FIG. 1; 
     FIG. 3 illustrates in logic diagram form a latch circuit with a metastability trap according to the present invention; 
     FIG. 4 illustrates a timing diagram of various signals in the latch circuit of FIG. 3; and 
     FIG. 5 illustrates a timing diagram of signals useful in understanding the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     FIG. 3 illustrates in logic diagram form a latch circuit  100  with a metastability trap according to the present invention. Latch circuit  100  includes a buffer  102 , three input D-type flip-flops  104 ,  106 , and  108 , a three-input NOR gate  110 , a three-input AND gate  112 , an SR-type flip-flop  114 , and an output D-type flip-flop  116 . Buffer  102  has an input for receiving an input signal labeled “INPUT”, and an output. Flip-flop  104  has a D input terminal connected to the output terminal of buffer  102 , a clock input terminal for receiving a clock signal labeled “A”, and a Q output terminal for providing a signal labeled “X”. Flip-flop  106  has a D input terminal connected to the output terminal of buffer  102 , a clock input terminal for receiving a clock signal labeled “B”, and a Q output terminal for providing a signal labeled “Y”. Flip-flop  108  has a D input terminal connected to the output terminal of buffer  102 , a clock input terminal for receiving a clock signal labeled “C”, and a Q output terminal for providing a signal labeled “Z”. NOR gate  110  has a first input terminal connected to the Q output terminal of flip-flop  104 , a second input terminal connected to the Q output terminal of flip-flop  106 , a third input terminal connected to the Q output terminal of flip-flop  108 , and an output terminal for providing a first intermediate signal labeled “0”. AND gate  112  has a first input terminal connected to the Q output terminal of flip-flop  104 , a second input terminal connected to the Q output terminal of flip-flop  106 , a third input terminal connected to the Q output terminal of flip-flop  108 , and an output terminal for providing a second intermediate signal labeled “1”. Flip-flop  114  has an S input terminal connected to the output terminal of AND gate  112 , an R input terminal connected to the output terminal of NOR gate  110 , and a Q output terminal for providing a first output signal labeled “OUT 1 ”. Flip-flop  116  has a D input terminal connected to the Q output terminal of flip-flop  114 , a clock input terminal for receiving a clock signal labeled “CAPTURE”, and a Q output terminal for providing a second output signal labeled “OUT 2 ” to internal logic (not shown in FIG.  3 ). 
     Latch circuit  100  provides a metastability trap by accepting the metastable condition as an input but protecting the system from its effects. Input latches  104 ,  106 , and  108  capture successive values of the INPUT signal. Signals X, Y, and Z output from input latches  104 ,  106 , and  108  represent values of the INPUT signal sampled at three successive times by successive clock signals A, B, and C. These successive times are selected to eliminate metastability in at least two of those values and thus, as will be described more fully below, clock signals A, B, and C are selected to have transitions distributed over a predetermined period which is less than half of a minimum fundamental period of the INPUT signal. 
     In alternate embodiments, other techniques can be used to capture three or more successive values of the INPUT signal. In one such alternate embodiment, a common clock signal could be used to clock three latch circuits and the INPUT signal could be successively delayed using delay lines. In another alternate embodiment three different clock signals A, B, and C could themselves be generated using delay lines. Each of these embodiments share the characteristic that they capture values of the INPUT signal at three successive points in time, the points in time occurring over a predetermined period which is less than half of the minimum fundamental period of the INPUT signal. 
     Signals X, Y, and Z are input to NOR gate  110  and AND gate  112  to detect whether they have all assumed the same logic state, either all logic low as detected by NOR gate  110  or all logic high as detected AND gate  112 . Since only one signal can be metastable at any one time within the capturing window, the other two values are used to “protect” the final output value from the metastability. Note that in other embodiments NOR gate  110  and AND gate  112  could be replaced with an OR gate and a NAND gate, respectively, to implement the same functions in negative logic. 
     The outputs of NOR gate  110  and AND gate  112  are intermediate signals that are used to change the value of the first output signal OUT 1 . Thus the output signal from AND gate  112  is used to set the output of flip-flop  114  to a “1” state (logic high), and the output signal from NOR gate  110  is used to reset the output of flip-flop  114  to a “0” state (logic low). It should be apparent that many other types of sequential circuits may be used in place of SR flip-flop  114 , such as a JK flip-flop, a clocked D-latch in which the D input is tied to a logic high level, the output of AND gate  112  is connected to the clock input, and the output of NOR gate  110  is connected to the reset input, etc. 
     The operation of latch circuit  100  is better understood with respect to FIG. 4, which illustrates a timing diagram  120  of various signals in latch circuit  100 . In FIG. 4 the horizontal axis represents time and the vertical axis the amplitude, in volts, of several relevant signals. Signals A, B, and C are activated successively and are related to the CAPTURE clock as follows. Clock signal A is activated on a first rising edge of the CAPTURE clock signal; clock signal B is activated on a second rising edge of the CAPTURE clock signal; and clock signal C is activated on a third rising edge of the CAPTURE clock signal. Each of these signals has a fifty percent duty cycle over a period equal to three periods of the CAPTURE clock, and is generated from the CAPTURE clock by a clock circuit, not shown in FIG.  3 . 
     As shown in FIG. 4, the INPUT signal gradually changes from a logic low to a logic high around time t 1 . Also around t 1  the CAPTURE and A clock signals make a low-to-high transition. Signal X output from flip-flop  104  is in a metastable state, which is shown as an oscillating signal overlying a signal at an intermediate logic state because the form in which the metastable condition actually takes will vary based on the circuit implementation. Since the INPUT signal has reached its logic high state by the low-to-high transition of the next successive clock signals B and C at times t 2  and t 3 , respectively, corresponding outputs Y and Z transition to a logic high in sequence. 
     In the example shown in FIG. 4 the metastable event in latch  104  persists until the next rising edge of the A clock signal at time t 4 . Between t 3  and t 4  AND gate  112  sees logic highs on its second and third inputs and an indeterminate level on its first input. If the metastability event causes oscillation on the output of latch  104 , AND gate  112  resolves to a logic high during the high phase of the oscillation shortly after t 3  and sets flip-flop  114 . If however the metastability event causes an intermediate level on the output of latch  104 , then AND gate  112  may not resolve to a logic high and flip-flop  114  may not be set until t 4 . At the next rising edge of the A clock at time t 4 , latch  104  recognizes a logic high input and since all three inputs of AND gate  112  are at logic high states, flip-flop  114  will assume a logic high state if it has not already done so. The second output, OUT  2 , follows OUT 1  on the next rising edge of the CAPTURE clock and so will change from a logic low to a logic high at t 4  or t 5  depending on the character of the metastability event. 
     As mentioned above there is a restriction on the A, B, and C clocks to ensure that only one latch sees a metastability event. This restriction is better understood with respect to FIG. 5, which illustrates a timing diagram  140  of signals useful in understanding the present invention. In FIG. 5 the horizontal axis represents time and the vertical axis the amplitude, in volts, of several relevant signals. As shown in FIG. 5 the INPUT signal starts out at a logic low and makes a transition to a logic high around a time labeled “t 1 ” by passing through a logic high threshold value labeled “V lH ”. This transition occurs a setup time labeled “t SU ” before a low-to-high transition of the A clock. The INPUT signal falls below V lH  around a time labeled “t 2 ” while making a high-to-low transition. This transition occurs a hold time labeled “t HOLD ” after a low-to-high transition of the C clock. To avoid the possibility of two metastable events being captured by latches  104 ,  106 , and  108 , signals A, B, and C need to be activated successively while the INPUT signal is at a logic high, that is between (t 1 +t SU ) and (t 2 −t HOLD ). Thus a MINIMUM WINDOW SIZE is defined as the difference between t 1  and t 2 . Since the high time of the INPUT signal represents the minimum high time, a full period of the INPUT signal represents its minimum fundamental period. The MINIMUM WINDOW SIZE is thus about half of the fundamental period of the INPUT signal, and when clocks A, B, and C all transition within this amount of time (less setup and hold times) no more than one metastable event can occur. 
     Thus latch circuit  100  traps metastability events from reaching internal circuitry by taking at least three successive samples of an input signal and determining when the samples indicate the same logic state. If the clock signals used to trigger corresponding input latches occur in less than about half of the minimum fundamental period of the INPUT signal, then at most one sample can be metastable at any given time. The metastable output of any one latch is protected by the remaining latches. It should be apparent that in other embodiments more than three latches may be used. The latch circuit may also be implemented with positive logic as shown or with corresponding negative logic. Also various types of latches and flip-flops may be substituted for the ones shown to achieve the same results. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.