Abstract:
An asynchronous signal synchronizing circuit for sampling and external asynchronous signal in a quarter of the period of a clock. A first latch circuit latches asynchronous input signal in accordance with a first clock, and a second latch circuit latches the output of the first latch circuit in accordance with a second clock having a phase shift 180° out of phase with the first clock. A third latch circuit latches the output signal of the second latch signal in accordance with a clock signal that represents the inverse of the first clock. A fourth latch circuit latches the output signal of the third latch circuit under the control of a clock that corresponds to the inverse of the second clock. The asynchronous input signal is sampled at the tailing edge of the first clock signal and validated by the tailing edge of the second clock signal. The synchronization of the asynchronous signal can thus be performed in a quarter of a clock cycle.

Description:
The present invention relates to a circuit for synchronizing an input signal which changes without being synchronous to a clock. 
     In recent years, the operating speed of logic LSI, such as microprocessors has risen. Now, a logic LSI adapted to operate at a clock frequency of 10 odd or several tens of MHz has appeared, and the signal delay time in the LSI has been shortened to approximately 1 ns. On the other hand, a signal delay time outside the LSI has reached on the order of several tens ns though packaging techniques. In the internal circuit of the LSI, accordingly, it has become difficult to synchronously perform the transmission and reception of signals outside the LSI. Therefore, the input signal from outside the LSI is inevitably handled as an asynchronous signal, and an asynchronous signal synchronizing circuit is required as the signal input circuit of the LSI. 
     In general, an asynchronous signal can be sampled using a shift register of two stages which are controlled by a clock φ1 and an inverted signal φ1 shifted in phase by 180° from the former. However, the output of a flip-flop at the second stage is unstable at the leading edge of the clock φ1. In this regard, by latching the output signal again with a clock lagging φ1, a signal which is valid completely during 1 machine cycle can be produced. Heretofore, φ1 shifted in phase by 180° has been employed as the clock lagging φ1. An example thereof is shown in FIG. 11 of U.S. Pat. No. 4,349,873. 
     The circuit of FIG. 11 of U.S. Pat. No. 4,349,873 is illustrated in FIG. 1. This circuit is such that three latch circuits 1, 2 and 3 are connected in series, among which the latch circuits 1 and 3 are controlled by the clock signal φ1, while the latch circuit 2 is controlled by the clock signal φ1. 
     The clock signals φ1 and φ1 are signals which have the relationship of inversion from each other. Accordingly, a half clock cycle after the sampling of the external signal is required before a synchronized signal is obtained from the latch circuit 3. 
     SUMMARY OF THE INVENTION 
     A first object of the present invention is to provide an asynchronous signal synchronizing circuit of high speed in which the period of time from the sampling of an external signal until the settling of an internal signal is shortened without raising a clock frequency. 
     A second object of the present invention is to provide an asynchronous signal synchronizing circuit which is arranged so as not to output an intermediate value (a value not settled to be &#34;high&#34; or &#34;low&#34;) when an asynchronous signal is synchronized. 
     In order to accomplish the first object, the present invention uses a new clock φ2 having a phase shift of below 180° from a clock φ1 and comprises a first latch circuit which latches an asynchronous input signal in accordance with the clock φ1, a second latch circuit which latches an output signal of the first latch circuit in accordance with the clock φ2, a third latch circuit which latches an output signal of the second latch circuit in accordance with an inverted signal of the clock φ1, and a fourth latch circuit which latches an output signal of the third latch circuit in accordance with an inverted signal of the clock φ2. Namely, the new clock φ2 is a clock having a period that is synchronized between the period of the first clock and the inverted clock of the first clock. Thus, a synchronized signal can be quickly derived without raising the clock frequency. 
     In order to accomplish the second object, the present invention adds a latch to be controlled by a clock φ2, between latches of clocks φ1 and φ1 and constructs a feedback loop in the two latches of the clocks φ1 and φ2 before the settling of an internal signal by a clock φ2, thereby to prevent the latch of an intermediate value. More specifically, the intermediate value which might be held in each the feedback loop of each latch is a value inherent depending upon the logical threshold values of gates which constitute the loop, and when a signal deviating from this value has been input, an output signal is infallibly rendered logical &#34;0&#34; or &#34;1&#34;. Therefore, the intermediate values of the asynchronous signal can be prevented from being latched in such a way that the intermediate values which might be held in respective feedback loops are set at unequal values beforehand. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a prior-art synchronizing circuit. 
     FIG. 2 is a block diagram of an embodiment of an asynchronous signal synchronizing circuit according to the present invention. 
     FIG. 3 is a timing diagram for the asynchronous signal synchronizing circuit of FIG. 2. 
     FIG. 4 is the logic diagram of a flip-flop which is employed in the circuit of FIG. 2. 
     FIG. 5 is a detailed circuit diagram of the flip-flop of FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now, one embodiment of the present invention will be described with reference to the drawings. 
     FIG. 2 is a block diagram of a synchronizing circuit for an asynchronous signal to which the present invention is applied. Flip-flops 102-105 are latch circuits each of which transmits input data D to an output terminal Q when a control signal input C is asserted, i.e., at a first logic level and holds and delivers an output signal Q when the same is negated, i.e. is at the other logic level. Each of circuits elements 225 and 226 is an inverter. The operation of the asynchronous signal synchronizing circuit will be described with reference to the timing diagram of FIGS. 2 and 3. An input signal 110 applied from the input terminal 101 of an LSI chip is latched by the flip-flop 102 when a timing clock φ1 goes &#34;low&#34;, whereupon a signal 120 is output. The flip-flop 102 passes the input signal 110 while the clock φ1 is &#34;high&#34;, so that the output value 120 during this period varies depending upon the input signal 110 and is indefinite. The signal 120 is then latched by the flip-flop 103 in accordance with a timing clock φ2 which is shifted in phase by 90° from the clock φ1, whereupon a signal 130 is output. An output signal 140 produced by latching the signal 130 in accordance with the inverted signal of the clock φ1 becomes a signal which holds a value during one cycle of the clock φ1 as shown in the timing diagram. However, the signal 140 is unstable at the tailing edge of the clock φ1. The reason is that, since gate delays are involved in the flip-flops 103 and 104 at the second and third stages, a time interval is required until the settling of the signal 140 even when the signal 120 has been settled at the tailing edge of the clock φ1. 
     Therefore, when the signal 140 is latched again in accordance with the inverted signal of the clock φ2; the resulting output signal 150 holds a value during one cycle of the clock φ2 and affords a value valid from the tailing edge of the clock φ2 as illustrated in the timing diagram. 
     FIG. 4 shows an example of arrangement of each of the flip-flops 102-105. A logic gate 200 outputs on line 330 the inverted signal of the input signal D on line 201 when the control signal C on line 203 is asserted, i.e., a &#34;high&#34; signal, and no signal when the same is negated because the logic gate 200 becomes a high impedance state. To the contrary a logic gate 210 outputs the inverted signal of the output signal Q of the flip-flop on line 207 when the control signal C is negated, and no signal when the same is asserted because the logic gate 210 becomes a high impedance state. When the two sorts of logic gates 200 and 210 and an inverter 220 are connected as shown in FIG. 4, a flip-flop which latches the input data D in accordance with the control signal C results. 
     As explained above, with the present embodiment, the asynchronous signal is sampled at the tailing edge of the clock φ1, and the internal signal can be validated at the tailing edge of the clock φ2. That is, the synchronization of the asynchronous signal can be done in a time interval of 1/4 of a clock cycle. Here, the clocks φ1 and φ2 whose phases shift by 90° can be readily generated in such a way that a parent clock φ o  having a frequency double higher is divided by 2 in a divider 224. 
     Therefore, the illustrated embodiment is well suited for use as a synchronizing input for the response signal, interrupt request signal etc. of an asynchronous transfer bus. 
     Next, an arrangement which prevents the asynchronous signal synchronizing circuit from outputting an intermediate value will be explained. 
     FIG. 5 shows an example of the more detailed circuit arrangement of the flip-flop. Circuit elements 301, 302, 311, 312, and 321 are P-channel MOSFETs, while circuit elements 303, 304, 313, 314 and 322 are N-channel MOSFETs. The MOSFETs 301, 302 303 and 304 that are connected in series constitute the logic gate 200 in FIG. 4, while the MOSFETs 311, 312, 313 and 314 similarly constitute the logic gate 210. The MOSFETs 301, 304, 311 and 314 are arranged outside of the respective series and the MOSFETs 302, 303, 312 and 313 are arranged inside. A circuit element 221 is an inverter. In addition, the MOSFETs 321 and 322 constitute the inverter 220. The operation of the flip-flop will be described with reference to FIG. 5. First, when the control signal C is asserted (rendered &#34;high&#34;), the circuit elements 302 and 303 fall into ON states, and the logic gate 200 effects the same function as that of an inverter and outputs the inverted signal of the input signal D. On the other hand, the logic gate 210 outputs no signal because the circuit elements 312 and 313 are in OFF states. Accordingly, a signal line 330 becomes the inverted signal of the input signal D, and the inverter 220 inverts it again and transmits the input signal D to the output terminal Q. 
     Next, when the control signal C is negated (rendered &#34;low&#34;), the logic gate 200 outputs no signal because the circuit elements 302 and 303 fall into OFF states. On the other hand, the circuit elements 312 and 313 fall into ON states, and the logic gate 210 effects the same function as that of an inverter and outputs the inverted signal of the output signal Q. Accordingly, the signal line 330 comes to have the inverted signal of the output signal Q, and is inverted again by the inverter 220, whereby the output signal Q is provided, and the value is held. 
     When, in this flip-flop, the gate width of the P-MOSFET 311 of the logic gate 210 is made greater than in an ordinary case, the logical threshold level of the logic gate 210 becomes higher. Accordingly, in the case where the control signal C is negated to latch the intermediate value in the feedback loop constructed of the logic gate 210 and the inverter 220, the value of the output signal Q at the input of the logic gate 210 becomes stable at a level which is higher than with the ordinary gate width of the P-MOSFET 311. That is, when a value somewhat higher than an ordinary one has been given as the input signal D with the control signal C asserted, this flip-flop latches the intermediate value and its output signal Q becomes a level higher than an ordinary one. Conversely, when the gate width of the N-MOSFET 314 of the logic gate 210 is enlarged, the logical threshold level of the logic gate 210 lowers. Accordingly, when the intermediate value is latched in the feedback loop, the value of the output signal Q becomes a lower level. That is, when a value somewhat lower than an ordinary one has been input with the control signal C asserted, this flip-flop latches the intermediate value, and its output signal Q becomes a level lower than an ordinary one. 
     In a case where the flip-flop has latched a value other than the intermediate value, the output signal Q is amplified to the perfectly high or low level owing to the feedback loop which is constructed of the logic gate 210 and the inverter 220. 
     When, in the asynchronous signal synchronizing circuit of FIG. 2, the flip-flop with the gate width of the P-MOSFET 311 enlarged is employed as the first-stage flip-flop 102 for latching the input signal 110 and the flip-flop with the gate width of the N-MOSFET 314 enlarged is employed as the second-stage flip-flop 103 for latching the output signal 120, the intermediate value can be amplified to the high or low level without fail, because even when the first-stage flip-flop 102 has latched the intermediate value, the output signal 120 at that time becomes the level somewhat higher than the intermediate value which is produced by the ordinary flip-flop, it can be settled to the perfectly high level by the second-stage flip-flop 103. The intermediate value is latched in the second-stage latch 103 in a case where a somewhat lower intermediate value is received as the input signal 120. However, the intermediate value becomes a somewhat higher level in the first-stage flip-flop 102 which is delivering the signal 120, and any other value is settled to the perfectly high or low level by the feedback loop. Therefore, the somewhat lower intermediate value is not delivered as the signal 120. 
     Accordingly, the logical threshold values of the gates constituting the respective flip-flops are controlled so that the value to be output when the first-stage flip-flop 102 has latched the intermediate value may differ from the value to be input when the second-stage flip-flop 103 latches the intermediate value, whereby when the first- and second-stage flip-flops 102 and 103 fall into the feedback states, the output signal 130 can be settled to the perfectly high or low level. The output signal 150 in FIG. 2 transmits the signal 140 to the output when the clock φ2 has been negated. More specifically, the output signal 150 is delivered after the first- and second-stage latches 102 and 103 have fallen into the feedback stages, so that it can provide a value perfectly settled to the high or low level. 
     While, in the above description, the sizes of the MOSFETs of the logic gate 210 have been altered in order to control the intermediate value which the flip-flop might latch, the sizes of the MOSFETs of the logic gate 200 or the inverter 220 may be altered alternatively. 
     The present invention produces the effect that a time interval from the sampling of an asynchronous signal until the settlement of a synchronized signal can be shortened without raising the frequency of an internal clock for sampling. 
     Another effect is that, by making a first-stage latch and a second-stage latch a feedback type and by controlling intermediate values, which might be latched, to unequal values, a signal perfectly settled to a high or low level can be derived as the synchronized signal of an asynchronous signal.