LATCH CIRCUIT

Abstract: A latch circuit having two complementary hold loops for improved noise tolerance. The latch circuit includes a first gate (G11, G16) receiving data (D) and a first clock signal (CL), for outputting at least one signal gating the data and the first clock signal in response to a change in the clock signal; a second gate (G12, G15) receiving a second clock signal having an inverted polarity to that of the first clock signal; and a third gate (G13, G14); and operatively connected to output terminals of the first and second gates, for outputting at least one latch output (SQ). The latch circuit also includes a first hold line (HL1) supplying the latch output to the second gate and a second hold line (HL2) supplying another latch output of the latch circuit having an inverted polarity to that of the latch output to the second gate. The second gate may have an inverted input terminal receiving the other latch output. The second gate may include a differential operation gate circuit operating in response to a signal on the first hold line and a signal on the second hold line. The latch circuit may be an "in-phase" hold type latch circuit or an "inverted-phase" hold type latch circuit.

Description: The present invention relates to a latch circuit. A variety of latch circuits, such as a delay (D) type flip-flop (FF) or a set-reset (R-S) type FF, are extensively known. These latch circuits have hold loops to feedback either a normal output Q or an inverted output Q , and to hold the status latched therein. However, each latch circuit has only a single hold loop. Consequently, this type of latch circuit is easily caused to malfunction by instantaneous electrical noise, such as noise due to alpha-ray exposure. This will be described in more detail with reference to specific examples. To overcome the above disadvantage, many countermeasures have been proposed, but disadvantages such as a low noise tolerance, a complex circuit configuration, etc., still remain. A. H. Seidman, "Integrated Circuits Applications Handbook", Wiley, 1983, discloses on page 82 an ECL latch circuit having first to third gate circuits, the first gate circuit receiving data, the third gate circuit outputting complementary latch outputs, and the second gate circuit receiving an inverted-OR of enable signals as well as the latch outputs from the third gate circuit. First and second hold lines supply the complementary latch outputs to the second gate circuit. The second gate circuit consists of an emitter-coupled pair of transistors whose bases are supplied with the latch output via respective "hold lines" in the form of emitter follower circuits, and the third gate circuit consists of an independent pair of emitter follower circuits. EP-A-0 167 339 by Sony Corporation discloses various ECL logic circuits in which the need for a reference voltage is avoided by using two voltage levels to represent logic "L" and two voltage levels to represent logic "H". In one such logic circuit, first to third gate circuits are arranged to form an R-S flip-flop provided with a single hold line for supplying a latch output to the second gate circuit. DE-B-1 230 079 (in particular Fig. 3) discloses a bistable multivibrator (latch) having an emitter-coupled transistor pair which is cross-coupled via two emitter followers. The emitter followers apparently act like hold loops although this document does not address the problem of noise due to alpha-ray exposure. The circuit of Fig. 3 includes, in addition to the basic multivibrator circuit, set and reset gates for setting and resetting the multivibrator in response to a clock signal. A special feature of this circuit is that the latching transistors of the emitter-coupled pair are precharged by means of voltage dividers, so that the input voltage needed to switch the latching transistors is half the sum of their base and emitter voltages. An embodiment of the present invention may provide a latch circuit having a remarkably improved noise tolerance. An embodiment of the present invention may provide a latch circuit having a simple circuit configuration. According to the present invention, there is provided a latch circuit comprising a first gate connected to receive data and a clock signal, a second gate connected to receive an inverted clock signal, and a third gate operatively connected to output terminals of said first and second gates, a first hold line being provided between an inverted output terminal of said third gate and a third input terminal of said second gate, and a second hold line being provided between a non-inverted output terminal of said third gate and an inverted input terminal of said second gate; wherein:- said first gate is formed by an emitter-coupled logic gate circuit consisting of first to third emitter-coupled transistors whose coupled emitters are connected to a second power source via a current source, resistors connecting collectors of said first to third transistors to a first power source, a first output transistor having a base connected to the collectors of said first and second transistors and a collector connected to said first power source, and a resistor connecting an emitter of said first output transistor to said second power source; said second gate is formed by an emitter-coupled logic gate circuit consisting of fourth to sixth emitter-coupled transistors whose coupled emitters are connected to a second power source via a current source, resistors connecting collectors of said fourth to sixth transistors to said first power source, a second output transistor having a base connected to said collectors of said fourth and fifth transistors and a collector connected to said first power source, and a resistor connecting an emitter of said second output transistor to said second power source; and said third gate is formed by an emitter-coupled logic gate circuit consisting of seventh to ninth emitter-coupled transistors whose coupled emitters are connected to a second power source via resistors connecting collectors of said seventh to ninth transistors to said first power source, a third output transistor having a base connected to said collectors of said seventh and eighth transistors and a collector connected to said first power source, a fourth output transistor having a base connected to said collector of said ninth transistor and a collector connected to said first power source, and a resistor connecting an emitter of said fourth output transistor to said second power source; wherein:- said first hold line is provided between an emitter of said third output transistor and a base of said fourth transistor; said second hold line is provided between the emitter of said fourth output transistor and a base of said sixth transistor; the emitter of said first output transistor and a base of said eighth transistor are commonly connected; and the emitter of said second output transistor and a base of said seventh transistor are commonly connected. Reference is made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a circuit diagram of an "in-phase" hold type latch circuit of the prior art; Figs. 2a and 2e are timing charts illustrating the operation of the "in-phase" hold type latch circuit shown in Fig. 1; Fig. 3 is a circuit diagram of an "inverted-phase" hold type latch circuit of the prior art; Fig. 4 is a circuit diagram of an embodiment of an "inverted-phase" hold type latch circuit according to the present invention; Figs. 5a to 5e are timing charts illustrating the operation of the latch circuit shown in Fig. 4; Fig. 6 is a specific circuit diagram of the "inverted-phase" hold type latch circuit shown in Fig. 4. Before describing an embodiment of the present invention, some examples of prior art latch circuits will be described with reference to the drawings for comparison. Referring to Fig. 1, an "in-phase" hold type latch circuit of the prior art consists of a first OR gate G1 receiving a data D and a clock signal CL and outputting an inverted output SQ and a non-inverted internal output, a second OR gate G2 receiving an inverted clock signal CL at an input terminal thereof and outputting an inverted output connected to the inverted output SQ from the OR gate G1 and a non-inverted internal output, and a wired-AND gate G3 receiving the non-inverted internal outputs from the OR gates G1 and G2 and outputting a non-inverted (normal) output SQ. A hold line HL is provided between an output terminal of the wired-AND gate G3 and another input terminal of the OR gate G2. The operation of the latch circuit in Fig. 1 will be described with reference to Figs. 2a to 2e. The basic operation of the latch circuit is that the data D is latched in response to the trailing of the clock signal CL. At a time t1, when the clock signal CL is changed from a high level "H" to a low level "L" a high level "H" of the data D is taken in the latch circuit. As a result, the OR gate G1 outputs a low level signal at the inverted output terminal and a high level signal at the non-inverted output terminal. At the same time, the inverted output of the OR gate G2 becomes low level, because the inverted clock signal CL applied to the OR gate G2 is at a high level. Accordingly, the inverted output SQ of the latch circuit becomes low level. Conversely, as two output signals from the OR gate G1 and G2 applied to the AND gate G3 are high level, the AND gate G3 outputs a high level non-inverted output SQ. At a time t2, the clock signal CL is restored to a high level "H" from a low level "L", but, as the data D is still at a high level, the OR gate G1 still outputs a high level signal at the non-inverted output terminal, and the AND gate G3 still outputs the high level non-inverted output SQ. As a result, the output signals from the OR gates G1 and G2, and accordingly, the outputs SQ and SQ , are maintained, regardless of whether or not the clock signal CL is restored. At a time t3, the data D is changed from a high level "H" to a low level "L", but the feedback signal on the hold loop HL is maintained at a high level "H". As a result, the output SQ and SQ are not changed. At a time t4, when the clock signal CL is again changed from a high level "H" to a low level "L", a low level "L" of the data D is taken in the latch circuit. The OR gate G1 outputs a high level signal at the inverted output terminal and a low level signal at the non-inverted output terminal, and the AND gate G3 outputs a low level signal. That is, the non-inverted output SQ of the latch circuit is changed to a low level. On the other hand, the output signals from the inverted output terminals of the OR gate G1 and G2 are high level, and thus the inverted output SQ of the latch circuit is changed to a high level. At a time t5, when the clock signal CL is restored to a high level "H", and the inverted clock signal CL is synchronously changed to a low level, the OR gate G2 outputs a high level signal at the inverted output terminal and a low level signal at the non-inverted output terminal. The output of the AND gate G3 becomes low level, and as a result, a low level of the non-inverted output SQ and a high level of the inverted output SQ are maintained. As described above, the hold loop HL has the above data latch function. When a semiconductor circuit substrate including the above latch circuit is exposed to alpha (α) - rays, pairs of holes and electrons are generated in the substrate along exposure lines traversing the semiconductor layers formed in the substrate. The holes in the semiconductor are extracted to a P-type semiconductor layer of a N-P-N transistor applied with a negative voltage, and the electrons are extracted to a depletion layer at a P-N junction. As a result, a potential at a collector of the N-P-N transistor may fall in a short period. If a transistor connected to the hold line HL is exposed to alpha-rays when the non-inverted output SQ, i.e., the feedback signal on the hold line HL, is high, a voltage at a collector of the transistor, i.e., a voltage of the feedback signal on the hold line HL, may fall in a short period due to the above phenomenon. If the level of the voltage on the hold line HL becomes lower than a threshold level V TH of a transistor in, for example, the OR gate G2, the OR gate G2 may output a low level signal from the non-inverted output terminal thereof in a short time. As a result, the output SQ may be changed to a low level, and the feedback signal on the hold line HL also may be changed to a low level. The inverted output SQ of the latch circuit is changed to a high level. This means that the latch circuit in Fig. 1 will malfunction due to the noise of the alpha-rays. Referring to Fig. 3, an "inverted phase" hold type latch circuit of the prior art consists of two NOR gates G5 and G6, and an OR gate G4. The NOR gate G6 receives the data D and the clock signal CL, and the NOR gate G5 receives the inverted clock signal CL . A hold line HL is provided between an inverted output terminal of the OR gate G4 and an input terminal of the NOR gate G5. The OR gate G4 outputs an inverted output SQ and a non-inverted output SQ. The operation of the latch circuit in Fig. 3 is the same as that of the latch circuit in Fig. 1, as described with reference to Figs. 2a to 2e, and thus a description thereof is omitted. The latch circuit of Fig. 3 also suffers from the disadvantage of a malfunction due to noise . An embodiment of a latch circuit according to the present invention will be described with reference to Fig. 4. The latch circuit in Fig. 4 is a so-called "inverted phase" hold type latch circuit and corresponds to the latch circuit of Fig. 3. The latch circuit of Fig. 4 includes a first NOR gate G16 receiving data D and a clock signal CL, a second NOR gate G15 receiving an inverted clock signal CL , and an OR gate G14. The latch circuit is provided with two hold lines HL1 and HL2. The hold line HL1 is provided between an inverted output terminal of the OR gate G14 and a third input terminal of the NOR gate G15, and the hold line HL2 is provided between a non-inverted output terminal of the OR gate G14 and an inverted input terminal of the NOR gate G15. A signal on the hold line HL1 has an inverted phase relationship to a signal on the hold line HL2. The operation of the latch circuit of Fig. 4 will be described with reference to Table 1 below and Figs. 5a and 5e. TIME t1 ∿ t2 t4 ∿ t5 D H L CL H → L* → H H → L* → H R L H* CL (S) L → H* → L L → H* → L SQ H L SQ L H In Table 1, asterisks (*) indicate short period signals. At the time t1, the data D is at a high level "H" and the clock signal CL is changed to a low level "L", and accordingly, the NOR gate G16 outputs a signal S16 having a low level "L". The NOR gate G15 receives the inverted clock signal CL having a high level "H" and outputs an signal S15 having a low level "L". As a result, the OR gate G14 outputs a non-inverted output SQ having a high level "H" at an inverted output terminal and an inverted output SQ having a low level "L" at another output terminal. A signal having a high level on the hold line HL1 and/or a signal having a low level on the hold line HL2 ensure that a low level of the signal S15 from the NOR gate G15 is maintained when the clock signal CL is restored to a high level and the inverted clock signal CL is also restored to a low level at the time t2. At the time t4, the data D is a low level "L" and the clock signal CL is again changed to a low level "L", then the NOR gate G16 outputs the signal S16 having a high level "H". As a result, the OR gate G14 receiving the signal S16 having a high level outputs the inverted output SQ having a high level and the non-inverted output SQ having a low level. At the time t5, the clock signal CL is restored to a high level, and accordingly, the signal S16 from the NOR gate G16 is changed to a low level "L". On the other hand, the inverted clock signal CL having a low level, the inverted output SQ having a high level to the inverted input terminal of the NOR gate G15 and the non-inverted output SQ having a low level are supplied to the NOR gate G15, and the signal S15 having a high level is output, and the output status of the outputs SQ and SQ is maintained. As can be seen from the above description, the OR gate G14, the NOR gate G15, and the hold lines HL1 and HL2 form a set-reset (R-S) type flip-flop. The NOR gate G16 provides a reset signal R, and the inverted clock signal CL supplied to the NOR gate G15 corresponds to a set signal S. An improvement of the noise tolerance due to the provision of the hold line HL2 in addition to the latch circuit of Fig. 3 is obtained. In this embodiment, the provision of the inverted input terminal at the NOR gate G15 for directly receiving an additional feedback signal on the hold line HL2 results in a greater noise margin than that of the latch circuit using an inverter INV as shown by dotted line. A more concrete circuit of the "inverted-phase" hold type latch circuit of Fig. 4 will be described with reference to Fig. 6. The "inverted-phase" hold type latch circuit includes the NOR gate G16 formed by an ECL gate circuit consisting of transistors Q32, Q33, and Q34, a current source I16 and resistors R21 and R22, an output transistor Q31 and a resistor R23, the OR gate G14 formed by an ECL gate circuit consisting of transistors Q11, Q12, and Q13, a current resistor I14 and resistors R11 and R12, output transistors Q14 and Q15, and resistors R13 and R14, and the NOR gate G15 formed by an ECL gate circuit consisting of transistors Q16, Q17, and Q18, a current source I15 and resistors R15 and R16, 97 output transistor Q19, and a resistor R17. The latch circuit is supplied with the first and second power sources V CC and V EE . The data D is supplied to a base of the transistor Q32, and the clock signal CL is supplied to a base of the transistor Q33. The inverted clock signal CL is supplied to a base of the transistor Q17. A base of the transistor Q11 is supplied with a signal from an emitter of the transistor Q19, and a base of the transistor Q12 is supplied with a signal from an emitter of the transistor Q31. The base of the transistors Q34 and Q13 are supplied with the reference voltage Vref. The first hold line HL1 is provided between an emitter of the transistor Q14 and a base of the transistor Q16, and the second hold line HL2 is provided between an emitter of the transistor Q15 and a base of the transistor Q18. The signals on the hold lines HL1 and HL2 have inverted polarities. In the latch circuit, feedback signals, i.e., the signals from the transistors Q14 and Q15, are directly supplied to the bases of the transistors Q16 and Q18 without providing an inverter. This also results in a high noise tolerance and a simple circuit configuration. In Fig. 4, if the gate G16 is removed, a reset signal R is supplied to the gate G14 instead of the signal S16 and a set signal S is supplied to the gate G15 instead of the inverted clock signal CL , the latch circuit may operate as a set-reset type flip-flop. The operation is shown in Table 1.

Claims: A latch circuit comprising a first gate (G16) connected to receive data (D) and a clock signal (CL), a second gate (G15) connected to receive an inverted clock signal ( CL ), and a third gate (G14) operatively connected to output terminals of said first and second gates, a first hold line (HL1) being provided between an inverted output terminal of said third gate (G14) and a third input terminal of said second gate (G15), and a second hold line (HL2) being provided between a non-inverted output terminal of said third gate (G14) and an inverted input terminal of said second gate (G15); wherein:- said first gate (G16) is formed by an emitter-coupled logic gate circuit consisting of first to third emitter-coupled transistors (Q32, Q33, Q34) whose coupled emitters are connected to a second power source (V EE ) via a current source (I16), resistors (R21, R22) connecting collectors of said first to third transistors (Q32, Q33, Q34) to a first power source (V CC ), a first output transistor (Q31) having a base connected to the collectors of said first and second transistors (Q32, Q33) and a collector connected to said first power source (V CC ), and a resistor (R23) connecting an emitter of said first output transistor (Q31) to said second power source (V EE ); said second gate (G15) is formed by an emitter-coupled logic gate circuit consisting of fourth to sixth emitter-coupled transistors (Q16, Q17, Q18) whose coupled emitters are connected to a second power source (V EE ) via a current source (I15), resistors (R15, R16) connecting collectors of said fourth to sixth transistors (Q16, Q17, Q18) to said first power source (V CC ), a second output transistor (Q19) having a base connected to said collectors of said fourth and fifth transistors (Q16, Q17) and a collector connected to said first power source (V CC ), and a resistor (R17) connecting an emitter of said second output transistor (Q19) to said second power source (V EE ); and said third gate (G14) is formed by an emitter-coupled logic gate circuit consisting of seventh to ninth emitter-coupled transistors (Q11, Q12, Q13) whose coupled emitters are connected to a second power source (V EE ) via a constant current source (I14) and resistors (R11, R12) connecting collectors of said seventh to ninth transistors (Q11, Q12, Q13) to said first power source (V CC ), a third output transistor (Q14) having a base connected to said collectors of said seventh and eighth transistors (Q11, Q12), a collector connected to said first power source (V CC ), and an emitter connected via a resistor (R13) to said second power source (V EE ), a fourth output transistor (Q15) having a base connected to said collector of said ninth transistor (Q13) and a collector connected to said first power source (V CC ), and a resistor (R14) connecting an emitter of said fourth output transistor (Q15) to said second power source (V EE ); wherein:- said first hold line (HL1) is provided between an emitter of said third output transistor (Q14) and a base of said fourth transistor (Q16); said second hold line (HL2) is provided between the emitter of said fourth output transistor (Q15) and a base of said sixth transistor (Q18); the emitter of said first output transistor (Q31) and a base of said eighth transistor (Q12) are commonly connected; and the emitter of said second output transistor (Q19) and a base of said seventh transistor (Q11) are commonly connected.

Inventors: ANDO, NAOYUKI, KANAI, YASUNORI, SETO, MOTOHIRO, SUGIYAMA, EIJI, TSUNOI, HIROYUKI
Applicants: FUJITSU LIMITED
Application year: 1987
Main classification: H03K19
IPC codes: H03K3, G11C11