Spike filtering circuit for logic signals

Abstract: A spike filtering circuit for a logic signal comprises a signal trasfer circuit formed by a first transfer (TG₁) gate followed by a pair of inverters (INV1,INV2) , functionally connected in series between the input terminal and the output terminal of the circuit and a second transfer gate (TG₂) connected between the output terminal and the input node of the first of said two inverters (INV1) . The two transfer gates (TG₁,TG₂) are driven in phase opposition to each other by means of a pair of control signals (CK, CK ) in phase opposition to each other which are generated by a control circuit functioning in a feedback mode. Basically the control circuit is formed by an exclusive-OR gate (XOR) having two inputs connected to the output terminal of the circuit directly and through a delay network (d) , respectively. Through an output node of the exlusive-OR gate (XOR) is produced a first control signal (2) from which the pair of control signals (CK, CK ) in phase opposition to each other are derived by means of inverting stages. The delay network (d) introduces a delay after a transition of the signal on the output terminal of the circuit has occurred during which said first transfer gate (TG₁) is momentarily disabled and said second transfer gate (TG₂) is enabled in order to maintain on the output terminal the state reached with the first transition for a period of time sufficiently long to allow the decay of spikes which may have been generated by said transition of the logic signal. By employing a NAND gate (NAND) and an inverter (INV3) connected in cascade to the output of said exclusive-OR gate (XOR) , the filtering circuit may be initialled by applying an enabling signal to a second input of said NAND gate.

Description: 1. Field of the invention The present invention relates to a filtering circuit for a logic signal affected by spikes generated in consequence of the switching of the signal from a logic level to another. 2. Description of the prior art Within integrated circuits comprising logic circuitry sections or in circuits operating in a switching (on/off) mode, it is fundamental to make the system immune from spikes and/or spurious oscillations in order to prevent improper switchings or a system's latch, the effect of which could be catastrophic. On the other hand, especially in power circuits comprising logic circuitry sections, the switchings are often accompanied by fast chargings and dischargings of reactances and therefore by the generation of spikes. Several solutions are known for avoiding the undesired effects of these spikes, basically resting upon the creation of a "masking time" for the spikes, that is of a preestablished period of time following each switching during which the system is made temporarily insensitive to the input signal (containing spikes). The prior document US-A-4,138,613 discloses a switching circuit for producing a logic output signal free of chattering (switching noise) according to this approach. These known "filtering" circuits generally utilize SET-RESET type flip-flops or similar devices, wherein a certain input terminal is sensitive to one only of the two possible switching fronts (i.e. from low-to-high level or vice-versa) so that after a transition as occurred, the output terminal state may change only through an enabling intervention through a different input terminal to which a suitable clock signal may be fed. In general it is necessary to use a plurality of flip-flops and delay blocks for solving the problem, with a remarkable circuit complexity and synchronization is notably critical because an erroneous implementation may lead not only to miss the expected result, but to a nonrecoverable latching condition of the system. Known spike filtering circuits are rather complex and the trimming thereof is critical. The temporary storing of a logic value is a function that is frequently implemented in logic circuitry by the use of so-called latches. CMOS logic circuits offer an intrinsically low power dissipation characteristics, in view of the fact that current is absorbed from the supply rails only at switchings. In order to further reduce absorption of current, it is important to reduce the leakages of current at switchings. The document EP-A-0206462 discloses a method and relative circuit for precharging the nodes of a dynamic CMOS logic circuit, implementing a so-called cascode voltage switch (CVS) that reduces susceptibility to internal noise if compared with the prior art feedback arrangements implemented in these cascode voltage switcting circuits. The method consists in isolating the output from the precharging voltage by means of a pass transistor connected between the logic gate and the output inverter. SUMMARY OF THE INVENTION It has been found now that the filtering of spikes which may be present in a logic signal as a consequence of transitions of the signal from a level to another level may be effectively performed by means of a filtering circuit far simpler than the known circuits and employing a limited number of components while being practically immune from unrecoverable latching phenomena. The improved filtering circuit is inherently fast because it employs a limited number of logic gates and the trimming thereof does not have any substantially critical aspect as compared to the known circuits. The filtering circuit of the invention comprises a signal transfer circuit that connects an output terminal to the input terminal of the filter circuit. A transition detector senses when the logic level of the output signal to the filter circuit changes state. The transition detector then opens the transfer circuit for a predetermined period so that spikes on the input signal cannot reach the output terminal of the filter circuit. At the same time, the transition detector also maintains the output terminal at the logic level it was in after the detected transition for the predetermined period. The predetermined period has a duration that is sufficiently long to allow decay of the spikes occurring after the detected transition. After the expiration of the predetermined period, the transfer circuit once again connects the output terminal to the input terminal. In one embodiment of the invention, the transfer circuit is formed by a first transfer gate driven by a first control signal and an inverted replica signal thereof, operatively connected between an input terminal of the filtering circuit and the input node of a first of two inverters connected in cascade. The output of the second inverter is connected to an output terminal of the filtering circuit. A second transfer gate driven in phase opposition to the first transfer gate by the same pair of control signals is operatively connected between the output terminal of the circuit and the input node of the first of said two inverters connected in cascade. The pair of control signals for the two trasfer gates is generated by a control circuit, functioning in a feedback mode, which advantageously may be initialled by an enabling signal for generating said pair of control signals. These control signals are characterized by having a first front which coincides with the switching instant from a level to another level of the logic signal to be filtered and by a second front of opposite direction in respect to the first front, occurring after a preestablished delay after the first front. The delay is sufficiently long to ensure that the switching spikes on the logic signal have decayed. The control signal and the respective inverted replica signal thereof enable the first transfer gate until a transition has occurred on the output terminal of the circuit. This latter event disables the first transfer gate and enables the second transfer gate in order to maintain on the output terminal the level reached after the preceding switching for such a preestablished delay time. At the end of the preestablished delay time, the pair of control signals switch and enable again the first transfer gate and disable the second trasfer gate until another switching of the logic signal occurs. BRIEF DESCRIPTION OF THE DRAWINGS The different aspects and advantages of the circuit of the invention will be more evident through the following description of a preferred embodiment and by reference to the attached drawings, wherein: Figure 1 is a logic diagram of one embodiment of the filtering circuit of the invention; Figure 2 is a diagram of a delay network used in the circuit of Fig. 1; Figure 3 is a diagram of an alternative form of the delay network used in the circuit of Fig. 1; and Figure 4 are timing diagrams representative of the various signals of the circuit of Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to the circuit depicted in Fig. 1, the logic signal to be filtered is fed to the input terminal (INPUT) of the filtering circuit and a replica signal, free of spikes which may have been generated as a consequence of a transition (switching) of the logic signal from a level to another level, is delivered through the output terminal (OUTPUT) of the circuit. The transfer path of the logic signal is evidenced in the figure by using a thick line for wirings. The transfer path of the logic signal through the filtering circuit comprises a first transfer gate TG 1 , functionally connected between the input terminal and an intermediate input node of the first of two inverters, INV1 and INV2 , connected in cascade between said intermediate node and the output terminal of the circuit and a second transfer gate TG 2 , which is connected between the output terminal of the circuit and the same intermediate input node. The transfer gates are familiar devices for a skilled technician, which commonly are formed by a pair of complementary transistors, typically a p-channel and an n-channel transistor, functionally connected in parallel between a common input node and a common output node. The two transistors are driven through their respective control terminals by a pair of control signals, one of which is the inverted replica of the other. For a certain logic state of the pair of control signals, the transfer gate represents a closed switch while by inverting the state of the pair of control signals, the transfer gate behaves as an open switch. The control circuit for generating said pair of control signals comprises essentially a delay network d and an exclusive-OR gate XOR and most preferably also a NAND gate and an inverter INV3 , as depicted in Fig. 1. The delay network d may be realized with any suitable circuit means. In Fig. 2 an embodiment utilizing a common RC network is shown, while in Fig. 3 an alternative form of the delay network is shown, wherein a chain of inverters, connected in cascade, is used for determining a overall delay of propagation as needed. The XOR gate delivers to an output node 2 a logic low signal ( ⊘ ) when the signals applied to the respective input nodes of the XOR gate are at the same level or a logic high signal (1) when the signals applied to the input nodes are not at the same logic level. Basically the signal produced by the XOR gate on its output node represents one of the two control signals, the inverted replica of which may be easily derived therefrom by employing a suitable inverting stage. Most preferably however, the pair of control signals are both derived from the signal produced by the XOR gate by further employing a NAND gate between the XOR gate and the inverter INV3 . In this way it becomes possible to initial the filtering circuit by means of an enable signal (START), which is applied to a second input of the NAND gate. The circuit operates as follows. The logic signal which is fed to the input terminal INPUT of the circuit is assumed to be affected by spikes, as depicted in the first diagram of Fig. 4 . The enable signal START provides to initialize the filtering circuit by imposing a control signal CK = ⊘ (low) and of course also an inverted control signal CK = 1 (high). Therefore the first transfer gate TG 1 is enabled and lets the signal through, while the second transfer gate TG 2 , which is driven in phase opposition in respect to the first transfer gate TG 1 , is disabled. Therefore the input signal, after going through two inversions which leave it unaltered, passes to the output terminal OUTPUT . Such a first switching (from high to low) of the input signal, and the transfer of the signal to the output terminal, has an effect on the control circuit because the delay block d delays the switching from high to low of the signal on the wire 1 , depicted in the third diagram of Fig. 4 , which is applied to one of the two input nodes of the XOR gate and this causes the delivery of a logic high signal through the output of the XOR gate on the wire 2 , depicted in the fourth diagram of Fig. 4 . The signal on the wire 2 of the control circuit represents a first control signal ( CK ) generated by the control circuit. Therefore for a delay time ( d ) after the initial transition of the output determined by the delay network d , the pair of control signals derived from said first control signal becomes: CK = ⊘ and CK = 1 . Therefore the transfer gate TG 1 becomes an open switch while the transfer gate TG 2 , which is driven in phase opposition in respect to the first transfer gate, becomes a closed switch . This together with the presence of the two inverters INV1 and INV2 ensures the maintenance on the output terminal ( OUTPUT ) of the circuit of the logic level reached after the first transition, as shown by the relative diagram of the output signal in Fig. 4 . It is evident that the spikes which follow the first switching front, within the interval of time ( d ), cannot pass through to the output terminal of the filtering circuit. At the end of the interval of time ( d ), which is established by the delay network d of the circuit of Fig. 1 , the starting condition is reset and the signal present at the input terminal is again directly transferred to the output terminal until a new switching occurs, when the cycle of generation of the pair of control signals CK and CK is repeated.

Claims: A method of filtering spikes contained in a logic signal as a consequence of the changes from a logic level to another of said signal, comprising feeding said spike containing signal to an input of a filtering circuit and recovering a signal free of spikes from an output of the circuit, characterized by the fact that it uses a transfer gate for disconnecting said output terminal from said input terminal for a predetermined period of time after a first transition of said signal has switched the level of the output to a new logic level; and maintains the level of the output at said new logic level during said period of time of disconnection; said period of time of disconnection being sufficiently long for said spikes to decay. A filtering circuit for a logic signal having an input terminal to which said signal to be filtered is fed and an output terminal through which said logic signal is delivered free of spikes consequent to a switching of said logic signal from a logic level to another, which comprises a signal transfer circuit formed by a first transfer gate (TG1) , driven by a pair of control signals (CK, CK!) in phase opposition to each other, functionally connected between said input terminal and an input node of a first of two inverters (INV1, INV2) connected in cascade between said first transfer gate (TG1) and said output terminal, and a second transfer gate (TG2), driven in phase opposition to said first transfer gate (TG1) by means of the same pair of control signals (CK, CK!), functionally connected between said output terminal and the input node of said first inverter (INV1); a control circuit (d, XOR, NAND, INV3), capable of generating said pair of control signals (CK, CK!) each having a first front coincident with the switching instant of said logic signal from a level to another level and a second resetting front occurring after a pre-established delay (d) from the occurrence of said first front sufficiently long for said spikes to decay; said pair of control signals (CK, CK!) enabling said first transfer gate (TG1) until the signal on said output terminal undergoes a transition from one level to another, which event causes a switching of said control signals (CK, CK!) which disable said first transfer gate (TG1) and enable said second transfer gate (TG2) for maintaining at the output terminal the logic level reached after said first transition for said preestablished delay (d) before enabling again at the end of said delay time (d) said first transfer gate (TG1) and disabling again said second transfer gate (TG2) until a new switching of said logic signal occurs. The filtering circuit according to claim 2, wherein said control circuit operating in a feedback mode for generating said pair of control signals (CK, CK!) is formed by an exclusive-OR gate having two inputs which are connected to said output terminal directly and through a delay network (d), respectively, and an output through which a first control signal (2) is produced; said pair of control signals (CK, CK!) being derived from said first control signal (2) by means of at least one inverting stage. The filtering circuit according to claim 3, wherein said first control signal (2) is fed to an input of a NAND gate having a second input to which an enabling signal (START) is applied for initialling the circuit; the signal (CK) produced at an output of said NAND gate and fed to the input of a third inverter (INV3), together with the signal produced at the output of said third inverter (CK!), constituting said pair of control signals. The circuit according to claim 2, which comprises an RC network for determining said delay (d). The circuit according to claim 2, wherein said delay network is formed by a chain of inverters connected in cascade.

Inventors: ALZATI ANGELO, BAIOCCHI ANTONELLA, NOVELLI ALDO, ALZATI, ANGELO, BAIOCCHI, ANTONELLA, NOVELLI, ALDO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1991
Main classification: H03K5
IPC codes: H03K5