An initialization circuit for memory registers

Abstract: An initialization circuit (1), particularly for memory registers (2), being of a type which comprises a signal input (I) to which a supply voltage (Vp) is applied, and an initialization output (P1) at which a voltage signal (V1) is produced which is equal to the supply voltage up to a predetermined tripping value (Vs) for the circuit, further comprises a second output (P2) connected to the register (2) and being also an initialization output driven to a null voltage value upon the supply voltage dropping below the tripping value (Vs).

Description: Technical field This invention relates to an initialization circuit for memory registers, being of a type comprising a signal input terminal, for receiving a supply voltage increasing from a null voltage upon initialization, and a first circuit portion connected to the signal input terminal for producing on a first initialization output a first initialization voltage signal following the supply voltage until a predetermined tripping value is attained for the circuit and then falling to ground and being pulsed when the supply voltage drops below the tripping value. Background of the Invention As is known, many electronic circuits depend for proper configuration on the reception of an initialization or enable signal which will bring the circuit to its design operating condition, when combined with an appropriate voltage supply. The initialization signal may be provided by a purposely arranged circuit. In general, an initialization circuit produces a signal which follows the rise in the supply voltage up to a predetermined value, then drops to null voltage as that value is exceeded. The threshold value for this change is referred to as the tripping voltage of the initialization circuit, and it matters that the value of this tripping voltage be appropriate for the design of the circuit to be driven to the "on" state. It is a well-recognized fact, for instance, that analog circuits generally require higher initialization voltage values than digital circuits. Further, the tripping voltage should be at all times a value that will properly set all the circuits affected by the initialization. With integrated circuits of the MOS or CMOS types, that voltage is also tied to the transistor thresholds. It should be further observed that most initialization circuits become automatically operative in those situations where the supply voltage drops to a level below the tripping voltage and no longer ensures proper operation of the circuit networks. The circuit network would also have to be re-initialized when the supply voltage is restored to the steady-state value. However, with some electronic devices, this mode of operation of the initialization circuits may be an untoward one. Let us consider the instance of circuit networks making up memory registers programmable by a microprocessor unit. With many registers, a requisite is that their configurations upon turning on meet certain specifications, commonly referred to as the default values, which are designed to set a circuit network for operation in its normal mode. Accordingly, the initialization circuit should be capable of activating the memory register in the default state provided for by the specifications. It happens, however, that upon the supply voltage attaining the steady-state value, the contents of the memory register may undergo changes, and if significant surges contemporaneously occurred in the power supply, the initialization circuit would reconfigure the memory register so as to re-establish the default state, as explained above. Thus, what is beneficial to standard circuits to be initialized would instead harm memory registers for which restoration to the default state represents a stable error that is retained until the next write operation by the program unit. For example, in the European patent application No. 0342735 a circuit for generating a pulse-shaped signal is disclosed. Such a circuit may be used to produce a reset signal for bistable circuits and memory cells when the supply voltage shortly decreases below a predetermined threshold level. In another European application No. 0096531 it is provided a reset signal inhibiting circuit which inhibits the transmission of a reset signal RST, generated in response to the drop of the power supply below a predetermined level, during the check mode. However, in both cases, it may happen that memory cells and registers may keep the information stored therein even when the supply voltage drops under a much lower value. As a practical example, it may suffice to consider that an elementary memory cell implemented in CMOS technology and operated at 5 volts is apt to retain the logic value which has been stored therein, even at very low supply values of less than 1 volt. This feature is retained over relatively long time periods and even on the occurrence of sharp supply voltage drops due to abrupt perturbation and/or having very short duration. Therefore, should the initialization circuit associated with the memory become operative under such conditions, the value contained therein would be lost even if the memory as such would be normally tolerant of the momentary perturbation. Summary of the Invention The technical problem underlying this invention is to provide an initialization circuit, particularly intended for memory registers, which has such structural and functional features as to overcome the aforesaid drawback of prior art circuits. This problem is solved by a circuit as previously indicated an further defined in the characterizing portion of Claim 1. The features and advantages of a circuit according to the invention will become apparent from the following detailed description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. Brief Description of the Drawings Figure 1 shows the circuit of the invention in diagram form. Figures 2 through 8 show schematically respective waveforms of signals having the same time base and appearing in the circuit of Figure 1. Detailed Description of the Invention With reference to the drawing figures, generally and schematically shown at 1 is an initialization circuit embodying this invention and being particularly, but not solely, intended for memory registers 2. The circuit 1 has an input terminal I to which a supply voltage Vp is applied. The circuit 1 comprises a pair of N-channel MOS transistors MN1, MN2 which are connected serially between a circuit node A and ground. Both transistors MN1, MN2 have their gate terminals G1, G2 connected to their corresponding drain terminals D1, D2. Advantageously, the width W and length L of the channel region have been selected for the transistors MN1 and MN2 to be identical and given by W=100≤ and L=6≤, thereby to provide a low impedance. The node A is coincident with the drain terminal D1 of the first transistor MN1 which is connected to the positive voltage supply pole Vp through an RC circuit consisting of a 350 kΩ resistor R1 and a 2 pF capacitor C1 in parallel. Said node A is also connected to the gate terminal G3 of a third transistor MP1, also a MOS type but of the P-channel variety, which has its source terminal S3 connected to the supply pole Vp and its drain terminal D3 connected to ground through an RC circuit consisting of a 350 kΩ resistor R2 and a 2 pF capacitor C2 in parallel. The channel region of transistor MP1 has the following dimensions: W-5Oµ' and L=8µ; and the drain D3 of that transistor forms a second circuit node denoted by B. The circuit 1 of this invention is provided with a pair of terminals P1 and P2 constituting initialization outputs, of which the former, P1, is connected to a circuit network 5 and the latter, P2, is connected to the memory register 2. Between the circuit node B and the output P1 there are three cascade-connected inverters I1, I2 and I3. In addition, said second node B is connected to the gate terminal G4 of a fourth N-channel MOS transistor MN3 having W=30µ and L=3µ and its source S4 connected to ground. A third circuit node C consists of the drain D4 of the fourth transistor MN3 and is connected to the second output P2 through a pair of inverters I4, I5 in series with each other. The circuit 1 construction is completed by further MOS transistors. Specifically, a first pair 3 of P-channel and N-channel transistors, respectively MP2 and MN4, are provided which are connected together into an inverter configuration. Transistor MP2 has dimensions given by W=12µ and L=1.5µ, while the other transistor, MN4, has W=2µ and L=18µ. A second pair 4 of transistors MP3, MN5, also connected into an inverter configuration, with the former being a P-channel type and the latter an N-channel type, are connected in the circuit 1 by a feedback cross-connection to the first pair 3. In essence, the gate terminals of the first pair 3 are connected to a node D, the point of drain-to-drain contact of the second pair 4. Conversely, the gate terminals of the second pair 4 are connected to the point of drain-to-drain contact of the first pair 3 as well as to the third circuit node C. The transistors MP3 and MN5 of the second pair 4 are respectively characterized by the following dimensional parameters: W=2µ and L=6µ for MP3; W=4µ and L=1.5µ for MN5. It is evinced from the above that the dimensions of the transistors MP3 and MN5 in the second pair 4 are effective to make the P channel much more resistive than the N channel, whereby a low tripping voltage is provided for the inverter. Conversely, the first transistor pair 3 will have the P channel much more conductive than the N channel, which imparts a high tripping voltage to the inverter. The operation of the circuit 1 according to the invention will now be described with reference to Figures 2 to 8, which show waveforms on a common base of voltage signals present in this circuit. As the power supply voltage Vp rises toward the steady state value (Figure 2), the voltage Va at the node A also rises up to a value, at time t2 (as shown in Figure 3), which is the sum of the threshold voltages Vt of transistors MN1 and MN2 of Figure 1. Since the resistance of R1 is quite high, a further small increment δN in voltage will establish at the node A a stable voltage value Vn=2Vt+δN, and this even if the power supply voltage, Vp, continues to rise. Referring to Figure 1, the third transistor MP1 is held off until the voltage between the pole Vp and the gate G3 attains the conduction threshold Vh for that transistor; at this time, a small voltage increment δP will also impart an impedance to the P channel of the third transistor which is negligible compared to that of resistor R2. Thus, before the voltage supply Vp attains the tripping threshold Vs, given by the following equation: Vs=Vn+Vh+δP=2Vt+δN+Vh+δP, the voltage at node B will stay null and the voltage output of the inverter I1 will rise. The voltage at the output of inverter I1 will eventually change to null value on the tripping voltage Vs being exceeded. This situation is illustrated by Figure 5, wherein V1 denotes the voltage value at the circuit output P1, which value substantially corresponds to the output from the inverter I1, since the function served by the other inverters I2, I3 is merely one of speeding up the change-over edge on attainment of the threshold Vs for tripping. As a result, so long as the power supply is held below the tripping voltage Vs, the output P1 of the circuit 1 will keep initializing the circuit 5 coupled thereto. It matters to observe here that the circuit 1 retains the same behavior over time, and accordingly, should the supply voltage Vp drop once again below the tripping threshold Vs, the signal V1 at the output P1 would act to re-initialize circuits 5, as shown in Figure 5. Within a context such as this, the provision of the capacitors C1 and C2 ensures for the circuit 1 ready response features to surges in the power supply. The voltage level changes of the other nodes C and D of the circuit 1 will now be discussed. As previously mentioned, the inverter pairs 3 and 4 are unbalanced, they having different tripping voltages. This implies that the feedback connection of such inverters 3 and 4 prevents the nodes C and D from attaining the same voltage value at the end of the transient. That is, starting from an initial state wherein both nodes C and D are at null voltage, the circuit will evolve to bring the voltage Vc at node C to the supply potential Vp and the voltage Vd at node D to ground potential (Figures 6 and 7). The imbalance between the inverters 3 and 4 is apt to persist until the fourth transistor MN3 goes into conduction, which would occur at time t3 when the voltage Vb at the second node B attains the tripping value Vs. Under this condition, the situation on the nodes C and D is reversed, and whereas the third node C is driven to ground, the voltage Vd at the node D rises sharply toward the supply value Vp. The voltage V2 present at the circuit output P2 and shown in Figure 8 substantially corresponds to the voltage Vc at the third node C because the inverters 14 and 15 only act to speed up the down edge of the signal Vc at node C. It can, therefore, be appreciated from the foregoing discussion that as the supply voltage Vp drops below the tripping threshold Vs, the third node C is held at ground potential by the presence of the supply voltage Vp at the other node D. Thus, the signal V2 at the second output of the circuit 1 will be held at zero volts. This signal V2 will only become operative again in case the power supply is cut off and then restored. Advantageously, the provision of the resistors R1 and R2 in the circuit 1 causes the voltage at the second node B to be held effectively null until time t3, when the power supply reaches the tripping threshold Vs. Thus, the initialization circuit 1 of this invention affords a major advantage in that it can either operate conventionally, by virtue of the output P1 reinitializing the circuit networks connected to it even when the power supply drops below the threshold Vs, or in a mode effective to initialize the memory registers, through the output P2. The time evolution of the signal at the second output P2 sustains the memory circuit programs and only performs the appropriate initialization when this becomes actually necessary. The inventive circuit may be implemented in integrated form, in which case the resistors R1 and R2 would be provided in the N wells or P wells beneath the layers of any metallization path provided.

Claims: An initialization circuit for memory registers, being of a type comprising a signal input (I) terminal for receiving a supply voltage (Vp) increasing from a null voltage upon initialization, and a first circuit portion connected to the signal input terminal for producing on a first initialization output (P1) a first initialization voltage signal (V1) following the supply voltage until a predetermined tripping value (Vs) is attained for the circuit and then falling to ground and being pulsed when the supply voltage drops below the tripping value (Vs), characterized in that it comprises a second circuit portion connected to the first circuit portion for producing on a second initialization output (P2), adapted to be connected to said memory registers (2), a second initialization voltage (V2) following the supply voltage until the supply voltage attains the tripping value (Vs) and then turning to a stable null voltage value which is maintained even when said supply voltage drops below the tripping value (Vs). A circuit according to Claim 1, characterized in that said second circuit portion comprises, between said input (I) and said second output (P2), a pair of inverters (3,4) in cross-connection feedback relationship with each other. A circuit according to Claim 2, characterized in that said inverters (3,4) comprise each a pair of MOS transistors (MP2,MN4;MP3,MN5) of the P-channel and N-channel types, respectively, a drain terminal of each P-channel transistor being connected to a drain terminal of its corresponding N-channel transistor with the gate terminals of the first pair (3) being connected to the point of drain-to-drain contact of the second pair (4), and vice versa. A circuit according to Claim 2, characterized in that said inverters (3,4) are unbalanced, having different tripping voltages. A circuit according to Claim 1, characterized in that, provided between said signal input (I) and said first output (P1), there are:a pair of MOS transistors (MN1,MN2) connected in series between the gate terminal (G3) of a third transistor (MP1) and ground,a first RC circuit (R1,C1) connected in parallel between said gate (G3) and said input (I), a second RC circuit (R2,C2) connected in parallel between the drain (D3) of the third transistor (MP1) and ground, andat least one inverter (I1) connected between said drain (D3) and said first output (P1). A circuit according to Claim 5, characterized in that a series of three inverters (I1,I2,I3) are connected between said drain (D3) and said first output (P1). A circuit according to Claim 5, characterized in that the drain (D3) of said third transistor (MP1) is also connected to the gate (G4) of a fourth transistor (MN3) having its source terminal (S4) connected to ground and its drain terminal (D4) connected to the second output (P2) of the circuit (1) through at least one inverter (I4). A circuit according to Claim 5, characterized in that said third transistor is a P-channel MOS type. A circuit according to Claim 6, characterized in that said fourth transistor is an N-channel MOS type. A circuit according to Claim 7, characterized in that two serial inverters (I4,I5) are connected between the drain terminal (D4) of the fourth transistor (MN3) and said second output (P2).

Inventors: CONFALONIERI PIERANGELO, CONFALONIERI, PIERANGELO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1992
Main classification: G11C7
IPC codes: G11C7, G06F1, H03K3, H03K17, G06F12