CMOS LATCH CIRCUITS

Abstract: A CMOS latch circuit is provided which eliminates the lowering of a high voltage level from a precharge/­ discharge data bus line caused by charge-sharing effect. The CMOS latch circuit is formed of a P-channel precharge transistor (P1), a P-channel drive transistor (P2), an N-­ channel drive transistor (N1), an N-channel enable transistor (N2), and a transmission gate (TG) for loading a complementary data input signal to a storage node (A) in response to true and complementary load signals. The latch circuit further includes output transistor devices formed of a pair of P-channel output transistors (P3, P4) and a pair of N-channel output transistors (N3, N4) which are all connected in series, and are responsive to true and complementary load signals and to true and complementary data output signals for maintaining the latch circuit in one of two states.

Description: This invention relates generally to static latches employed in digital logic systems and more particularly, it relates to an improved CMOS latch for implementation as part of an integrated circuit so as to provide a greater operational reliability and higher speeds of operation. It is generally known that in microprocessor designs storage units are required for temporarily storing digital information. One such type of storage unit is referred to as a static latch. One implementation of such a static latch includes a pair of inverters, a feedback path coupling the output of one inverter, and a CMOS transmission gate. The transmission gate may be formed of an N-channel MOS transistor and a P-channel MOS transistor having their common electrodes (source or drain leads) connected to a precharge/discharge data line and the other one of the common electrodes connected to an inverter's input node. The transmission gate is operable in response to a strobe signal for transferring data from the data line to the input node for latching. One disadvantage of this type of static latch is encountered when a low voltage logic state is stored and a floating high logic state is written which causes a charge-sharing effect between the parasitic capacitances, thereby reducing the high voltage level of the logic state at the input node. Consequently, a problem of reliability in its operation is experienced when the high voltage level at the input node fails to reach the trip point of the latch. The larger the size of the device in the latch that is required to achieve a given performance, the greater is the high voltage level reduction due to this charge-sharing phenomenon. An article in Electronic Design Vol. 33, No. 10, 2nd May 1985, pages 51-54 describes a CMOS latch circuit, as part of a CMOS flash a-d converter cell. An IBM Technical Disclosure Bulletin, Vol. 24, No. 6, November 1981, pages 3039-3040, describes a "FET gated data latch circuit" which may be employed as a RAM cell. It would therefore be desirable to provide an improved CMOS latch which eliminates the lowering of the high voltage level of the data bus line caused by charge-sharing effects. It would also be expedient to provide a CMOS latch which operates at higher speeds so as to reduce logic delays. Accordingly, there is described in the present specification an improved CMOS latch which overcomes the disadvantages of the prior art. The improved CMOS latch operates at higher speeds so as to reduce logic delays to a level less than that which has been traditionally available. The improved CMOS latch also eliminates the lowering of the high voltage level of a data bus line caused by charge-sharing effects. According to the invention there is provided a CMOS latch circuit comprising:- a P-channel precharge transistor having a source connected to a supply potential, a gate, and a drain; a P-channel drive transistor having a source, a gate and a drain; an N-channel drive transistor having a drain connected to the drain of said P-channel drive transistor, a gate, and a source; an N-channel enable transistor having a drain connected to the source of said N-channel drive transistor, a gate, and a source connected to a ground potential; transmission gate means responsive to a true load signal and a complementary load signal, and output transistor means responsive to said true and complementary load signals and to true and complementary data output signals for maintaining said latch circuit in one of two states; characterised in that the P-channel drive transistor has a source connected to the supply potential, a gate connected to the drain of the P-channel pre-charge transistor, and a drain connected to a storage node and a true data output terminal; the gate of the N-channel drive transistor is connected to a first input terminal for receiving a complementary data input signal, the gate of the N-channel enable transistor has a gate connected to a second input terminal for receiving a true load signal, and the transmission gate means connects the complementary data input signal to the gate of the P-channel drive transistor. In the accompanying drawings, by way of example only:- Figure 1 is a schematic circuit diagram of a CMOS latch embodying the present invention; and Figure 2 is a schematic circuit diagram of a second CMOS latch embodying the present invention. Referring now to the drawings, there is shown in Figure 1 a CMOS latch 10. The CMOS latch 10 includes a P-channel precharge transistor P1, a P-channel drive transistor P2, a N-channel drive transistor N1, and a N-channel enable transistor N2. The precharge transistor P1 has its source connected to a supply voltage or potential VCC, which is typically at +5.0 volts for CMOS technology, its gate connected to the gate of the enable transistor N2, and its drain connected to the gate of the drive transistor P2. The drive transistor P2 has its source also connected to the supply potential VCC. The transistor P2 has its drain connected to the drain of the drive transistor N1, to storage node A at an output terminal 12, and to the input of an inverter I. The output of the inverter I is connected to an output terminal 14. The output terminal 12 provides a true data output signal Q, and the output terminal 14 provides a complementary data output signal Q . The drive transistor N1 has its gate connected to an input terminal 16 for receiving a complementary data signal DATA . This data signal DATA is from a precharge/discharge complementary data bus line (not shown) which is normally precharged to a logic "1" state ( DATA =1 or DATA=0) and can be discharged to a logic "0" state ( DATA =0 or DATA=1). The drive transistor N1 has its source connected to the drain of the enable transistor N2. The enable transistor N2 has its gate also connected to an input terminal 18 for receiving a true load signal LOAD and its drain connected to a ground potential. The CMOS latch 10 further includes a conventional CMOS transmission gate TG formed of a P-channel MOS transistor and an N-channel MOS transistor. The transmission gate TG has a signal input line 21, a signal output line 22, a true control line 24, and a complementary control line 26. The signal input line 21 is connected to the complementary data signal DATA via the input terminal 16, and the signal output line 22 is connected to the gate of the drive transistor P2. The true control line 24 is connected to the gate electrode of the N-channel MOS transistor of the gate TG and to the true load signal LOAD via the input terminal 18. The complementary control line 26 is connected to the gate electrode of the P-channel MOS transistor of the gate TG and to a complementary load signal LOAD via an input terminal 20. When the true load signal LOAD is at a high or logic "1" level and the complementary load signal LOAD is at a low or logic "0" level, the complementary data input signal DATA applied to the signal input line 21 will be coupled to the signal output line 22. Thus, the transmission gate TG is defined to be closed or turned on. then the true load signal LOAD is at a low logic level and the complementary load signal LOAD is at a high logic level, the complementary data signal DATA applied to the signal input line 21 will not be coupled to the signal output line 22. Thus, the transmission gate TG is defined to be opened or turned off. The CMOS latch 10 further includes output transistor latch means formed of a pair of P-channel output MOS transistors P3, P4, and a pair of N-channel output MOS transistors N3, N4 all connected in a series arrangement. The transistor P3 has its source connected to the supply potential VCC, its gate connected to the output of the inverter I, and its drain connected to the source of the transistor P4. The transistor P4 has its gate connected to the gate of the transistor N2. The transistor P4 has its drain connected to the drain of the transistor P2, to the drain of the transistor N1 to the output terminal 12, and to the drain of the transistor N3 at the storage node A. The transistor N3 has its gate connected to the input terminal 20 for receiving the complementary load signal LOAD and its source connected to the drain of the transistor N4. The transistor N4 has its gate connected to the output of the inverter I and its source connected to the ground potential. Before the operation of the CMOS latch 10 is explained, it will be helpful to illustrate how the true and complementary data output signals Q and Q at the respective output terminals 12 and 14 will indeed be latched and retain their original states indefinitely when the true load signal LOAD is at a low logic level (LOAD=0) and the complementary load signal LOAD is at a high logic level ( LOAD =1). Initially, it will be assumed that the complementary data input signal DATA is at a high logic level ( DATA =1), the true data output signal Q is at a low logic level (Q=0), and the complementary data output signal Q is at a high logic level ( Q =1). Under these conditions, the transmission gate TG will be turned off; the transistors P1, P4, N1, N3 and N4 will be turned on; and the transistors P2, P3 and N2 will be turned off. Thus, the true data output signal Q will be maintained at a low logic level since the transistors N3 and N4 are both turned on. Due to the inverter I, the complementary data output signal Q will be maintained at a high logic level which keeps the transistor P3 turned off and the transistor N4 turned on. It can be seen that the true and complementary data output signals would still be latched in their original states even if the complementary data input signal DATA is allowed to be discharged to a logic "0" level ( DATA =0). This will only have the effect of turning off the transistor N1, but will not change the states of the true and complementary data output signals. With the true load signal LOAD still being at a low logic level (LOAD=0), it will now be assumed that the complementary data input signal DATA is again at a high logic level ( DATA =1) but that the true data output signal Q is at a high logic level (Q=1) and the complementary data output signal Q is at a low logic level ( Q =0). Under these conditions, the transmission gate TG will again be turned off; the transistors P1, P3, P4, N1 and N3 will be turned on; and the transistors P2, N2 and N4 will be turned off. Thus, the true data output signal Q will be maintained at a high logic level since the output transistors P3 and P4 are both turned on. Due to the inverter I, the complementary data output signal Q will be maintained at a low logic level which keeps the transistor P3 turned on and the transistor N4 turned off. Again, it can be seen that the true and complementary data output signals would still be latched in the original states even if the complementary data input signal DATA is allowed to be discharged to a logic "0" level ( DATA =0). This will again only have the effect of turning off the transistor N1. Therefore, it has been proved that the output transistors P3, P4, N3 and N4 will function as an output transistor latch means to retain the true and complementary data output signals in one of the two original states indefinitely when the transmission gate TG is turned off by the load signals (LOAD=0 and LOAD =1). This will be true regardless of whether the complementary data input signal DATA is maintained at its precharged high logic level or is discharged to a low logic level. In order to explain the operation of the latch, it will now be necessary to assume that the true load signal LOAD is at a high logic level (LOAD=1) and the complementary load signal LOAD is at a low logic level ( LOAD =0). In a first case, it will be further assumed that the complementary data input signal DATA is precharged by the precharge/discharge complementary data bus line to a high logic level ( DATA =1), the true data output signal Q is at a high logic level (Q=1), and the complementary data output signal Q is at a low logic level ( Q =0). Prior to the time when the true load signal LOAD switches to a high logic level, the transmission gate TG will be turned off; the transistors P1, P3, P4, N1 and N3 will be turned on; and the transistors P2, N2 and N4 will be turned off. When the true load signal LOAD switches to the high logic level and the complementary load signal LOAD switches to the low logic level, this causes the transmission gate TG to be turned on; the transistors P1, P4 and N3 to be turned off; and the transistor N2 to be turned on. Since the complementary data input signal DATA is at a high logic level, the transistor N1 will also be turned on so as to pull or force the true data output signal Q to a low logic level. The inverter I will cause the complementary data output signal Q to go to a high logic level which turns off the transistor P3 and turns on the transistor N4. When the true load signal LOAD switches back to the low logic level, the complementary load signal LOAD will again cause the transistor N3 to turn on, thereby maintaining the true data output signal Q in the low logic level. In a second case, it will be further assumed that the complementary data input signal DATA is allowed to be discharged to a low logic level ( DATA =0), the true data output signal Q is at a low logic level (Q=0), and the complementary data output signal Q is at a high logic level ( Q =1). Prior to the time when the true load signal LOAD switches to a high logic level, the transmission gate TG will be turned off; the transistors P1, P4, N1, N3 and N4 will be turned on; and the transistors P2, P3 and N2 will be turned off. When the true load signal LOAD switches to the high logic level and the complementary load signal LOAD switches to the low logic level, this causes the transmission gate TG to be turned on; the transistors P1, P4 and N3 to be turned off; and the transistor N2 to be turned on. The transistor N1 will be initially turned on since the complementary data input signal DATA will be at a high logic level. However, when the complementary data input signal DATA is discharged to the low logic level, this will turn off the transistor N1 and the transistor P2 will be turned on so as to pull or force the true data output signal Q to the high logic level. The inverter I will cause the complementary data output signal Q to go to the low logic level which turns on the transistor P3 and turns off the transistor N4. When the true load signal LOAD switches back to the low logic level, this true load signal LOAD will again turn on the transistor P4, thereby maintaining the true data output signal Q in the high logic level. Further, the transmission gate TG will then be turned off and the transistor P1 will be turned on. As a result, the gate of the transistor P2 will be driven to a high logic level, thereby causing it to be turned off as well. It should be noticed that whenever the transmission gate TG is turned on both its input and output nodes are at a high logic level so as to avoid any charge-sharing effect to the latch circuit regardless of its previous state. In Figure 2, there is depicted an alternate embodiment 10A of the invention differing only in that there is provided a means for loading the latch storage node A at the true data output terminal 12 with an additional or second complementary data input signal DATA2. This is accomplished by the provision of a second data source driver circuit 28 and a feedback driver circuit 30 formed of a P-channel transistor P5 and an N-channel transistor N5. The driver circuit 28 serves to drive the latch storage node A with the second data input signal DATA2 applied to input terminal 31. As can be seen, the transistor P5 has its source-drain conduction path interconnected between the drain of the transistor P3 and the source of the transistor P4, and the transistor N5 has its source-drain conduction path interconnected between the drain of the transistor N4 and the source of the transistor N3. The gate of the transistor P5 is connected to an input terminal 32 for receiving a second true load signal LOAD2, and the gate of the transistor N5 is connected to an input terminal 34 for receiving a second complementary load signal LOAD2 . The transistors P5 and N5 function to disable the drive to the storage node A when the second data signal DATA2 is being loaded. It should be understood to those skilled in the art that only one of the data signals DATA1 or DATA2 is to be loaded at a time. Except for these differences, the functionality and advantages of the present invention remain in this alternate embodiment. From the foregoing detailed description, it can be seen that the present CMOS latch circuit eliminates the charge-sharing effects, which reduces the high voltage level from the complementary data bus line, since the P-channel drive transistor P2 is used to force the true data output signal Q to the high logic level via the supply potential VCC. Further, the RC delay due to the transmission gate TG is minimized since the N-channel drive transistor N2 is being driven directly from the complementary data bus line and thus only the drive transistor P2 contributes a load to the RC across the transmission gate TG. As a result, it is possible to fabricate larger physically the drive transistors P2 and N1 for increased performance without adversely affecting its reliability.

Claims: A CMOS latch circuit comprising:- a P-channel precharge transistor (P1) having a source connected to a supply potential (VCC), a gate, and a drain; a P-channel drive transistor (P2) having a source, a gate and a drain; an N-channel drive transistor (N1) having a drain connected to the drain of said P-channel drive transistor (P2), a gate, and a source; an N-channel enable transistor (N2) having a drain connected to the source of said N-channel drive transistor (N1), a gate, and a source connected to a ground potential; transmission gate means (TG) responsive to a true load signal and a complementary load signal, and output transistor means (P3,P4,N3,N4) responsive to said true and complementary load signals and to true and complementary data output signals for maintaining said latch circuit in one of two states; characterised in that the P-channel drive transistor (P2) has a source connected to the supply potential, a gate connected to the drain of the P-channel pre-charge transistor (P1), and a drain connected to a storage node (A) and a true data output terminal (12); the gate of the N-channel drive transistor (N1) is connected to a first input terminal (16) for receiving a complementary data input signal, the gate of the N-channel enable transistor (N2) has a gate connected to a second input terminal (18) for receiving a true load signal, and the transmission gate means (TG) connects the complementary data input signal to the gate of the P-channel drive transistor (P2). A CMOS latch circuit as claimed in claim 1, wherein the transmission gate means (TG) is formed of a P-channel transistor and an N-channel transistor. A CMOS latch circuit as claimed in claim 1, wherein said output transistor means comprises a pair of P-channel output transistors (P3,P4) and a pair of N-channel output transistors (N3,N4), all being connected in series. A CMOS latch circuit according to claim 1 or 2 wherein the transmission gate means (TG) has a signal input line (21) connected to the first input terminal (16), a signal output line (22) connected to the gate of said P-drive transistor (P2), a true control line (24) coupled to the second input terminal (18), and a complementary control line (26) connected to a third input terminal (20) for receiving the complementary load signal; and the output transistor means comprise: a first P-channel output transistor (P3) having a source connected to the supply potential, a gate connected to a complementary data output terminal (14) and a drain; a second P-channel output transistor (P4) having a source connected to the drain of said first P-channel output transistor (P3), a gate connected to the gate of said N-channel enable transistor (N2), and a drain connected to the storage node (A); a first N-channel output transistor (N3) having a drain connected to the storage node (A), a gate connected to the third input terminal, and a source; a second N-channel output transistor (N4) having a drain connected to the source of said first N-channel output transistor (N3), a gate connected to the complementary data output terminal (14) and a source connected to the ground potential; and an inverter (I) having an input connected to the storage node (A) and an output connected to the complementary data output terminal (14). A CMOS latch circuit as claimed in claim 4, wherein the true control line (24) is connected to the gate electrode of the N-channel transistor of the transmission gate (TG) and the complementary control line (26) is connected to the gate electrode of the P-channel transistor of the transmission gate means (TG). A CMOS latch circuit as claimed in claim 5, wherein the transmission gate means (TG) is turned on when the true load signal is at a high logic level and the complementary load signal is at a low logic level. A CMOS latch circuit as claimed in claim 6, wherein the complementary data input signal is normally precharged to provide a high logic level and is discharged to provide a low logic level. A CMOS latch circuit as claimed in claim 7, wherein when the true data output signal is at a high logic level and the complementary data input signal is at a high logic level the N-channel drive transistor (N1) will be turned on so as to pull the true data output signal to a low logic level. A CMOS latch circuit as claimed in claim 7, wherein when the true data output signal is at a low logic level and the complementary data input signal is at a low logic level the P-channel drive transistor (P2) will be turned on so as to pull the true data output signal to a high logic level. A CMOS latch circuit as claimed in claim 4, further comprising means for loading the storage node (A) with a second complementary data input signal. A CMOS latch circuit as claimed in claim 10, wherein the loading means comprises driver circuit means (28) coupled to the storage node (A) for driving the true data output terminal (12) with a second data input signal, and feedback driver means (30) coupled to the storage node (A) and being responsive to second true and complementary load signals for disabling of its drive when said second complementary data input signal is being loaded. A CMOS latch circuit as claimed in claim 11, wherein the feedback driver means comprises a P-channel transistor (P5) having its source-drain conduction patch interconnected between the drain of the first P-channel output transistor (P3) and the source of the second P-channel output transistor (P4), and an N-channel transistor (N5) having its source-drain conduction path interconnected between the drain of the second N-channel output transistor (N4) and the source of the N-channel output transistor (N3), the gate of the transistor (P5) being connected to the second true load signal, the gate of the transistor (N5) being connected to receive said second load signal.

Inventors: WALTERS, DONALD MONROE, JR.
Applicants: ADVANCED MICRO DEVICES, INC.
Application year: 1988
Main classification: H03K3
IPC codes: H03K3