Abstract:
A circuit and method reduces the number of nodes that must be forced during a standby mode when using clocked latches. The circuit and method can be used for half-cycle latches and full cycle latches in conjunction with alternate power-gated circuitry, even when many stages are cascaded in a pipeline structure. The data state on a single forcing node can be passed through one or more cascaded latch stages as well as through additional circuitry. By forcing latch transmission gates to be conductive during standby mode, multiple stages can be set to a specific state, as determined by an earlier stage being set by a forcing transistor. A clock generation. circuit and method is also provided for controlling transmission gates within the latches.

Description:
BACKGROUND 
     An embodiment of the present invention relates to integrated circuits, and more particularly, to a circuit and method for implementing a standby power-down state in the integrated circuit. 
     Referring now to FIG. 1, a circuit  10  is shown that can be used to power-gate serially-coupled CMOS inverters U 1 , U 2 , U 3 , U 4 , and U 5  to reduce standby current due to subthreshold leakage. Every other inverter stage U 1 -U 5  uses power-gating devices M 5  and M 6  to coupled the power terminals of the inverter stages to either VCC or VSS. However, the alternate power-gating method of operating circuit  10  requires that the input and output node voltages for each inverter stage be forced to a specific state for proper functionality. If the wrong data state is used, this technique will not work since the alternate power supply terminals are turned off. In FIG. 1, transistor M 3  turns on and forces a low data state at the input node of inverter U 1 , which results in all subsequent stages U 2 -U 5  going to the necessary standby levels. Transistor M 4  is used to avoid contention when transistor M 3  is on during standby. Transistor M 5  is the positive VCC power supply gating device and transistor M 6  is the ground or negative VSS power gating device. The NPG control node is high (goes to VCC or slightly higher than VCC) when circuit  10  is in non-standby mode and goes low when circuit  10  is in standby mode. PPG is low when circuit  10  is in non-standby mode and goes high when circuit  10  is in standby mode. 
     Asynchronous (ripple-through) logic, as illustrated in circuit  10  of FIG. 1, is relatively easy to use to enable standby states by forcing an early stage node (usually an input stage). However, with synchronous logic, especially where clocked flip-flops or pipelined stages are used, multiple points must be forced when power-gating during low power standby. 
     Referring now to FIG. 2, circuits  20  and  22  are operated such that nodes within a latch stage are forced to specific logic states during standby mode. The method of operation for circuits  20  and  22  is relatively complicated since it uses a NAND gate U 1  in circuit  20  or a NOR gate U 4  in circuit  22  as the feedback element and requires an additional transmission gate (M 3 , M 4  is circuit  20  and M 7 , M 8  in circuit  22 ) in the feedback path. 
     Circuits  30 , 40 , and  50  is corresponding FIGS. 3,  4 , and  5  show an obvious method for setting logic states within latch stages. Each of circuits  30 ,  40 , and  50  addresses how to force the stages based on different conditions of the clock during a low power standby mode. In circuits  30 ,  40 , and  50 , one or more “forcing” transistors must be added to set node states where the clock blocks signal propagation from a previous stage. In circuit  30  forcing transistors M 7  and M 11  are shown. In circuit  40  forcing transistors M 7  and M 12  are shown. In circuit  50  forcing transistors M 7 , M 12 , and M 11  are shown. Adding two or more forcing transistors as shown in FIGS. 3-5 may not be a problem in some cases. In other cases, however, where latches are placed within a tight physical pitch or when many latches are clocked in parallel, such as for input data latches for embedded memories, it is desirable to minimize unnecessary transistors. 
     What is desired, therefore, is a circuit and method of operation for implementing a low power supply standby mode in an integrated circuit that uses a single forcing transistor, yet is able to work with synchronous signal paths including latch circuits as well as other logic circuits. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a signal path in an integrated circuit includes a single forcing transistor coupled to an initial portion of the signal path, at least one transmission gate in the signal path, a clock circuit for forcing the clock inputs of the transmission gate to be conductive during a standby mode, and additional alternate power-gated circuitry in the signal path, wherein a forced data state generated by the single forcing transistor propagates through the entire signal path. The circuit of the present invention works with synchronous circuits including full cycle latches, half cycle latches, as well as combinatorial logic circuits, or combinations thereof. 
     In a specific embodiment of the invention, a full cycle latch includes a forcing transistor having a gate for receiving a control signal and a drain for receiving an input signal, a first transmission gate having an input for receiving the input signal, a first latch stage having an input coupled to the output of the first transmission gate, a second transmission gate having an input coupled to the output of the first latch stage, a second latch stage having an input coupled to the output of the second transmission gate and an output for providing an output signal, and a clock circuit for forcing the first and second transmission gates to be conductive during a standby mode of operation. 
     This circuit of the present invention is operated according to a method that reduces the number of nodes that must be forced during a standby mode when using clocked latches. The circuit and method of the present invention can be used with is half-cycle and full cycle latches, even when many stages are cascaded in a pipeline structure. 
     The circuit of the present invention modifies the clock signals to the transmission gates in a synchronous latch circuit so that a single forcing node data state can be passed through one or more cascaded latch stages. 
     Other features, objects, and advantages of the present invention are apparent from the accompanying drawings and from the detailed description that follows below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements, and in which: 
     FIGS. 1-5 are schematic diagrams of prior art power-gating circuitry for implementing a power-down or standby mode of operation; 
     FIG. 6 is a schematic diagram of a full cycle latch according to the present invention; 
     FIG. 7 is a schematic diagram of a clock circuit for use with the full cycle latch of FIG. 6; 
     FIG. 8 is a schematic diagram of a half cycle latch according to the present invention; and 
     FIG. 9 is a schematic diagram of a clock circuit for use with the half cycle latch of FIG.  8 . 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 6, a full cycle synchronous latch circuit  60  is shown. Separate clocks (CLK&lt; 0 : 1 &gt; and CLKB&lt; 0 : 1 &gt;) are used for each latched stage. During normal operation (non-standby), CLK&lt; 0 &gt; and CLK&lt; 1 &gt; are 180 degrees out of phase. However, when in standby mode, CLK&lt; 0 &gt; and CLK&lt; 1 &gt; are both low, which enables both transmission gates (including N-channel transistor M 3  and P-channel transistor M 4  in a first transmission gate and P-channel transistor M 5  and N-channel transistor M 6  in a second transmission gate) to allow a forced data state, generated by transistor M 7 , under command of the PWD control signal, to propagate through the latch stages. A first latch stage includes cross-coupled inverters U 1  and U 3 . A second latch stage includes cross-coupled inverters U 2  and U 4 . Note that the first and second latch stages each include power-gated VCC and VSS power supply terminals, as well as non-gated VCC and ground power supply terminals. By forcing the clock inputs of the first and second transmission gates to be conductive during the standby mode, multiple stages can be set to a data specific state. The initial data state is determined by an earlier stage being set to that state by a single forcing transistor M 7 . 
     Referring now to FIG. 7, a clock circuit  70  is shown for generating the clock signals used in both the normal operating mode and the standby mode for the full cycle latch circuit  60  shown in FIG.  6 . Clock circuit  70  includes a CLK input for receiving a system clock signal, as well as a PWDB power-down control signal. Clock circuit  70  also has four outputs CLK&lt; 0 &gt;, CLKB&lt; 0 &gt;, CLK&lt; 1 &gt;, and CLKB&lt; 1 &gt; for generating the four separate clock signals required for latch  60 . A first NAND gate U 1  receives the CLK and PWDB signal and generates the CLKB&lt; 0 &gt; output. A second NAND gate U 3  is coupled to the output of gate U 1  and also receives the PWDB signal. NAND gate U 3  also generates the CLKB&lt; 1 &gt; clock signal. A first inverter U 2  is coupled to the output of NAND gate U 1  and generates the CLK&lt; 0 &gt; signal. A second inverter U 4  is coupled to the output of NAND gate U 3  and generates the CLK&lt; 1 &gt; signal. In operation, clock circuit  70  generates clock signal that are 180 degrees out of phase for the first and second latch stages, whereas in the standby mode, the clock signal generation is changed so that the clocks have the same phase, and the inverted clock signals also have the same phase. Note in FIG. 7 that logic gates U 1  and U 3  are powered through VCC and PDVSS (switched during standby mode via transistor M 1 ), and inverters U 2  and U 4  are powered through PDVCC (switched during standby mode via transistor M 2 ) and ground. 
     A half cycle latch circuit  80  is shown in FIG.  8 . Latch circuit  80  also includes a number of input inverter stages to illustrate how a forced data state can ripple through additional alternate power-gated circuitry as well as the latch circuits of the present invention. From left to right in FIG. 8, circuit  80  includes a DIN data input and an input inverter stages including P-channel transistors M 2  and M 25 , and N-channel transistor M 1 . A single forcing transistor M 22  has a gate for receiving a PWD power-down control signal, a drain coupled to the next inverter stage, and a source coupled to ground The next three inverter stages include: P-channel transistor M 4  and N-channel transistor M 5 ; P-channel transistor M 19  and N-channel transistor M 20 ; and P-channel transistor M 6  and N-channel transistor M 7 . Note that the alternate power-gating technique is used whereby the drains of transistors M 25  and M 19  are coupled to the gated PDVCC power supply and the drains of transistors M 4  and M 6  are coupled to the non-gated VCC power supply. Similarly, the sources of transistors M 5  and M 7  are coupled to the gated PDVSS power supply and the sources of transistors M 1  and M 20  are coupled to the non-gated ground terminal. A transmission gate includes P-channel transistor M 8  in parallel with N-channel transistor M 9 . The clock inputs receive the DICLK and DICLKB clock signals. A latch stage includes a first inverter including P-channel transistor M 10  and N-channel transistor M 11 . The latch stage also includes a second cross-coupled inverter including P-channel transistors M 12  and M 13 , and N-channel transistors M 14  and M 15 . Note that the first inverter is powered between PDVCC and ground, and the second inverter is powered between VCC and PDVSS. A final inverter in circuit  80  includes P-channel transistor M 16  coupled to VCC and a N-channel transistor M 17  coupled to PDVSS. The data state provided by forcing transistor M 22  is passed through the entire signal path of circuit  80  when it is placed into the standby mode. Since the data state itself is carried throughout the data path, additional forcing transistors are not needed and precious integrated circuit area is preserved. 
     While circuit  80  includes only one transmission gate, a special clock circuit is nonetheless required to provide the DICLK and DICLKB signal in both the normal and standby operating modes. Such a clock circuit  90  is shown in FIG.  9 . Referring now to FIG. 9, a clock circuit  90  is shown for generating the clock signals used in both the normal operating mode and the standby mode for the half cycle latch circuit  80  shown in FIG.  8 . Clock circuit  90  includes a CLK input for receiving a system clock signal, a MACTB input for receiving a mask signal, as well as a PWDB power-down control signal. The MACTB input stands for Memory Active Low True, which causes all phases of the clock to toggle only when the memory is active. Clock circuit  90  also has two outputs DICLK and DICLKB for generating the two separate clock signals required for latch circuit  80 . A first NOR gate U 2  receives the CLK and MACTB signals. A first transistor M 1  is coupled to the output of gate U 2  and also receives the PWDB signal. A second transistor M 2  is coupled to a power terminal of gate U 2  and also receives the PWDB signal. Transistor M 1  is powered via VCC and transistor M 2  is powered via PDVSS. A first inverter U 3  is coupled to the output of NOR gate U 2  and generates the DICLK signal. A second inverter U 4  is coupled to the output of NOR gate U 2 . A third inverter U 5  is coupled to the output of inverter U 4  and generates the DICLKB signal. In operation, clock circuit  90  generates clock signals that are 180 degrees out of phase in a normal mode of operation, whereas in the standby mode, the clock signal generation is changed so that the clocks have the same phase. Note in FIG. 9 that logic gates U 1  and inverter U 5  are powered through VCC and PDVSS (switched during standby mode via transistor M 1 ), and inverters U 3  and U 4  are powered through PDVCC (switched during standby mode via transistor M 2 ) and ground. 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof It will, however, be appreciated by those skilled in the art that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.