Abstract:
An apparatus and method for reducing the power consumption of a memory integrated circuit during a period of power down mode operation by interrupting the clocking transitions of a delay line. A memory integrated circuit may include a delay lock loop including a plurality of delay elements connected to one another in series and adapted to delay propagation of the signal of a free running clock. When the delayed signal is not required, as during a period of power down mode operation, the free running clock signal is prevented from reaching the delay lock loop. Consequently the delay elements do not toggle, and power associated with delay element toggling is saved.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a memory integrated circuit, and more specifically to a method and apparatus for reducing power consumption by a memory integrated circuit. 
     2. Description of the Related Art 
     In a double data rate (DDR) random access memory (RAM), it is desirable to have output data retrieved from memory and placed on an output bus synchronously with an external clock. Conventionally, this is achieved by including a data buffer, referred to as a DQ buffer, among the functional circuitry of the integrated circuit. The DQ buffer includes a set of latches having respective data inputs attached to an internal data bus, a set of respective data outputs attached to an external data bus, and one or more clock inputs. In response to a signal transition at the clock input of a data latch, the latch loads data available at the data input of the latch and makes it available at the latch data output, in effect transferring data from the internal data bus to the external data bus. 
     Also conventional is the use of a signal delay device, such as a delay locked loop (DLL) circuit timed by a free running external clock. The delay locked loop produces a plurality of delayed clock signals corresponding to, but lagging in phase, the free running external clock. Each data latch receives a delayed DLL clock signal produced by the delay locked loop to clock the clock input of the latch. 
     The delay locked loop includes a delay line. The delayed DLL clock signals of the delay locked loop are produced by passing the external clock signal through this delay line. The delay line consists primarily of a group of delay elements connected in series such that the output of a first delay element is operatively connected to the input of a second delay element and the output of the second delay element and is operatively connected to the input of a third delay element and so on. Typically a delay element includes at least one logic gate. Each of the of delay elements has a characteristic delay such that a signal introduced at its input produces a signal at its output after a period of time equal to the characteristic delay. Accordingly, by placing a number of delay elements in series and feeding a digital signal sequentially through the series of delay elements a delay of finite duration may be introduced into the propagation of the signal. When a signal transition is applied at the input of the DLL, each of the series of delay elements undergoes a state transition after a delay related to the number of preceding delay elements in the series. It is thus apparent that the delay is cumulative with the number of elements that must transition, and the result is a signal delay proportional to the number of delay elements in the delay line. In a delay locked loop circuit, this number of delay elements can be varied to insure that the delay produced corresponds to the period of the external clock, less a small time allowed for signal transmission of a clock signal from the DLL to a latch of the DQ buffer. 
     It is a general characteristic of transistors, and hence of electronic gates and delay elements constructed with transistors, that state transitions cause a dissipation of energy. Accordingly, in making the state transitions described above, the delay elements of a DLL dissipate a certain amount of power. 
     Because, there is a relatively large number of delay elements in a delay line, and because each of these elements must transition, or toggle, with each transition of the external clock, the DLL circuit represents a locus of some power dissipation in a DRAM integrated circuit, particularly in a double data rate (DDR) DRAM. Generally, this power loss is tolerable, and is necessary for functioning of the chip. Nonetheless, saving all, or a portion, of the energy dissipated in toggling of the delay elements of the DLL would be advantageous. 
     SUMMARY OF THE INVENTION 
     The present invention reduces the power consumption of an integrated circuit by eliminating unnecessary cycling of a delay locked loop circuit during a power down mode of operation of a DRAM. Power down mode is a mode of operation during which system power is conserved. In a preferred embodiment, when the integrated circuit is placed in a power down mode, the integrated circuit of the invention interrupts transmission of a free running external clock signal that is otherwise received by an input of the delay locked loop. Because the delay locked loop does not receive the external clock signal, the delay elements of the delay line of the delay locked loop do not toggle. Rather, the delay elements of the delay line remain in a constant state as long as the integrated circuit continues to receive power and the delay locked loop does not receive the clock signal. Since operation of electronic logic gates in a constant state requires less power than operation of the same gates in transition, a power savings is realized. 
     In one aspect of the invention, the integrated circuit is manufactured with a switch. The switch is most commonly a circuit, made of gates, implemented with transistors. The switch circuit is electrically connected in series with a conductive line that connects the external clock to the external clock input of the delay locked loop. The switch circuit can be switched to be non-conductive during power down. When the switch is nonconductive, transmission of the external clock signal is interrupted by the switch circuit, and the delay locked loop does not receive the external clock signal. 
     Accordingly, the invention includes a method of reducing the power consumption of a random access memory integrated circuit by preventing a delay locked loop, or other internal clock producing circuit, from receiving an external clock signal during a power down mode. In another aspect, the invention includes an embodiment of a memory integrated circuit including a switch, e.g. a transistor based gate circuit, electrically connected in series with a conductive trace adapted to operatively connect an external clock with an external clock input of a delay locked loop or other internal clock producing circuit. 
     In various other aspects, the invention includes methods and embodiments adapted to otherwise prevent toggling of the delay line of a delay locked loop when the DLL clock signal produced by that loop is not required, as during operation in power down mode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other advantages and features of the invention will become more apparent from the detailed description of the preferred embodiments of the invention given below with reference to the accompanying drawings in which: 
     FIG. 1 shows an external clock and a portion of a conventional integrated circuit, including a delay locked loop portion, in block diagram form; 
     FIG. 2A shows a portion of the integrated circuit of the present invention, along with an external clock and a control device, in block diagram form; 
     FIG. 2B shows two alternative embodiments of the switch of the present invention implemented using logic gates; and 
     FIG. 3 shows the state timing of signals indicating one aspect of operation of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A memory device having a conventional delay locked loop circuit is illustrated in FIG.  1 . The present invention will be described as set forth in the exemplary embodiments illustrated in FIGS. 2A,  2 B and  3 . Other embodiments may be utilized and structural or logical changes may be made without departing from the spirit or scope of the present invention. Like items are referred to by like reference numerals throughout the drawings. 
     FIG. 1 illustrates a portion of a conventional memory device circuit. As shown, a DQ buffer  10  formed as a plurality of latches has a plurality n of data inputs  20  connected to a first multi-bit internal data path  30  of a RAM integrated circuit  40 . As also shown, n data outputs  50  of the DQ buffer are connected to a second external data path  60 , for example a bus external to the illustrated memory device. The DQ buffer  10  also includes a delay locked loop (DLL) clock input  70  that, when triggered, causes a latching of data and thus transfer of data across the DQ buffer from the data input paths  30  to the data output paths  60 . A RAM integrated circuit  40  also includes a delay locked loop (DLL)  80  with an input  72  adapted to receive a signal from a substantially periodic external clock  74 . Generally, the external clock  74  is a free running clock which is received from a memory controller or other external circuit. The DLL circuit  80  provides a signal to the DLL clock input  70  of the DQ buffer  10  at precisely the correct time to insure the latching of output data in a particular relationship with the timing of external clock  74 . To this end, the DLL circuit  80  delays the passage of an external clock signal by a time almost equal to one full period of the external clock signal. Accordingly, a clock transition of the external clock signal simultaneously initiates activity in a memory logic array  82  of the RAM integrated circuit  40 , and also begins propagating through the DLL circuit  80 . 
     The DLL circuit  80  includes a delay line  100 . The delay line incorporates a plurality of delay elements  110 ,  112 ,  114 ,  116 , for example, connected in series such that an output  120  of one delay element is connected to an input  130  of the next delay element in the series. The external clock input  72  of the delay line is operatively connected to an input  140  of the first delay element in the series. As previously described, an external clock  74  supplies a clock signal to the input  72  of the DLL circuit  80 . A power supply  86  supplies power to the system. The external clock signal is applied to the input  140  of the first delay element  110  of the delay line  100 . The first delay element  110  undergoes a state transition in response to the signal transition applied at its input  140 , and changes the state of its output  120  accordingly. During the state transition, the power dissipated by the delay element  110  increases for a finite time. As noted above, the output  120  of the first delay element  110  is operatively connected to an input  130  of a second delay element  112  so that when the output  120  of the first delay element changes state, the input  130  of the second delay element  112  detects this change, and the second delay element  112  also changes state. Consequently the second delay element, like the first, experiences an increase in power dissipation for a finite time. 
     Generally a delay line  100  includes many delay elements, each of which experiences a state transition in response to a clock transition at DLL input  72 . It should be noted that the number of delay elements in the delay line  100  may be large, and may in fact vary automatically according to the requirements of the system and the frequency of the external clock  74 . The combination of a free running clock operating at high frequency, and a large number of delay elements, means that a significant amount of power, taken in the context of the integrated circuit, may be dissipated in state transitions of the delay elements. As mentioned previously, it is unnecessary for the DQ buffer to transfer data from its data input  20  to its data output  50  during a power down mode. Consequently the DLL circuit need not operate when the system is in power down mode. Since not operating the DLL circuit means that the delay elements of the delay line  100  need not transition, the power associated with such transitions may be saved by not operating the delay line during power down mode. 
     It is thus a feature of the present invention that the DLL circuit  80  is disabled during a power down mode. According to one embodiment of the invention, as shown in FIG. 2A, this is achieved by disposing a switch circuit  160 , or other switch, having a control input  162  designated enable (en). The switch is in series with conductive traces  170 ,  180  that conduct the external clock signal from an external clock  74  to the input  72  of the DLL circuit  80 . In a first state this switch  160  is closed, or conductive, and conducts the external clock signal through to the DLL input  72 . In a second state, operative during power down mode, this switch  160  is open or non-conductive, and prevents the external clock signal from reaching the DLL input  72 . As a result, when the switch  160  is open, no external clock signal is received by the input  72  of the DLL circuit  80 , and no state transitions take place within the delay elements of the delay line  100  of the DLL. 
     As a practical matter, the implementation of such a switch may reduce the current requirements of a RAM integrated circuit during power down mode by from about 1 to about 15 milliamps, or more. 
     As will be apparent to one of skill of the art, the switch  160  may be implemented as any of a variety of circuits, such as, for example, a gate circuit including insulated gate field effect transistors. FIG. 2B illustrates two alternative embodiments of a logic-gate based circuit adapted to serve as a switch  160 . In a first circuit, a Nand gate  193  is connected in series with a Not gate  194 . In a second circuit, a Nor gate  195  is connected in series with a Not gate  196 . The Nand and Nor gates each have a first input serving as an external clock input, and a second input serving as an enable or control input. In both embodiments of the circuit, the output of the Not gate,  197 ,  198 , serves as an output of the switch circuit. 
     The use of the gates shown in FIG. 2B is merely exemplary, and one of skill of the art will readily ascertain that another switching device, such as a microfabricated relay or a mechanical switch, would also serve. In alternative embodiments, switching devices could be incorporated within the delay line  110 , or elsewhere within the DLL circuit  80 . Such switches might be external to, or part of, the delay elements  110 ,  112 ,  114 ,  116  of the delay line  110 . Accordingly it should be understood that many different types of switches or methods might be used to prevent cycling of the delay line during power down mode. 
     As shown in FIG. 2A, a control device  192  may be provided which is operatively connected to a control (enable) input  162  of the switch  160 . The control device may be adapted to give a user automatic or manual control over the switch, and thereby permit a user to allow or prevent the external clock input  72  of the DLL from receiving the external clock signal generated by the external clock  74 . Typically the power supply  86  of the system is operatively connected to the control device. 
     Operation of the invention is illustrated in FIG. 3 which shows the relative timing of state and signal transitions for an exemplary embodiment of the invention. The external clock signal  200  is shown as a free running periodic square wave. Also shown is the controlled external clock signal  260 , as controlled by the switch  160  and received at the input  72  of the DLL. During a period of conventional power up operation  210  the controlled external clock signal  260 , is substantially identical to the external clock signal  200 . During conventional operation  210  the DLL clock signal  270  is shown to follow  220  the controlled external clock signal  260  at the DLL input  72  with a phase delay of 360 degrees, less a small delta  230 . This delta represents time allowed in the design of the integrated circuit for the transmission of the DLL clock signal from the output  82  of the DLL circuit to the input  70  of the DQ buffer. Accordingly during a period of conventional operation  210  the DQ buffer transfers data  240  to its outputs. 
     During a period of power down mode operation  230 , the external clock signal continues to transition normally  250 . According to the invention, however, the controlled external clock signal  260  does not undergo any state transition. Hence the external clock input  72  of the DLL circuit does not receive any state transition  260 . Consequently, the DLL circuit does not cycle, and in particular the delay elements of the DLL delay line undergo no state transitions. This is reflected in the constant value of the DLL clock output  270  resulting from power down  230 . The absence of state transitions indicated by the DLL clock  270  and DQ output  280  signals during power down mode saves energy as discussed above. 
     While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, deletions, substitutions, and other modifications can be made without detracting from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.