Patent Publication Number: US-7915920-B2

Title: Low latency, power-down safe level shifter

Description:
This application is a division of U.S. application Ser. No. 11/610,236, which was filed on Dec. 13, 2006 now U.S. Pat. No. 7,652,504. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention is related to the field of integrated circuits and, more particularly, to supplying power to integrated circuits having multiple voltage domains. 
     2. Description of the Related Art 
     As the number of transistors included on a single integrated circuit “chip” has increased and as the operating frequency of the integrated circuits has increased, the management of power consumed by an integrated circuit has continued to increase in importance. If power consumption is not managed, meeting the thermal requirements of the integrated circuit (e.g. providing components required to adequately cool the integrated circuit during operation to remain within thermal limits of the integrated circuit) may be overly costly or even infeasible. Additionally, in some applications such as battery powered devices, managing power consumption in an integrated circuit may be key to providing acceptable battery life. 
     Power consumption in an integrated circuit is related to the supply voltage provided to the integrated circuit. For example, many digital logic circuits represent a binary one and a binary zero as the supply voltage and ground voltage, respectively (or vice versa). As digital logic evaluates during operation, signals frequently transition fully from one voltage to the other. Thus, the power consumed in an integrated circuit is dependent on the magnitude of the supply voltage relative to the ground voltage. Reducing the supply voltage generally leads to reduced power consumption. However, there are limits to the amount by which the supply voltage may be reduced. 
     Reducing the supply voltage often reduces the performance of the circuits supplied by that supply voltage. If some circuits in the integrated circuit are busy (and thus need to perform at or near peak operation), the supply voltage must generally remain at a relatively high level. One technique to avoid this is to divide the integrated circuit into voltage “domains” that are supplied by separate supply voltages that may be independently adjusted. That is, the supply voltage for circuits in a given voltage domain is the corresponding supply voltage. Thus, some voltages may be reduced (or even powered down completely) while others remain high for full speed operation. 
     Once voltage domains that may be at different levels are introduced, it is often required to level shift signals from one domain to another to ensure proper operation in the receiving voltage domain. If the supply voltage from the source voltage domain of a level shifter is powered down, all input signals may be reduced to ground voltage, which may result in an inaccurate signal being provided to the receiving voltage domain by the level shifter. Inaccurate operation may result. 
     SUMMARY 
     In one embodiment, an apparatus comprises a circuit supplied by a first supply voltage during use, the circuit having at least a first input signal; and a level shifter supplied by the first supply voltage during use and coupled to provide the first input signal to the circuit. The level shifter is coupled to receive a second input signal sourced from circuitry supplied by a second supply voltage during use, and is configured to generate the first input signal by level shifting the second input signal. Coupled to receive a power control signal indicating, when asserted, that the second supply voltage is to be powered down, the level shifter is configured to assert a predetermined level on the first input signal independent of the second input signal and responsive to an assertion of the power control signal. 
     In an embodiment similar to the above embodiment, a method comprises detecting that the second supply voltage is to be powered down; asserting a power control signal to the level shifter responsive to the detecting; and generating a predetermined level on the first input signal responsive to the asserting and independent of the second control signal, the generating performed by the level shifter. 
     In another embodiment, a level shifter comprises a first node on which either an output signal of the level shifter or its inverse is provided by the level shifter during use, an input on which an input signal is received by the level shifter during use, shifting circuitry coupled between the input and the first node and supplied by the first supply voltage during use, and a first transistor coupled to the first node and having a gate controlled responsive to a power control signal. The output signal is received by circuitry supplied with a first supply voltage during use, and the input signal is generated by circuitry supplied by a second supply voltage during use. The shifting circuitry is configured to level shift the input signal to generate the signal on the first node. The power control signal is asserted during use to indicate that the second supply voltage is to power down, and the first transistor drives a predetermined voltage on the first node responsive to assertion of the power control signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description makes reference to the accompanying drawings, which are now briefly described. 
         FIG. 1  is a block diagram of one embodiment of an integrated circuit. 
         FIG. 2  is a block diagram of one embodiment of a memory circuit shown in  FIG. 1 . 
         FIG. 3  is a circuit diagram of one embodiment of a level shifter shown in  FIG. 2 . 
         FIG. 4  is a circuit diagram of another embodiment of a level shifter shown in  FIG. 2 . 
         FIG. 5  is a flowchart illustrating one embodiment of a method. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Turning now to  FIG. 1 , a block diagram of one embodiment of an integrated circuit  10  is shown. In the illustrated embodiment, the integrated circuit includes a plurality of logic circuits  12 , a plurality of memory circuits  14 , and a power control circuit  16 . The logic circuits  12  are coupled to the memory circuits  14 . The power control circuit  16  is coupled to the memory circuits  14  (specifically, via a V L OFF signal). The logic circuits  12  are powered by a first supply voltage provided to the integrated circuit  10  (labeled V L  in  FIG. 1 ). The memory circuits  14  and the power control circuit  16  are powered by a second power supply voltage provided to the integrated circuit  10  (labeled V M  in  FIG. 1 ). In the illustrated embodiment, the memory circuits  14  are also powered by the V L  supply voltage, as will be explained in more detail for certain embodiments below. The integrated circuit  10  may generally comprise the logic circuits  12 , the memory circuits  14 , and the power control circuit  16  integrated onto a single semiconductor substrate (or chip). 
     The logic circuits  12  may generally implement the operation for which the integrated circuit is designed. The logic circuits  12  may generate various values during operation, which the logic circuits  12  may store in the memory circuits  14 . Additionally, the logic circuits  12  may read various values on which to operate from the memory circuits  14 . For example, in various embodiments, the memory circuits  14  may include memory used for caches, register files, integrated-circuit-specific data structures, etc. The memory circuits  14  may implement any type of readable/writeable memory. In an example below, an SRAM memory will be used. It is noted that, while the illustrated embodiment includes a plurality of logic circuits  12  and a plurality of memory circuits  14 , various embodiments may include at least one logic circuit  12  and at least one memory circuit  14 . 
     Generally, if a logic circuit  12  is to access a memory circuit  14 , the logic circuit  12  may generate various control signals to the memory circuit  14 . For example, the control signals may include an address identifying the memory location in the memory circuit  14  that is to be accessed, a read enable signal which may be asserted to perform a read, and a write enable signal which may be asserted to perform a write. For a read, the memory circuit  14  may output data to the logic circuit  12 . For a write, the logic circuit  12  may supply data to the memory circuit  14  for storage. 
     By separating the supply voltage for the logic circuits  12  and the memory circuits  14 , the supply voltage for the logic circuits  12  (V L ) may be reduced below the level at which the memory circuits  14  may operate robustly. The supply voltage for the memory circuits  14  (V M ) may be maintained at the minimum supply voltage that provides for robust memory operation (or greater, if desired). Thus, the V L  supply voltage may be less than the V M  supply voltage during use. At other times, the V L  supply voltage may exceed the V M  supply voltage during use (e.g. at times when higher performance is desired and higher power consumption is acceptable to achieve the higher performance). Alternatively, the V M  supply voltage may be increased to match the V L  supply voltage if the V L  supply voltage would otherwise exceed the V M  supply voltage. 
     In one embodiment, the V L  supply voltage may even be powered down (that is, reduced to the ground reference) while the V M  supply voltage remains active to retain data in the memory circuits  14 . The power control circuit  16  may monitor various inputs (internal, illustrated by arrow  17 , and/or external, illustrated by arrow  18 ) to determine that the V L  supply voltage is to be powered down. In one embodiment, the power control circuit  16  may determine that the V L  supply voltage is to be powered down and may issue a power down request to an external power source such as a voltage regulator (not shown in  FIG. 1 ) that supplies the V L  voltage. In other embodiments, internal or external inputs may indicate that the power down is to occur, and the power control unit  16  may detect the event by monitoring the inputs. In either case, the power control unit  16  may assert the V L OFF signal to the memory circuits  14 . The V L OFF signal may be active high (where the asserted state indicating that the V L  voltage is to be powered off is the logical one state) or active low (where the asserted state is the logical zero state). The deasserted state is the opposite of the asserted state in either case. Various level shifters in the memory circuits  14  may use the V L OFF signal to assert a predetermined output to other circuitry in the memory circuits  14 , as described in more detail below. 
     Generally, a supply voltage may be a voltage provided to a circuit to power the circuit, providing the electrical energy to permit the circuit to generate one or more outputs responsive to one or more inputs. At various points herein, supply voltages may be referred to as being greater than or less than other supply voltages. That is, the magnitude of the voltage may be greater than (or less than) the magnitude of the other voltage. 
     Turning now to  FIG. 2 , a block diagram of one embodiment of a memory circuit  14 A is shown. The memory circuit  14 A may be one of the memory circuits  14 . Other memory circuits  14  may be similar. In the embodiment of  FIG. 2 , the memory circuit  14 A includes a level shifter circuit  20 , a set of word line driver circuits  22 , a memory array  24 , a clock gater circuit  26 , a control signal generator circuit  28 , and a NAND gate  30 . The level shifter  20  and the word line drivers  22  are supplied by the V M  supply voltage. The memory array  24  and the control signal generator  28  are supplied by both the V M  and the V L  supply voltages. The clock gater  26  is supplied by the V L  supply voltage. The NAND gate  30  and the clock gater  26  are coupled to receive a clock input (gclk) and one or more enable inputs (En) from the logic circuits  12 . The output of the NAND gate  30  (which may be supplied by the V L  supply voltage, not shown in  FIG. 2 ) is coupled as an input to the level shifter  20 , which is also coupled to receive the V L OFF signal. The clock gater  26  is configured to generate a clock output (clk) to the word line drivers  22  and the level shifter  20  is also configured to generate a clock output (clk_e) to the word line drivers  22 . The word line drivers  22  are further coupled to receive one or more address inputs (Addr inputs) from the logic circuits  12 . The word line drivers  22  are configured to generate a set of word lines to the memory array  24  (WL 0  . . . WLN). The memory array  24  is further coupled to receive data (Din) and provide data (Dout) to/from the logic circuits  12 . Additionally, the memory array  24  is coupled to receive various control signals from the control signal generator  28 . For example, the control signals may include a write enable (WE) signal and a read enable (RE) signal. The control signals may also include a precharge (PChg) signal, and any other desired control signals. The control signal generator  28  may generate the control signals for the memory array  24  from corresponding control inputs from the logic circuits  12 , and may level shift control signals, in some embodiments. Thus, the control signal generator  28  may receive the V L OFF signal as well and may include one or more level shifters similar to the level shifter  20 . 
     The memory array  24  may comprise a plurality of memory cells that are supplied by the V M  supply voltage. However, the memory circuit  14 A is designed to provide for access to the memory array  24  by the logic circuits  12 , even if the logic circuits  12  are supplied with a V L  supply voltage that is less than the V M  supply voltage. Each memory cell is activated for access (read or write) by one of the word lines WL 0  . . . WLN coupled to that memory cell. One or more memory cells coupled to the same word line form a “word” for access in the memory array  24 . That is, the bits of the word may be read/written as a group. The width of the word may thus be the width of the Din and Dout signals from the memory array  24 . 
     Since the memory cells are supplied by the V M  supply voltage, the word lines may also be supplied by the V M  supply voltage. That is, when a word line is asserted high, the word line may be at approximately a V M  voltage. Thus, the word line drivers  22  are supplied with the V M  supply voltage. 
     The word line drivers  22  activate a given word line based on address inputs from the logic circuits  12 . The address identifies the word in the memory array  24  to be accessed for a given access generated by the logic circuits  12 . In some embodiments, the logic circuits  12  may include circuits that partially or fully decode the address, and the address inputs may be the partially or fully decoded address. Alternatively, the word line drivers  22  may implement the full decode function and the address inputs may encode the address. Generally, each different address causes a different word line WL 0  to WLN to be asserted. 
     Since the word line drivers  22  are supplied with the V M  supply voltage, inputs to the word line drivers  22  that are coupled to the gates of p-type metal oxide semiconductor (PMOS) transistors in the word line drivers  22  may be driven to a V M  voltage when driven high (to ensure that the PMOS transistors, which are supplied with a V M  supply voltage, are fully turned off when the gate is driven high). That is, if the gate of the PMOS transistor is driven to a voltage less than the V M  supply voltage on its source, the gate to source voltage of the PMOS transistor is still negative and thus the PMOS transistor may still be active even though it is logically intended to be inactive. If the word line drivers  22  were designed with static complementary MOS (CMOS) circuits, each input would be coupled to the gate of a PMOS transistor and would be driven to a V M  voltage when driven high. In one embodiment, the word line drivers  22  may be implemented with dynamic logic gates. Thus, the clock signal that precharges the circuit (clk_e) is coupled to the gate of a PMOS transistor and may be driven to a V M  voltage. Other signals, coupled to the gates of n-type MOS (NMOS) transistors, may be driven with the V L  voltage. Thus, the address inputs from the logic circuits  12  may be provided directly to the word line drivers  22  (without level shifting). Additionally, the clk signal from the clock gater  26  (supplied with the V L  voltage and thus driven to the V L  voltage when driven high) may be provided directly to the word line drivers  22 . 
     The level shifter  20  is configured to generate the clk_e signal responsive to the gclk signal and the En signal (supplied through the NAND gate  30 ). If the En signal (or signals) indicate that the clock is enabled for the current clock cycle, the level shifter  20  may generate the clk_e signal by level shifting the input signal such that the high assertion of the clk_e signal is at a V M  voltage. 
     Additionally, the level shifter  20  may be designed to provide a predetermined voltage level on the output signal (clk_e, in  FIG. 2 ) if the V L  supply voltage is to be powered down, independent of the input signal to the level shifter  20 . Since the input signal is generated by circuitry powered by the V L  supply voltage (and thus is powered down), the input signal may be at a ground voltage (or may even float at indeterminate levels). Furthermore, for the embodiments of  FIGS. 3 and 4 , the inverter  56  may be powered by the V L  supply voltage, and thus the input signal and its inversion are not complements of each other when the V L  supply voltage is powered down. By providing the predetermined voltage level during such time periods, the level shifter  20  may provide predictable behavior for the receiving circuitry. If predicable behavior was not provided, various undesired effects could occur. For example, for the embodiment of  FIG. 2  (in which the level shifter provides signals for the memory circuitry), data in the memory could be accidentally overwritten. Or, circuitry that normally is not active at the same time (e.g. bitline prechargers and word line drivers) could be active at the same time and thus cause contention, which could damage the memory circuits and/or cause undesirable power consumption. Still further, when the V L  supply voltage is powered up again, unpredictable logic circuit initializations and/or transitions on the input signals to the level shifter may occur, but the forced value on the output may prevent such unpredictable behavior from reaching the memory circuits. In the present embodiment, the assertion of the V L OFF signal indicates that the V L  supply voltage is to be powered down, and the V L OFF signal may remain asserted while the V L  supply voltage is off, in this embodiment. 
     The predetermined voltage level may be the level that is considered “safe” for the receiving circuitry. For example, in the case of  FIG. 2 , the clk_e signal is coupled to the gate of a PMOS transistor in the word line drivers  22 , as mentioned above. Accordingly, the safe voltage would be the high (V M ) voltage, so that the PMOS transistor is inactive. In other cases, the safe voltage may be the low (ground) voltage, or any other desired voltage level. Various level shifters may implement various predetermined voltage levels, even in the same design, dependent on the safe voltage levels for the receiving circuitry. 
     Generally, a level shifter circuit may be a circuit configured to level shift an input signal to produce an output signal. Level shifting a signal may refer to changing the high assertion of the signal from one voltage to another. Level shifting may be performed in either direction (e.g. the voltage after level shifting may be higher or lower than the voltage before level shifting). In some embodiments, the low assertion may remain the ground voltage supplied to the integrated circuit  10  (not shown in the figures, often referred to as V SS ). 
     The clock gater  26  generates the clk signal responsive to the En signal (or signals) and the gclk signal (similar to the discussion above for the level shifter). If the En signal (or signals) indicate that the clock is enabled for the current clock cycle, the clock gater  26  may generate the clk signal responsive to the gclk signal. If the En signal (or signals) indicate that the clock is disabled, the clock gater  26  may hold the clk signal steady at a low level (ground). In other implementations, the clock gater  26  may hold the clk signal steady at a high level (V L ) if the En signal (or signals) indicate that the clock is disabled. In other embodiments, the clock gater  26  may be eliminated and the clk_e signal may be used in place of the clk signal in the word line drivers  22 . 
     In some embodiments, the delay through the level shifter  20  may be approximately the same as the delay through the clock gater  26 . In such embodiments, the impact of the level shifter  20  on the critical timing path of the integrated circuit  10  (if any) may be minimized. 
     As mentioned above, the memory circuit  14 A is designed to provide read/write access to the memory array  24  even if the V M  supply voltage is higher than the V L  supply voltage. The level shifter  20  level-shifting input signals and the word line drivers  22  operating at the V M  voltage provide the start of an access. The Din and Dout signals provide the data in (for a write) or the data out (for a read), and thus are in the V L  domain used by the logic circuits  12  in this embodiment. The memory array  24  may also be supplied with the V L  voltage, and may be configured to operate with the Din and Dout signals in the V L  domain. In other embodiments, the Din and Dout signals may be level shifted between the V L  and V M  domains, or only the Din signals may be level shifted and the Dout signals may be in the V M  domain. 
     In one embodiment, at least the sense amplifier (senseamp) circuits in the memory array  24  that sense the bits read from the memory cells are supplied with the V L  voltage. Thus, the senseamps may also provide a level shift to the V L  domain for the Dout signals. In another embodiment, the senseamp circuits may be supplied with the V M  voltage and the Dout signals may be in the V M  domain. In another implementation, the bit lines coupled to the memory cells to communicate the bits into and out of the memory cells may be in the V L  domain and thus other circuitry that is coupled to the bit lines may be supplied with the V L  supply voltage (except for the memory cells themselves). 
     As mentioned previously, signals in the V L  domain that are coupled to the gates of PMOS transistors that are supplied by the V M  supply voltage may be level shifted. Thus, in various embodiments, some of the control signals provided to the memory array  24  may be level-shifted. The control signal generator  28  may provide the level shifting, as needed, in various embodiments. If a given control signal is not level shifted, the control signal generator  28  may generate the control signal using circuitry supplied by the V L  supply voltage. If a given control signal is level shifted, the control signal generator  28  may include a level shifter to shift to the V M  domain. The level shifters in the control signal generator  28  may be similar to the level shifter  20  and may respond to an assertion of the V L OFF signal by providing appropriate safe voltages on their outputs. 
     Turning now to  FIG. 3 , a circuit diagram of one embodiment  20   a  of the level shifter  20  is shown. In the embodiment of  FIG. 3 , the level shifter  20   a  includes a shifting stage comprising transistors T 1 -T 9  and an output inverter comprising transistors T 11 -T 13 . T 3  has a source coupled to the V M  supply voltage, a gate coupled to a node N 1 , and a drain coupled to the source of T 4 . The gates of T 4  and T 5  are coupled to receive an input signal (In), which may be the output of the NAND gate  30  in  FIG. 2 . Generally, however, any input signal may be used. The drains of T 1 , T 4 , and T 5  are coupled to the node N 2 . The source of T 5  is coupled to the drain of T 6 , and the source of T 6  is coupled to ground. The gate of T 6  is coupled to an inversion of the V L OFF signal, output from the inverter  50 . The output of the inverter  50  and is also coupled to the gate of T 1 , which has its source coupled to the V M  supply voltage. The In signal is input to an inverter  56  which has its output coupled to the gates of T 8  and T 9 . The source of T 9  is coupled to ground. The drains of T 8  and T 9  are coupled to the node N 1 . The source of T 8  is coupled to the drain of T 7 , which has its source coupled to the V M  supply voltage. The gate of the transistor T 7  is coupled to the node N 2 . The node N 1  is the output of the shift stage and supplies the input to the output inverter. The gates of T 11  and T 13  are coupled to the node N 1 , and the drains of T 11  and T 12  are coupled to the output signal (e.g. the clk_e signal in  FIG. 2 ). The source of T 11  is coupled to the V M  supply voltage. The source of T 12  is coupled to the drain of T 13 , which has its source coupled to ground. The gate of T 12  is coupled to the V L  supply voltage. The drain of T 2  is coupled to the node N 1 , and the source of T 2  is coupled to ground. The gate of T 2  is coupled to the output of an inverter  52 , which has its input coupled to the output of the inverter  50 . 
     Operation of the shift stage will first be described. When the input signal (In) transitions from low to high, T 5  is activated and begins discharging node N 2 . T 4  is also deactivated by the input signal transition, isolating the node N 2  from T 3 . As the node N 2  discharges, T 7  activates and begins charging node N 1  to the V M  supply voltage (T 8  is also activated, and T 9  is deactivated, by the transition to low on the output of the inverter  56  due to the transition high of the input signal). Accordingly, N 1  results in the same logical state as the input signal, at the V M  supply voltage. When the input signal transitions from high to low, the output of the inverter  56  transitions from low to high and T 9  is activated. T 9  begins discharging the node N 1 . T 8  is also deactivated by the input signal transition, isolating the node N 1  from T 7 . Thus, the node N 1  is discharged to ground. As the node N 1  discharges, T 3  activates and begins charging node N 2  to the V M  supply voltage (T 4  is also activated by the transition to low of the input signal), thus deactivating T 7 . 
     T 4  and T 8  may limit power dissipation during transition, by isolating the nodes N 2  and N 1 , respectively, from T 3  and T 7 , respectively. T 3  and T 7  may be delayed in deactivating with respect to the activation of T 5  and T 9 , respectively, since T 3  and T 7  are deactivated through the charging of nodes N 1  and N 2 , respectively. By isolating T 3  and T 7  from their respective nodes N 2  and N 1  when T 5  and T 9  are activated, T 3  and T 7  may be prevented from fighting the discharge of their respective nodes N 2  and N 1 . T 4  and T 8  are optional and may be deleted in other embodiments. In such embodiments, the drains of T 3  and T 7  may be coupled to the drains of T 5  and T 9 , respectively. 
     In this embodiment, the level shifter  20   a  also provides a forced, predetermined voltage on the output if the V L OFF signal is asserted (high). If the V L OFF signal is asserted, T 2  is activated and discharges node N 1  to ground. T 1  is also activated and charges node N 2  to the V M  voltage. This forces a value of ground on node N 1  (or a V M  voltage on the output signal). T 6  is deactivated, preventing T 5  from affecting the node N 2 . T 7  is deactivated as the node N 2  charges. Thus, the output node N 1  may be held steady at ground if the V L OFF signal is asserted, independent of the state of the input signal. The output signal (Out) may thus be held at the V M  voltage through the inverter formed from transistors T 11 , T 12 , and T 13 . If the V L OFF signal is deasserted (low), T 6  is activated via the output of the inverter  50  and thus the shift stage may operate as described above. T 2  is deactivated via the output of the inverter  52 , and T 1  is also inactive. Transistors T 1 , T 2 , and T 6  establish voltages on the internal nodes N 1  and N 2  that are consistent with the desired output signal level. 
     The output inverter provides output buffering, which may permit the transistors T 1 -T 9  to be smaller. The output inverter is optional and may be eliminated in other embodiments. T 11  and T 13  provide the inversion. In the illustrated embodiment, the transistors T 12  is provided to aid in matching the delay of the level shifter  20   a  to other circuitry supplied by the V L  supply voltage (e.g. the clock gater  26 ). The transistor T 12  is optional and may be eliminated in other embodiments that have the inverter formed from transistors T 11  and T 13 . 
     The embodiment of  FIG. 3  provides a logical one output (V M  voltage on the output signal) if the V L OFF signal is asserted. The embodiment of  FIG. 4  is an example of a second embodiment of the level shifter  20  (level shifter  20   b ) that provides a logical zero output (ground on the output signal). 
     The embodiment of  FIG. 4  includes a shift stage comprising T 3 -T 5  and T 7 -T 9  and an output inverter comprising T 11  and T 13 . The transistor T 12  has been eliminated in this embodiment, since the output signal (Out) is ground when V L  is powered down in this embodiment. In order to drive the ground voltage via T 13 , T 12  is eliminated. T 3 -T 5 , T 7 -T 9 , and T 10 -T 11  and T 13  are coupled to each other and to the nodes N 1  and N 2  in a manner similar to the embodiment of  FIG. 2  (except that the transistor T 12  has been removed and thus the drain of T 13  is coupled to the drain of T 11 ). T 3 -T 5 , T 7 -T 9 , T 10 -T 11 , and T 13  operate similar to the description of  FIG. 3  responsive to the high and low inputs on their gate terminals. Comments regarding portions that are optional may be similar to the description of  FIG. 3  as well. However, the source of T 9  is coupled to the drain of a transistor T 15 , which has its source coupled to ground and its gate coupled to the output of the inverter  52 , which has its input coupled to the V L OFF signal. The gate of a transistor T 16  is coupled to the V L OFF signal as well, and has its drain coupled to the node N 2  and its source coupled to ground. A transistor T 14  has its drain coupled to the node N 1  and its source coupled to the V M  supply voltage. The gate of T 14  is coupled to the output of the inverter  56 . Thus, if the V L OFF signal is asserted (high), the transistor T 16  discharges the node N 2  to ground, and the transistor T 14  charges the node N 1  to the V M  supply voltage (and the transistor T 15  prevents the transistor T 9  from having an effect on the node N 1 ). The output signal is thus a logical zero (ground voltage) in response to assertion of the V L OFF signal, independent of the input signal. Transistors T 14 , T 15 , and T 16  establish voltages on the internal nodes N 1  and N 2  that are consistent with the desired output signal level. 
     It is noted that the transistors in the embodiments of  FIGS. 3 and 4  may be PMOS or NMOS transistors, using the standard symbols for such transistors. That is, a transistor with a gate terminal having an open circle (e.g. the transistor T 1 ) is a PMOS and a transistor with a gate terminal having no open circle (e.g. the transistor T 2 ) is an NMOS. 
     As can be seen in the embodiments of  FIGS. 3 and 4 , various transistors may have gate terminals coupled to receive the V L OFF signal or its complement (inverse). Accordingly, the transistors may generally be controlled responsive to the V L OFF signal. It is noted that, while various nodes/signals are described as being charged/discharged to a given voltage level, there may be some variation in voltage levels on nodes. Accordingly, a node may generally be charged/discharged (or driven) to approximately the given voltage level. 
     It is noted that, while logic circuits that may be powered down and memory circuits that may remain powered, with level shifters as described above in between, are shown in this embodiment, other embodiments may be used in other fashions. Generally, there may be source circuitry powered by one supply voltage (that may be powered down) that supplies signals to level shifters and receiving circuitry powered by another supply voltage that receives the level shifted outputs from the level shifters (and that may remain powered when the source circuitry&#39;s supply voltage is powered down). 
     In some embodiments, the level shifter may be configured to receive an enable along with the input signal. For example, the gclk and En signals input to the NAND gate  30  could be incorporated into the level shifter. In such an embodiment, additional transistors may be included. Such transistors may have the enable signal or its complement coupled to the gate terminal and may be placed in parallel or series with the transistors having gate terminals coupled to the input signal or its inversion. Parallel or series coupling is selected to prevent the effect of the input signal if the enable signal is not asserted to indicate enabled. 
     In some embodiments, an additional NMOS transistor may be coupled in series with the transistor T 9  of  FIG. 3  or the transistor T 5  of  FIG. 4 , with the gate terminal coupled to the V M  supply voltage, to make the circuit symmetrical. Other embodiments may be asymmetrical, as shown. 
     Turning now to  FIG. 5 , a flowchart is shown illustrating one embodiment of a method for using the V L OFF signal and level shifters as described above to protect receiving circuitry such as the memory circuits when source circuitry has its supply voltage powered down and/or powered up. For example, the method may be implemented by the power control circuit  16  (in conjunction with an external voltage regulator to power the supply voltage up or down, in one embodiment). 
     The power control circuit  16  may monitor various signals and determine if the V L  supply voltage is to be powered down (decision block  70 ). If so (decision block  70 , “yes” leg), the power control circuit  16  may assert the V L OFF signal (block  72 ). Optionally, the power control circuit  16  may delay for a period of time to allow the V L OFF signal to propagate and the level shifters to establish their predetermined voltage levels. Once the delay has expired (decision block  74 , “yes” leg), the power control circuit  16  may cause the V L  supply voltage to be powered off (block  76 ). For example, the power control circuit  16  may transmit a request to a voltage regulator or other power circuit that supplies the V L  supply voltage. The V L OFF signal may remain asserted. 
     At some point, it may be desirable to power the V L  supply voltage back up (or restore the voltage—decision block  78 ). Similar operation may occur at initial power up of the V L  supply voltage (e.g. when a device containing the integrated circuit  10  is powered on). When the V L  supply voltage is to be restored (decision block  78 , “yes” leg), the power control circuit  16  may cause the V L  supply voltage to be powered up (block  80 ). For example, the power control circuit  16  may transmit a request to the voltage regulator or other power circuit that supplies the V L  supply voltage, as mentioned above. Optionally, the power control circuit  16  may delay for a time period to allow the V L  supply voltage to power up and stabilize, and for the circuitry powered by the V L  supply voltage to initialize. When the delay expires (decision block  82 , “yes” leg), the power control circuit may deassert the V L OFF signal (block  84 ), and the level shifters may begin regular operation. 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.