Patent Publication Number: US-2023139283-A1

Title: Memory with efficient dvs controlled by asynchronous inputs

Description:
TECHNICAL FIELD 
     This application relates to memories, and more particularly to a memory with a dynamic voltage stress (DVS) scan controlled by asynchronous inputs. 
     BACKGROUND 
     A dynamic voltage stress (DVS) scan is an important tool for an integrated circuit manufacturer to test their embedded memory function. During a DVS scan, the power supply voltage is increased to unmask faults. For example, memory transistors may have a weak dielectric layer that will eventually fail. But despite the weak dielectric layer, the memory is functioning and thus could be sold to a customer, whereupon it will eventually fail and lead to costly returns. A DVS scan exposes such faults so that a high-quality product may be delivered. 
     Despite the importance of a thorough DVS scan, existing memory designs limited the power supply voltage increase that could be applied. Accordingly, there is a need in the art for memories configured for improved DVS scans. 
     SUMMARY 
     In accordance with an aspect of the disclosure, a memory is provided that includes: a bitcell array; a bitcell array head switch coupled between the bitcell array and a node for a memory power supply voltage; a memory periphery including a memory power domain portion; a memory periphery head switch coupled between the memory power domain portion and the node for the memory power supply voltage; and a power management circuit configured to switch off the bitcell array head switch and the memory periphery head switch to power off the bitcell array and the memory power domain portion during a sleep mode without retention for the memory, the power management circuit being further configured to switch off only the bitcell array head switch to power off the bitcell array and to maintain a power on of the memory power domain portion during a scan of the memory. 
     In accordance with another aspect of the disclosure, a method of operation for a memory is provided that includes: powering down a bitcell array and a memory periphery in the memory responsive to an assertion of a sleep mode without retention control signal while a dynamic voltage stress scan control signal is not asserted; powering down the bitcell array in the memory while powering the memory periphery responsive to an assertion of both the sleep mode without retention control signal and the dynamic voltage stress scan control signal; and performing a dynamic voltage stress scan of the memory periphery while the memory periphery is powered and the bitcell array is powered off following the assertion of both the sleep mode without retention control signal and the dynamic voltage stress scan control signal. 
     In accordance with yet another aspect of the disclosure, a memory is provided that includes: a memory periphery including a memory power domain portion; a periphery head switch coupled between a node for a memory power supply voltage and the memory power domain portion; and a power management circuit including a first logic gate configured to assert a dynamic voltage stress scan control signal responsive to an assertion of a core power domain control signal, wherein the power management circuit is configured to switch off the periphery head switch responsive to an assertion of a sleep mode without retention control signal while the dynamic voltage stress scan control signal is de-asserted, and wherein the power management circuit is further configured to keep the periphery head switch on responsive to an assertion of both the sleep mode without retention control signal and the dynamic voltage stress scan control signal. 
     These and additional advantageous features may be better appreciated through the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates an integrated circuit with an embedded memory configured for an improved DVS scan in accordance with an aspect of the disclosure. 
         FIG.  2    illustrates a first portion of a power management circuit in the embedded memory of  FIG.  1    in accordance with an aspect of the disclosure. 
         FIG.  3    illustrates a second portion of the power management circuit in the embedded memory of  FIG.  1    in accordance with an aspect of the disclosure. 
         FIG.  4    illustrates a third portion of the power management circuit in the embedded memory of  FIG.  1    in accordance with an aspect of the disclosure. 
         FIG.  5    is a flowchart for a method of operation of a memory configured for an improved DVS scan in accordance with an aspect of the disclosure. 
         FIG.  6    illustrates some example electronic systems each incorporating a memory with configured for an improved DVS in accordance with an aspect of the disclosure. 
     
    
    
     Implementations of the present disclosure and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures. 
     DETAILED DESCRIPTION 
     An integrated circuit memory with an improved DVS scan is provided. To better appreciate the advantageous features of this improved DVS scan, consider the challenges that a successful DVS scan should overcome. For example, an integrated circuit memory is typically segregated into its own power domain denoted herein as an MX power domain whereas a core logic of the integrated circuit is segregated into a core power domain denoted herein as a CX power domain. An MX power supply voltage powers the MX power domain. Similarly, a CX power supply voltage powers the CX power domain The memory includes a bitcell array that is controlled by a memory periphery (write drivers, address decoders, and so on). A portion of the memory periphery is in the CX power domain whereas a remaining portion is in the MX power domain. 
     While a memory is tested by a typical DVS scan, both the bitcells and the memory periphery are powered. Due to the elevated MX supply voltage, substantial leakage current typically conducts from the bitcells during the DVS scan. In addition, the memory periphery consumes current from both the MX and CX power domains. The bitcell leakage combined with the current draw by the memory periphery may cause damaging temperature spikes. To prevent a DVS scan from damaging the memory, it is thus conventional to limit the MX power supply voltage increase. Although this limit on the power supply voltage increase keeps the memory temperature in a safe range, the weakened DVS then fails to unmask faults, leading to undesirable failures during customer use and costly returns. 
     To limit complexity and DVS scan latency, it is advantageous that a DVS scan be compatible with the control signals for memory sleep modes using during normal (non-DVS scan) operation. In that regard, an integrated circuit with a core logic power domain (the CX power domain) and a memory power domain (the MX power domain) will typically include a power management circuit to control whether the bitcell array and the memory periphery are powered on or off during memory sleep modes. The power management circuit responds to sleep control signals for the sleep modes that originate in the core logic power domain. For example, should the core logic determine that the operating conditions are such that the memory may be placed into a sleep mode, the core logic may then assert a sleep mode control signal to the power management circuit. 
     With respect to the power management circuit responding to an asserted sleep mode control signal, a signal is deemed herein to be “asserted” when the signal is logically true, regardless of whether the logical true state is represented by an active-high or active-low convention. In an active-high convention, a CX power domain control signal is asserted by being charged to the CX power supply voltage. Such a signal is thus de-asserted by being discharged to ground. But in an active-low convention, a CX power domain control signal is asserted by being discharged to ground. An active-low signal is thus de-asserted by being charged to the power supply voltage. The following discussion will assume that the core logic uses an active-low convention for the sleep mode control signals without loss of generality. 
     In general, there are two types of sleep modes for an integrated circuit memory. In a sleep mode without retention, both the bitcells and the memory periphery are powered down. To activate the sleep mode without retention, the core logic may assert an active-low sleep mode without retention control signal (slp_nret_n) that is also denoted herein as the “slp_nret_n control signal” for brevity. The power management circuit responds to the asserted sleep mode without retention control signal by powering down the bitcell array and the memory periphery. Since the bitcells are powered down, they cannot retain their stored bits; hence the “without retention” designation of the sleep mode without retention. In a sleep mode with retention, the core logic may assert an active-low sleep mode with retention control signal (slp_ret_n) that is denoted herein as the “slp_ret_n control signal” for brevity. The power management circuit responds to the asserted sleep mode with retention control signal by powering down only the memory periphery whereas the bitcell array remains powered. Since the bitcell array remains powered, the bitcells may retain their stored binary content; hence the “with retention” aspect of the sleep mode with retention. 
     Neither sleep mode control signal is asserted during a traditional DVS scan. For example, should the slp_nret_n control signal be asserted, then both the bitcell array and memory periphery are powered down and cannot be tested by any scanned-in test vectors. Similarly, if the slp_ret_n control signal was asserted, then the memory periphery is powered down and cannot be tested. Since neither the sleep mode with retention nor the sleep mode without retention can be active during a traditional DVS scan, both the bitcell array and the memory periphery are powered and hence a damaging temperature increase as noted earlier may occur. 
     To address the combination of the bitcell leakage and the current drawn by the memory periphery causing memory damage during a DVS scan, an integrated circuit memory is provided in which an improved DVS scan may occur while the sleep mode without retention control signal slp_nret_n is asserted. To do so, the memory&#39;s power management circuit is configured to respond to an assertion of a control signal (e.g., an asynchronous control signal) by blocking the asserted slp_nret_n control signal from triggering a shutdown of the memory periphery. Although the powering off of the memory periphery is blocked, the power management circuit still responds to the asserted slp_nret_n control signal by powering off the bitcell array. The bitcell leakage is thus eliminated and cannot combine with the current draw by the memory periphery to cause damaging temperature spikes during the DVS scan. The power supply voltage(s) may thus be sufficiently boosted and for a sufficient duration during the DVS scan to uncover faults in the periphery. Note that a DVS scan is just one example use of the advantageous powering of the memory periphery while the bitcell array is powered off. It will thus be appreciated that the memory control discussed herein is applicable to other types of memory scans in addition to DVS scans. 
     Turning now to the drawings, an example system-on-a-chip (SoC) integrated circuit  100  with a core logic circuit  105  and an associated embedded static random-access memory (SRAM)  106  is shown in  FIG.  1   . Core logic circuit  105  is located within a core (CX) power domain and is thus powered by a CX power supply voltage. SRAM  106  includes a bitcell array  110 , a memory periphery  115 , and word line (WL) driver  125 . Bitcell array  110  and WL driver  125  are located within a memory (MX) power domain and are thus powered by an MX power supply voltage. Memory periphery  115  has an MX power domain portion in the MX power domain and a CX power domain portion in the CX power domain Memory periphery  115  includes the components for reading and writing from bitcell array  110  such as write drivers, address decoders, and so on. 
     During normal operation of SRAM  106 , core logic  105  does not invoke any memory sleep modes. But during dormant periods, core logic  105  may completely power down SRAM  106  by asserting the sleep mode without retention control signal (slp_nret_n). Power management circuit  120  responds to the assertion of the sleep mode without retention control signal by asserting head switch control signals to switch off the head switches powering SRAM  106 . Each head switch is represented by a single p-type metal-oxide semiconductor (PMOS) transistor. For example, bitcell array  110  couples to a node for the MX power supply voltage through a PMOS transistor P 1 . Transistor P 1  is an example of a bitcell array head switch. Similarly, WL driver  125  couples to a node for the MX power supply voltage through a PMOS transistor P 4 . The CX portion of memory periphery  115  couples to a node for the CX power supply voltage through a PMOS transistor P 2  whereas the MX portion of memory periphery  115  couples to a node for the MX power supply voltage through a PMOS transistor P 3 . Transistors P 2  and P 3  are examples of periphery head switches. 
     It will be appreciated that each of the head switches may be implemented by multiple PMOS transistors instead of a single PMOS transistor as illustrated. To switch off transistor P 1  during a sleep mode without retention in response to the assertion of the slp_nret_n control signal, power management circuit  120  asserts an active-high MX power domain core sleep signal (slp_core) that drives a gate of transistor P 1 . Transistor P 1  will thus switch off to power down bitcell array  110 . Similarly, power management circuit  120  asserts an active-high MX power domain periphery sleep signal (slp_peri) that drives a gate of transistor P 3  to power down the MX portion of memory periphery  115  in response to the assertion of the slp_nret_n control signal. In the same fashion, power management circuit  120  asserts an active-high CX power domain periphery sleep signal (slp_peri_CX) that drives a gate of transistor P 2  to power down the CX portion of the memory periphery  115  in response to the assertion of the slp_nret_n control signal. Finally, power management circuit  120  also asserts an active-high MX power domain word line driver sleep signal (slp_wl) that drives a gate of transistor P 4  to power down WL driver  125  in response to the assertion of the slp_nret_n control signal. 
     Should core logic  105  instead determine that a sleep mode with retention should be invoked, core logic  105  may assert the slp_ret_n control signal. In response to the assertion of the slp_ret_n control signal, power management circuit  120  asserts the slp_wl, slp_peri_CX, and slp_peri signals to cut off the power to memory periphery  115  and WL driver  125 . But power management circuit  120  does not assert the slp_core signal in response to the assertion of the slp_ret_n control signal. Thus, bitcell array  110  remains powered so that its bitcells may retain their stored binary content. 
     To invoke a DVS scan of memory periphery  115 , a DVS tester or scan tool (not illustrated) may trigger an assertion of a DVS scan control signal (not illustrated in  FIG.  1    but discussed further below) while SRAM  106  is functional (no sleep mode being active). To trigger the assertion of the DVS scan control signal, it is convenient if the DVS tester or scan tool asserts asynchronous control bits in an asynchronous control signal (ACC) received over a plurality of terminals  140  that may be also used for other asynchronous control signals to integrated circuit  100 . In this fashion, the input/output interface to integrated circuit  100  need not be modified to accommodate the improved DVS scan disclosed herein. More generally, power management circuit  120  asserts the DVS scan control signal in response to an assertion of CX power domain control signal. It will thus be appreciated that alternative implementations may use other types of control signals beside the ACC control signal. In addition to asserting the ACC signal, the DVS tester may also trigger an assertion of the slp_nret_n control signal. During normal operation, power management circuit  120  would respond to the assertion of the slp_nret_n control signal by powering off not only bitcell array  110  but also memory periphery  115  and WL driver  125 . However, power management circuit  120  is configured to respond to the assertion of both the DVS scan control signal and the slp_nret_n control signal by powering off only bitcell array  110 . Memory periphery  115  thus remains powered during the DVS scan despite the assertion of the slp_nret_n control signal. In this fashion, only transistor P 1  switches off whereas transistors P 2 , P 3 , and P 4  remain on. As noted earlier, each of transistors P 1 , P 2 , P 3 , and P 4  may comprise multiple head switch transistors in alternative implementations. There is thus at least one head switch for each of bitcell array  110 , the CX portion of memory periphery  115 , the MX portion of memory periphery  115 , and WL driver  125 . With transistor P 1  off and transistors P 1 , P 3 , and P 4  on, a DVS scan of memory periphery  115  may proceed without requiring a limited increase in the power supply voltage(s). 
     Without this advantageous DVS scan as managed by power management circuit  120 , the CX and MX power supply voltages could not be raised as robustly during a DVS scan of SRAM  106 . The actual level of the increased power supply voltage used during a DVS scan of memory periphery  115  will depend upon the process node. In one example implementation, the increased power supply voltage during a traditional DVS scan could only be raised to 1.4 V. But with the power management circuit  120  responding to the DVS scan control signal as discussed herein, the DVS scan may instead be performed with a 1.6 V power supply voltage level and for a longer duration. In this fashion, the DVS scan of SRAM  106  may be sufficiently robust to uncover faults that would otherwise remain undetected. 
     An example implementation of power management circuit  120  will now be described in more detail.  FIG.  2    illustrates a first portion  200  of power management circuit  120  that generates the core sleep signal slp_core responsive to the slp_nret_n control signal. Note that the slp_nret_n control signal is a CX power domain signal. In a sleep mode with retention, the CX power domain may be powered down but the MX power domain remains powered. To keep first portion  200  from undesirably responding to CX power domain control signals when the CX power domain is powered down, SRAM  106  receives an active-high MX power domain control signal (clamp_mem) that is asserted when the CX power domain is powered down. 
     To assist in the retention of a desired binary state for the core sleep signal slp_core despite the CX power domain being powered off, power management circuit  120  includes a latching level-shifter  210  that latches and level-shifts the clamp_mem signal from the MX power domain to the CX power domain. Level-shifter  210  also receives an inverted version of the clamp_mem signal as inverted by an inverter  230 . The level-shifted version of the clamp_mem signal drives a gate of a PMOS transistor P 5  that has its source connected to a node for the CX power supply voltage. The drain of transistor P 5  serves as a node for a clamp_nor signal that will equal the CX power supply voltage when the clamp_mem signal is de-asserted. In this implementation, the clamp_mem signal is an active-high signal and is thus de-asserted by being grounded. The clamp_nor signal will then equal the CX power supply voltage only when the clamp_mem signal is a logic zero. 
     The clamp_nor signal powers an inverter  215 , a NOR gate  220  and a NOR gate  225 . The following discussion will assume that the clamp_mem signal is discharged to ground (the CX power domain not being powered down) to cause the clamp_nor signal to be charged to the CX power supply voltage. Inverter  215  functions to invert the sleep mode without retention control signal (slp_nret_n) into an inverted signal processed by NOR gate  225  with the clamp_mem signal. In addition, NOR gate  220  NORs the slp_nret_n control signal with the clamp_mem signal. NOR gates  220  and  225  will each act as an inverter while the clamp_mem signal is de-asserted. If the slp_nret_n control signal is then asserted (recall that the slp_nret_n control signal may be an active-low signal such that is grounded when asserted), the output of NOR gate  220  is asserted to the CX power supply voltage whereas the output of NOR gate  225  is grounded. A latching level-shifter  205  shifts the output of NOR gate  220  from the CX power domain to the MX power domain. An output signal  235  of level-shifter  205  is charged to the MX power supply voltage in response to the assertion the slp_nret_n control signal while the clamp_mem signal is de-asserted. A pair of inverters  240  and  245  buffer the output signal  235  to form the core sleep signal slp_core. The core sleep signal slp_core will thus be asserted to equal the MX power supply voltage in response to the assertion of the slp_nret_n control signal while the clamp_mem signal is de-asserted. The asserted core sleep signal slp_core shuts off the head switch(es) as represented by transistor P 1  ( FIG.  1   ) so that bitcell array  110  is powered down in response to the assertion of the slp_nret_n control signal. 
     The output signal of NOR gate  225  equals the slp_nret_n control signal so long as the clamp_mem signal is de-asserted. Conversely, the output signal of NOR gate  220  equals the complement of the slp_nret_n control signal so long as the clamp_mem signal is de-asserted. Since level-shifter  205  is level-shifting this complement of the slp_nret_n control signal, the output of level-shifter  205  is indicated with an “inversion bubble” to indicate that it is level-shifting the complement of the slp_nret_n control signal instead of level-shifting the slp_nret_n control signal itself. 
     If the slp_nret_n control signal is de-asserted (charged to the CX power supply due to its active-low implementation), the output of NOR gate  220  will be discharged whereas the output of NOR gate  225  will be charged to the CX power supply voltage. The output signal  235  of level-shifter  205  will thus be discharged to ground, which grounds the core sleep signal slp_core. Bitcell array  110  is thus powered while the slp_nret_n control signal is de-asserted. 
     A portion  300  of power management circuit  120  is shown in  FIG.  3    for the generation of the slp_peri and the slp_peri_CX signals responsive to the slp_nret_n control signal and also to the slp_ret_n control signal. Recall that output signal  235  was generated as discussed regarding portion  200  of power management circuit  120  responsive to the slp_nret_n control signal. A logic gate such as a NAND gate  330  processes output signal  235  with a DVS scan control signal that is also denoted herein as the DVS_SLP signal for brevity. As will be explained further herein, power management circuit  120  asserts the DVS_SLP signal during a DVS scan so that the asserted slp_nret_n control signal does not cause a cutoff of power to memory periphery  115 . It will be assumed herein without loss of generality that the DVS_SLP signal is an active-low signal so that it is de-asserted during normal operation by being charged to the MX power supply voltage. During normal (non-DVS scan) operation, NAND gate  330  thus functions as an inverter to drive a NAND gate  335  with an inverted version of output signal  235 . As will be explained further herein, NAND gate  335  NANDs the inverted version of output signal  235  with a level-shifted version (ls_slp_ret_n) of the sleep mode with retention control signal (slp_ret_n). During a sleep mode without retention, the ls_slp_ret_n signal is charged to the MX power supply voltage so that NAND gate  335  acts as an inverter. Thus, an output signal  340  of NAND gate  335  will equal output signal  235  during the sleep mode without retention (the DVS_SLP signal not being asserted). NAND gate  330  is an example of a second logic gate configured to process the DVS scan control signal to prevent the assertion of a periphery sleep signal. NAND gate  335  is an example of a third logic gate configured to process an output signal of the second logic gate with a memory power domain sleep mode with retention control signal (e.g., ls_slp_ret_n). 
     A NAND gate  345  NANDs output signal  340  with the slp_wl signal. As will be explained further herein, the slp_wl signal is asserted to the MX power supply voltage during a sleep mode, regardless of whether it is with retention or without. NAND gate  345  will thus function as an inverter during a sleep mode without retention to invert output signal  340  into a complement (slp_n_peri) of the slp_peri signal. An inverter  350  inverts the complement signal slp_n_peri to form the slp_peri signal. During the sleep mode without retention, the slp_peri signal will thus be asserted to the MX power supply voltage to cutoff power to the MX power domain portion of memory periphery  115 . 
     A latching level-shifter  360  level-shifts the slp_peri signal from the MX power domain to the CX power domain. A pair of serially-arranged inverters  365  and  370  buffer an output signal of level-shifter  360  to form the slp_peri_CX signal. During the sleep mode without retention, the slp_peri_CX signal will thus be asserted to the CX power supply voltage to cutoff power to the CX power domain portion of memory periphery  115 . Memory periphery  115  is thus powered down during the sleep mode without retention. 
     An inverter  310 , a NOR gate  320 , and as NOR gate  315  are powered by the clamp_nor signal. In a sleep mode with retention, the sleep mode with retention control signal (slp_ret_n) is asserted by being discharged to ground. Inverter  310  inverts the slp_ret_n control signal to drive NOR gate  315 , which also receives the clamp_mem signal. With the CX power domain being powered, NOR gate  315  thus acts as an inverter to invert the inverted output signal from inverter  310  to reproduce the slp_ret_n control signal. Another NOR gate  320  NORs the slp_ret_n control signal with the clamp_mem signal. With the CX power domain remaining powered, NOR gate  320  thus acts as an inverter to invert the slp_ret_n control signal to provide an inverted version of the slp_ret_n control signal. A latching level-shifter  305  levels shifts the inverted version of the slp_ret_n control signal from the CX power domain to the MX power domain (an output node of level-shifter  305  is thus indicated with an inversion bubble). An output signal of level-shifter  305  will then be asserted to the MX power supply voltage during the sleep mode with retention. A NOR gate  325  NORs the output signal of level-shifter  305  with the clamp_mem signal. NOR gate  325  thus acts as inverter while the CX power domain remains powered to invert the output signal of level-shifter  305  into the level-shifted version ls_slp_ret_n of the slp_ret_n control signal. 
     During normal operation (the DVS scan being inactive), the DVS_SLP signal is charged to the MX power supply so that NAND gate  330  inverts output signal  235 . Since output signal  235  is a level-shifted and inverted version of the slp_nret_n control signal, output signal  235  is thus discharged to ground during the sleep mode with retention. The output signal of NAND gate  330  will then be charged to the MX power supply voltage during the sleep mode with retention. This charging of the output signal of NAND gate  330  forces NAND gate  335  to function as an inverter. Output signal  340  of NAND gate  335  will thus be charged to the MX power supply voltage during the sleep mode with retention. As will be explained further herein, the slp_wl control signal is asserted during the sleep mode with retention so that NAND gate  345  inverts output signal  340  to cause the output signal of NAND gate  345  to be discharged to ground. The slp_peri and the slp_peri_CX signals will thus both be charged to cut off power to the memory periphery  115  during the sleep mode with retention analogously as discussed for the sleep mode without retention. However, bitcell array  110  remains powered because the slp_nret_n control signal is de-asserted (charged to the CX power supply voltage) while the sleep mode with retention is active. 
     The transition to the DVS scan occurs from a functional (non-sleep) mode of operation for SRAM  106  as will be explained further herein. During such a functional mode, bitcell array  110  and memory periphery  115  are powered and thus the slp_nret_n control signal and the slp_ret_n control signal are charged to the CX power supply voltage. But the slp_nret_n control signal is then discharged during the transition to the DVS scan. Since the slp_nret_n control signal is discharged, output signal  235  is charged to the MX power supply. The DVS_SLP signal is asserted by being discharged to ground in the transition from the functional mode to the DVS scan. This assertion of the DVS_SLP signal by being discharged to ground while output signal  235  is asserted forces the output signal of NAND gate  330  to be charged to the MX power supply voltage. The level-shifted version ls_slp_ret_n of the slp_ret_n control signal is also charged to the MX power supply during the DVS scan. Output signal  340  of NAND gate  335  will thus be discharged to ground during the DVS scan, which forces the output of NAND gate  345  to be asserted to the MX power supply voltage. The slp_peri and the slp_peri_CX signals will thus be de-asserted by being grounded during the DVS scan so as to keep memory periphery  115  powered despite the assertion of slp_nret_n control signal. This is quite advantageous with regard to limiting complexity as the DVS scan may function without requiring a rework or modification of the control of the sleep modes by the slp_nret_n and slp_ret_n control signals. 
     A portion  400  of power management circuit  120  is shown in  FIG.  4    for the generation of the DVS scan control signal (DVS_SLP) and the slp_wl signal. With regard to the generation of the DVS_SLP signal, an integrated circuit such as integrated circuit  100  will typically include a number of asynchronous control pins or terminals such as terminals  140  (shown in  FIG.  1   ) so that integrated circuit  100  may be configured as desired by a user. It is thus convenient for the DVS_SLP signal to be asserted responsive to an asynchronous control (ACC) signal as controlled by an external DVS tester (not illustrated) through terminals  140 . In portion  400 , three one-bit asynchronous control signals are used by the external DVS tester to generate the ACC signal. The ACC signal is thus three bits wide, but it will be appreciated that other bit widths may be used in alternative implementations. A NAND gate  430  NANDs the ACC control signal bits. An output signal of NAND gate  430  will thus be discharged responsive to an assertion of the ACC control signal bits to the CX power supply voltage. A NOR gate  425  NORs an active-low scan control signal scan_n with the output signal of NAND gate  430 . In this fashion, the assertion of the ACC signal during a non-DVS scan mode of operation is prevented from activating a DVS scan. An output signal of NOR gate  425  will be asserted to the CX power supply voltage in response to the assertion of the ACC control signal by the external DVS tester since the scan_n signal will be discharged. An inverter  420  inverts the output signal of NOR gate  425 . An output signal of inverter  420  will thus be discharged to ground in response to the assertion of the ACC and scan_n signals. A NOR gate  410  NORs the output signal of inverter  420  with the clamp_mem signal (the following discussion assumes that the clamp_mem signal is discharged due to the CX power domain being initially powered). An output signal of NOR gate  410  will thus be asserted to the CX power supply in response to the assertion of the ACC control signal to trigger a DVS scan. A NOR gate  415  NORs the clamp_mem signal with the output signal of NOR gate  425 . An output signal of NOR gate  415  will thus be discharged to ground in response to the assertion of the ACC control signal during a DVS scan. NAND gate  430 , NOR gate  425 , inverter  420 , NOR gate  415 , and NOR gate  410  are all powered by the clamp_nor signal and are thus powered so long as the clamp_mem signal is de-asserted. 
     A latching level-shifter  405  level-shifts the output signal of NOR gate  410  from the CX power domain to the MX power domain. An output signal of level-shifter  405  will thus be asserted to the MX power supply voltage in response to the assertion of ACC control signal and the scan_n signal. A NAND gate  460  NANDs the output signal of level-shifter  405  with an output signal of an inverter  450  to form the DVS scan control signal DVS_SLP. An inverter  455  inverts the output signal of inverter  450  to form the slp_wl signal. The output signal of inverter  450  is thus asserted to the MX power supply voltage while SRAM  106  is functional, which causes NAND gate  460  to function as an inverter. NAND gate  460  will thus discharge the DVS_SLP signal to ground in response to the assertion of ACC control signal while the scan_n signal is asserted. Level-shifter  405  may also be denoted herein as a first level-shifter. The output signal of level-shifter  405  may also be denoted herein as a memory power domain control signal. NAND gate  460  is an example of a first logic gate configured to process the memory power domain control signal to assert the DVS_SLP signal. Level-shifter  305  may also be denoted herein as a second level-shifter. 
     In a power up from a power down state, SRAM  106  may be programmed to power up in the sleep mode without retention. The slp_nret_n control signal is then charged during the power up of SRAM  106  to cause SRAM  106  to transition to the functional mode of operation. However, note that the ACC signal and the scan_n signal are CX power domain signals. Because the CX power domain is initially unstable during the memory power up it is thus possible that the ACC control signal is asserted while the slp_nret_n control signal and the scan_n signal are still discharged. The unintentional assertion of the ACC control signal could then cause an unintentional assertion of the DVS_SLP signal. The unintentional assertion of the DVS_SLP signal could then trigger a power-on of memory periphery  115  prior to the charging of the slp_nret_n control signal. Memory periphery  115  may then have increased leakage during the delay from the unintentional assertion of the DVS_SLP signal to the intentional charging of the slp_nret_n control signal. To prevent memory periphery  115  from powering on until the slp_nret_n control signal is charged during the power up of SRAM  106 , the assertion of the DVS_SLP signal is gated by a clock signal (clk). The DVS_SLP signal may thus only be asserted following a triggering clock edge (e.g., a rising edge) of the clock signal. 
     To perform this clock gating, portion  400  includes a set-reset latch  465  such as formed by a cross-coupled pair of NAND gates  440  and  435 . A drain of a PMOS transistor P 6  that is connected to a drain of an NMOS transistor M 2  that is also connected to the drain of a PMOS transistor P 7  functions as an input node to NAND gate  435 . An inverter  445  inverts output signal  340  from portion  300  of power management circuit  120  to drive NAND gate  440  in latch  465 . The source of transistor M 2  couples through an NMOS transistor M 1  to ground. The complement signal slp_n_peri drives the gates of transistors M 1  and P 7 . During the functional mode for SRAM  106 , the slp_wl signal is discharged to ground and the complement signal slp_n_peri is charged to the MX power supply voltage. At the transition from the functional mode to a DVS scan, transistor M 1  will thus be on whereas transistor P 7  will be off. 
     During the power-up of SRAM  106 , the slp_wl signal may initially be charged to the MX power supply voltage. The output signal from inverter  450  is thus discharged to ground. The output signal of inverter  450  drives a gate of a PMOS transistor P 8  and a gate of a PMOS transistor P 9 . The sources of transistors P 8  and P 9  couple to a node for the MX power supply voltage. A drain of transistor P 8  couples to a source of transistor P 6 . Similarly, a drain of transistor P 9  couples to a source of transistor P 7 . With the slp_wl control signal being charged, transistors P 8  and P 9  are thus conducting. Since the clock signal drives the gates of serially-coupled transistors P 6  and M 2 , transistors P 6  and M 2  function as an inverter while transistors P 8  and M 1  are conducting. Prior to the cycling of the clock signal, the clock signal will be discharged such that transistor P 6  is on to charge the input signal to NAND gate  435  to the MX power supply voltage. Since the output signal of inverter  450  is discharged, the output signal of NAND gate  440  is charged to the MX power supply voltage. An output signal of NAND gate  435  is thus discharged while the clock signal is low during the power up of SRAM  106 . This discharged output signal of NAND gate  435  forces the output signal of NAND gate  440  to be charged to the MX power supply regardless of whether the DVS_SLP signal is asserted by being discharged. The slp_wl signal will thus remain charged prior to the cycling of the clock signal. 
     At the rising edge of the clock signal (the clock signal transitioning from ground to the MX power supply voltage), the drain of transistor P 6  is discharged to ground. This causes the output signal of NAND gate  435  to be charged to the MX power supply voltage. In turn, the charging of the output signal of NAND gate  435  causes NAND gate  440  to function as an inverter. As discussed previously, the charging of the slp_nret_n control signal while the DVS scan control signal DVS_SLP is charged, causes output signal  340  to be discharged and in turn causes the output signal of inverter  445  to be charged to the MX power supply voltage. The output signal of NAND gate  440  will thus be discharged at the rising edge of the clock signal when the slp_nret_n control signal is charged, which causes the output signal of inverter  450  to be charged to the MX power supply voltage and causes the slp_wl signal to be discharged. The charging of the output signal of inverter  450  to the MX power supply voltage then causes NAND gate  460  to function as an inverter so that the DVS_SLP signal may be discharged. Since the DVS_SLP signal cannot be discharged until the rising edge of the clock (and also the charging of the slp_nret_n control signal), the potential leakage of memory periphery  115  from an unintentional assertion of the ACC control signal prior to the subsequent intentional charging of the slp_nret_n control signal is solved by the clock gating of the DVS_SLP signal. 
     A method of operation for a memory configured for an improved DVS scan will now be discussed with regard to the flowchart of  FIG.  5   . The method includes an act  500  of powering down a bitcell array and a memory periphery in the memory responsive to an assertion of a sleep mode without retention control signal while a dynamic voltage stress scan control signal is not asserted. The assertion of the slp_core, slp_peri_CX, and the slp_peri control signals to power down bitcell array  110  and memory periphery  115  during the sleep mode without retention is an example of act  500 . The DVS_SLP signal is an example of the dynamic voltage stress scan signal. 
     The method also includes an act  505  of powering down the bitcell array in the memory while powering the memory periphery responsive to an assertion of both the sleep mode without retention control signal and the dynamic voltage stress scan control signal. The assertion of only the slp_core control signal to power down bitcell array  110  while memory periphery  115  is powered during a DVS scan is an example of act  505 . 
     Finally, the method includes an act  510  of performing a dynamic voltage stress scan of the memory periphery while the memory periphery is powered and the bitcell array is powered off following the assertion of both the sleep mode without retention control signal and the dynamic voltage stress scan control signal. The DVS scanning of memory periphery  115  by an external DVS tester is an example of act  510 . 
     A memory configured for an improved DVS scan as disclosed herein may be incorporated into a wide variety of electronic systems. For example, as shown in  FIG.  6   , a cellular telephone  600 , a laptop computer  605 , and a tablet PC  610  may all include a memory configured for an improved DVS scan in accordance with the disclosure. Other exemplary electronic systems such as a music player, a video player, a communication device, and a personal computer may also be configured with memories constructed in accordance with the disclosure. 
     Some aspects of the disclosure will now be summarized in the following series of example clauses:
     Clause 1. A memory, comprising:   

     a bitcell array; 
     a bitcell array head switch coupled between the bitcell array and a node for a memory power supply voltage; 
     a memory periphery including a memory power domain portion; 
     a memory periphery head switch coupled between the memory power domain portion and the node for the memory power supply voltage; and 
     a power management circuit configured to switch off the bitcell array head switch and the memory periphery head switch to power off the bitcell array and the memory power domain portion during a sleep mode without retention for the memory, the power management circuit being further configured to switch off only the bitcell array head switch to power off the bitcell array and to maintain a power on of the memory power domain portion during a scan of the memory.
     Clause 2. The memory of clause 1, wherein the bitcell array head switch is configured to switch off responsive to an assertion of a sleep core signal and the memory periphery head switch is configured to switch off responsive to an assertion of a periphery sleep signal, and wherein the power management circuit is further configured to assert both the sleep core signal and the periphery sleep signal responsive to an assertion of a sleep mode without retention control signal during the sleep mode without retention for the memory,   

     the power management circuit being further configured to assert the sleep core signal and to not assert the periphery sleep signal responsive to an assertion of the sleep mode without retention control signal during the scan of the memory.
     Clause 3. The memory of clause 2, wherein the power management circuit further includes:   

     a first level-shifter configured to level-shift a core power domain control signal into a memory power domain control signal; and 
     a first logic gate configured to process the memory power domain control signal to assert a DVS scan control signal, wherein the power management circuit is further configured to respond to an assertion of the DVS scan control signal to prevent an assertion of the periphery sleep signal.
     Clause 4. The memory of clause 3, wherein the power management circuit further includes:   

     a second logic gate configured to process the DVS scan control signal to prevent the assertion of the periphery sleep signal.
     Clause 5. The memory of clause 2, wherein the memory periphery further includes a core power domain portion, the memory further comprising:   

     a core power domain head switch coupled between the core power domain portion and the node for the memory power supply voltage, the core power domain head switch being configured to switch off to power down the core power domain portion responsive to an assertion of a core power domain periphery sleep signal, wherein the power management circuit is further configured to assert the core power domain periphery sleep signal responsive to the assertion of the sleep mode without retention control signal while the DVS scan control signal is not asserted, the power management circuit being further configured to not assert the core power domain periphery sleep signal responsive to the assertion of the sleep mode without retention control signal while the DVS scan control signal is asserted.
     Clause 6. The memory of any of clauses 3-4, wherein the power management circuit further includes:   

     a second level-shifter configured to level-shift the sleep mode without retention control signal into a memory power domain signal, and wherein the second logic gate is configured to process the DVS scan control signal with the memory power domain signal to prevent the assertion of the periphery sleep signal.
     Clause 7. The memory of any of clauses 4-5, wherein the power management circuit is further configured to assert both assert the periphery sleep signal and to not assert the sleep core signal responsive to an assertion of a sleep mode with retention control signal.   Clause 8. The memory of clause 7, wherein the power management circuit further includes:   

     a second level-shifter configured to level-shift the sleep mode with retention control signal into a memory power domain sleep mode with retention control signal; and 
     a third logic gate configured to process an output of the second logic gate with the memory power domain sleep mode with retention control signal.
     Clause 9. The memory of any of clauses 4-8, wherein the power management circuit further includes:   

     a latch configured to prevent the assertion of the DVS scan control signal while a clock signal has not cycled, the latch being further configured to permit the assertion of the DVS scan control signal responsive to a rising edge of the clock signal.
     Clause 10. The memory of clause 9, wherein the latch comprises a pair of cross-coupled NAND gates.   Clause 11. The memory of clause 10, wherein the power management circuit further includes an inverter configured to invert the clock signal to drive an input to a first NAND gate in the pair of cross-coupled NAND gates.   Clause 12. The memory of any of clauses 1-11, wherein the bitcell array head switch and the memory power domain head switch each comprises at least one p-type metal-oxide semiconductor (PMOS) transistor.   

     Clause 13. A method of operation for a memory, comprising: 
     powering down a bitcell array and a memory periphery in the memory responsive to an assertion of a sleep mode without retention control signal while a dynamic voltage stress scan control signal is not asserted; 
     powering down the bitcell array in the memory while powering the memory periphery responsive to an assertion of both the sleep mode without retention control signal and the dynamic voltage stress scan control signal; and 
     performing a dynamic voltage stress scan of the memory periphery while the memory periphery is powered and the bitcell array is powered off following the assertion of both the sleep mode without retention control signal and the dynamic voltage stress scan control signal.
     Clause 14. The method of clause 13, further comprising:   

     level-shifting a core power domain control signal into a memory power domain control signal; and 
     asserting the dynamic voltage stress scan control signal responsive to an assertion of the memory power domain control signal.
     Clause 15. The method of clause 14, further comprising:   

     gating the assertion of the dynamic voltage stress scan control signal to occur only after an assertion of a clock signal.
     Clause 16. The method of any of clauses 13-15, wherein powering down the memory periphery comprising powering down a memory power domain portion and a core power domain portion.   Clause 17. The method of any of clauses 13-16, further comprising:   

     powering down a word line driver responsive to the assertion of the sleep mode without retention control signal while the dynamic voltage stress scan control signal is not asserted.
     Clause 18. The method of clause 17, further comprising:   

     maintaining a powering of the word line driver responsive to the assertion of both the sleep mode without retention control signal and the dynamic voltage stress scan control signal.
     Clause 19. A memory, comprising:   

     a memory periphery including a memory power domain portion; 
     a periphery head switch coupled between a node for a memory power supply voltage and the memory power domain portion; and 
     a power management circuit including a first logic gate configured to assert a dynamic voltage stress scan control signal responsive to an assertion of a core power domain control signal, wherein the power management circuit is configured to switch off the periphery head switch responsive to an assertion of a sleep mode without retention control signal while the dynamic voltage stress scan control signal is de-asserted, and wherein the power management circuit is further configured to keep the periphery head switch on responsive to an assertion of both the sleep mode without retention control signal and the dynamic voltage stress scan control signal.
     Clause 20. The memory of clause 19, wherein the memory is included in an integrated circuit including a plurality of terminals configured to receive the core power domain control signal, the power management circuit further including:   

     a first level-shifter configured to level-shift the core power domain control signal into a memory power domain control signal, wherein the first logic gate is further configured to assert the dynamic voltage stress scan control signal responsive to an assertion of the memory power domain control signal.
     Clause 21. The memory of clause 19, wherein the power management circuit further includes a second logic gate configured to process the dynamic voltage stress scan control signal to prevent a switching off of the periphery head switch responsive to the assertion of the dynamic voltage stress scan control signal.   Clause 22. The memory of clause 19, wherein the power management circuit is further configured to switch off the periphery head switch responsive to an assertion of a sleep mode with retention control signal.   Clause 23. The memory of any of clauses 19-22, wherein the memory is included in a cellular telephone.   

     As those of some skill in this art will by now appreciate and depending on the particular application at hand, many modifications, substitutions and variations can be made in and to the materials, apparatus, configurations and methods of use of the devices of the present disclosure without departing from the scope thereof. In light of this, the scope of the present disclosure should not be limited to that of the particular implementations illustrated and described herein, as they are merely by way of some examples thereof, but rather, should be fully commensurate with that of the claims appended hereafter and their functional equivalents.