Patent Application
20060215474
1. Field of the Invention
The invention generally relates to memory devices and, more particularly, to reducing current consumption during refresh operations.
2. Description of the Related Art
The evolution of sub-micron CMOS technology has resulted in an increasing demand for high-speed semiconductor memory devices, such as dynamic random access memory (DRAM) devices, pseudo static random access memory (PSRAM) devices, and the like. Herein, such memory devices are collectively referred to as DRAM devices.
DRAM devices utilize memory cells (also referred to as storage nodes) consisting of one transistor and one capacitor. The cells are accessed by activating a wordline, switching on the transistor and coupling the capacitor to a bit line. The stored charge on the capacitor is then sensed by a sense amplifier to determine if a logical ‘1’ or ‘0’ bit of data is stored in the accessed cell. Due to leakage current, charge stored in the capacitors may be lost to the point that the corresponding data is no longer valid.
As a result, DRAM devices need refresh operations to maintain their stored data. Refresh operations are typically performed at regular time intervals by means of activating a word line, or a number of word lines, followed by a pre-charge of the same word line or wordlines. This operation is repeated for the next word line or set of word lines until the whole chip is refreshed. Modern DRAM devices determine the word-line address for each refresh operation internally. Each refresh operation is either initiated externally, by means of an external command (e.g., a CAS before RAS or “CBR” refresh command) or internally when the device is in a “self refresh mode” or similar standby-like mode.
To optimize access to storage cells (e.g., to speed access, simplify signal routing, and/or facilitate layout), wordlines are sometimes grouped and controlled by master wordlines.
For special-purpose low power DRAM devices, such as those utilized in cellular telephones and personal digital assistants (PDAs), it is important to minimize current consumption, typically to increase battery life. As these devices often spend a large majority of their life in standby modes, requiring refresh to maintain their data (e.g., digital pictures, files, etc.), current consumption during refresh is particularly important. Unfortunately, each cycle of pre-charging and activating the master wordline during refresh operations of conventional DRAM devices results in additional current consumption.
Accordingly, it would be desirable to reduce the amount of current consumption caused by pre-charging and activating the master wordlines during refresh operations.
Embodiments of the invention generally provide techniques and apparatus for reducing the amount of current consumed during refresh operations of DRAM devices that utilize segmented wordlines.
One embodiment provides a method for refreshing memory cells accessed via wordline segments driven by master wordlines. The method generally includes in response to a first refresh command, activating a master wordline, activating and de-activating a first wordline segment driven by the master wordline; in response a second one or more refresh commands, activating and de-activating a corresponding second one or more wordline segments driven by the master wordline prior to de-activating the master wordline; and in response to a third refresh command, activating and de-activating a third wordline segment driven by the master wordline and de-activating the master wordline.
One embodiment provides a method for refreshing memory cells accessed via wordline segments driven by master wordlines. The method generally includes activating a master wordline in response to a refresh signal and, prior to de-activating the master wordline, sequentially activating a plurality of wordline segments driven by the master wordline to refresh memory cells connected thereto.
Another embodiment provides another method for refreshing memory cells accessed via wordline segments driven by master wordlines. The method generally includes (a) activating a master wordline selected based on an internally generated refresh address, (b) prior to de-activating the selected master wordline, activating a plurality of wordline segments driven by the master wordline to refresh memory cells connected thereto, (c) de-activating the master wordline, (d) adjusting the refresh address, and (e) repeating steps (a)-(c) to refresh memory cells connected to wordline segments driven by other master wordlines.
Another embodiment provides a memory device generally including a plurality of memory cells connected to wordline segments driven by master wordlines and refresh circuitry. The refresh circuitry is generally configured to activate a master wordline in response to a refresh signal and, prior to de-activating the master wordline, sequentially activate a plurality of wordline segments driven by the master wordline to refresh memory cells connected thereto.
Another embodiment provides a memory device generally including a plurality of memory cells connected to wordline segments driven by master wordlines and refresh circuitry. The refresh circuitry is generally configured to (a) activate a master wordline selected based on an internally generated refresh address, (b) prior to de-activating the selected master wordline, activate a plurality of wordline segments driven by the master wordline to refresh memory cells connected thereto, (c) de-activate the master wordline, (d) adjust the refresh address, and (e) repeat steps (a)-(c) to refresh memory cells connected to wordline segments driven by other master wordlines.
Another embodiment provides a dynamic random access memory (DRAM) device generally including a plurality of master wordlines, a plurality of wordline segments driven by the master wordlines, each having a plurality of memory cells connected thereto, and refresh circuitry. The refresh circuitry is generally configured to refresh each of the memory cells during a device refresh cycle during which each of the master wordlines is only activated once to refresh memory cells connected to wordline segments driven thereby.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
Embodiments of the invention generally provide techniques and apparatus that may be utilized to reduce current consumption during refresh cycles of DRAM devices that utilize wordline segments. Rather than activate and subsequently de-activate (pre-charge) a master wordline each time a corresponding wordline segment is refreshed, the master wordline may remain activated while corresponding wordline segments are refreshed. As a result, the current needed to charge and discharge the master wordlines and devices connected thereto may be reduced. In fact, each master wordline may only need to be activated one time while refreshing the entire device, which may reduce overall current consumption when the device is in standby mode.
As will be described in greater detail below, for some embodiments, a single wordline segment may be activated for each internally generated or externally supplied refresh command. For such embodiments, the wordline segment refreshed with each command may be determined by an internally generated row address count adjusted (e.g., incremented) with each command. Access control circuitry may be configured to activate a given master wordline when detecting the row address count corresponding to the first corresponding wordline segment driven by that master wordline. The access control circuitry may subsequently deactivate the master wordline only after detecting the row address count corresponding to the last wordline segment driven by the master wordline. For other embodiments, multiple wordline segments (e.g., all segments driven by a selected master wordline) may be activated with each refresh command.
An Exemplary Dram Device
Depending on such a standby mode, refresh operations for each wordline (or row) in the arrays 305 may be initiated internally or externally. For example, a memory controller 302 may issue refresh commands (e.g., CBR commands), via command lines 306 interpreted by command decoder 310, to the device 304 to initiate refresh operations. Alternatively, the memory controller 302 can place the device 304 in a self-refresh mode whereby refresh operations are generated by an internal self-refresh timer 316. In either case, the particular row of cells refreshed is typically determined by a refresh address generated by a row address counter 314 that is automatically incremented with each refresh operation.
A row address multiplexer 320 may select between the row address generated by the row address counter 314 and a row address generated by an address decoder and latch circuit 311 based on an externally supplied address. In other words, the multiplexer 320 may be controlled such that the row address counter value is selected during refresh operations, while the address generated by the address decoder and latch circuit 311 will be selected during active (read/write) accesses by the memory controller 302.
Master Wordline Refresh Activation
Access control circuitry 318 may be generally configured to activate rows corresponding to the row address received from the row address multiplexer 320. The access control circuitry 318 may be configured to keep master wordlines active, during refresh operations, while each wordline segment driven by the master wordline is refreshed. As previously described, depending on the embodiment, a single wordline segment or multiple wordline segments may be refreshed with each refresh command.
As illustrated in
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Various approaches may be used to achieve the “extended” activate cycle 404 for a selected master wordline. For example, as illustrated in
As illustrated in
As illustrated, wordline segment select logic 620 may generate wordline segment control signals (SEG<3:0>) based on the lower bits of the row address. For some embodiments, rather than control the pre-charge cycles based on the lower bits of the row address, the master wordline activate logic 610 may examine a wordline segment control signal (e.g., SEG3=“1”) for the final wordline segment being refreshed.
In some cases, a refresh counter may be configured to generate refresh row addresses in a manner different than simply incrementing through a counter. For example, in some cases, a Gray code counting technique may be employed in an effort to minimize the number of the row address bits that change each count (thereby reducing the amount of logic switching and corresponding current consumption). In such cases, rather than simply examine the lower bits of the row address to determine which segment is selected, some other type of decoding to translate a Gray code output may be utilized.
Exemplary Refresh Operations
At step 708, a loop of operations 710-714 for refreshing each wordline segment controlled by the selected master wordline while the selected wordline is kept active. At step 710, a wordline segment driven by the activated master wordline is activated in order to refresh any cells connected thereto. At step 712, the activated wordline segment is pre-charged (de-activated). At step 714, the refresh counter is incremented, thereby selecting the next wordline segment.
Once the operations 710-714 have been repeated to refresh every wordline segment driven by the selected master wordline, the selected master wordline is de-activated, at step 715 and the operations return to step 704, to select the next master wordline. The illustrated refresh operations may be repeated until the entire device has been refreshed, with each master wordline being activated just once for each device refresh cycle.
Of course, these operations may be interrupted, for example, if the device exits the standby mode and is accessed for normal operation (e.g., to read/write data). In this case, a master wordline with a different address than the currently selected master wordline may be accessed. In response, any master wordline currently activated for refresh may be de-activated (pre-charged), while the new master wordline is activated for the normal access. In other words, the refresh operation may be postponed, which may help ensure that certain timing specifications for the active cycle are met. Because the master wordline that was being refreshed may be forcefully pre-charged, the power savings may be reduced slightly. However, this is typically acceptable as much more current draw is typically allowed in an active mode than in a standby (refresh) mode. Because the refresh address counter typically changes only when refresh operations are being performed, once the standby mode is re-entered, refresh operations may resume at the same master wordline and wordline segment that was being refreshed when refresh operations were interrupted.
For some embodiments, refresh operations may be performed in what may be referred to as a “burst mode” with several wordline segments being refreshed rapidly. For example, rather than refresh a single wordline segment in response to each externally supplied refresh command or internal refresh signal (generated by the self refresh timer), all wordline segments driven to a master wordline may be refreshed in a burst. In other words, rather than space the refresh operations for each wordline segment at a period tREFRESH, the refresh operations may be performed for each master wordline at a period of tREFRESH×N, with N being the number of wordline segments driven by a master wordline. While each master wordline would still only be activated one time per total device refresh, this approach may result in shorter active periods for each master wordlines which, in some cases, may reduce corresponding leakage current.
By maintaining a master wordline active while wordline segments driven by the master wordline are refreshed, a number of current consuming pre-charge and re-charge cycles for the master wordline may be eliminated when compared to conventional refresh techniques. As a result, refresh current consumption caused by pre-charging and re-charging master wordlines may be reduced.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
