Abstract:
A method and circuit for refreshing dynamic data stored in an integrated circuit are disclosed. The integrated circuit receives a supply voltage and operates in a self-refresh mode of operation to refresh the dynamic data at a refresh time that defines how often the dynamic data is refreshed during the self-refresh mode. The method includes monitoring a magnitude of the supply voltage and adjusting the refresh time as a function of the monitored magnitude of the supply voltage. The integrated circuit may be any type of integrated circuit that stores dynamic data, such as a memory device like a dynamic random access memory, DDR DRAM, SLDRAM, or RDRAM, or other type of integrated circuit such as a microprocessor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of pending U.S. patent application Ser. No. 11/255,243, filed Oct. 20, 2005, which is about to issue as U.S. Pat. No. 7,747,815 on Jun. 29, 2010, which is a divisional of U.S. application Ser. No. 10/685,997, filed Oct. 14, 2003, issued Mar. 14, 1006 as U. S. Pat. No. 7,013,363, which is a continuation of U.S. patent application Ser. No. 09/973,998, filed Oct. 9, 2001, issued Nov. 11, 2003 as U.S. Pat. No. 6,646,942. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to integrated circuits, and more specifically to refreshing dynamic data stored in an integrated circuit, such as a dynamic random access memory (DRAM), as a supply voltage applied to the integrated circuit varies. 
       BACKGROUND OF THE INVENTION 
       [0003]    Many battery-powered portable electronic devices, such as laptop computers, Portable Digital Assistants, cell phones, and the like, require memory devices that provide large storage capacity and low power consumption. To reduce the power consumption and thereby extend the battery life in such devices, the devices typically operate in a low-power mode when the device is not being used. In the low-power mode, a supply voltage or voltages applied to electronic components such as a microprocessor, associated control chips, and memory devices are typically reduced to lower the power consumption of the components, as will be appreciated by those skilled in the art. Although the supply voltages are varied to reduce power consumption in the low-power mode, data stored in the electronic components such as the memory devices must be retained. 
         [0004]    Because large storage capacity is typically desired to maximize the amount of available storage in portable devices, it is typically desirable to utilize dynamic random access memory (DRAM), which has a relatively large storage capacity, over other types of memories such as static random access memories (SRAM) and non-volatile memories such as FLASH memory. In a DRAM, the data is “dynamic” because the data stored in memory cells in the DRAM must be periodically recharged or “refreshed” to maintain the data, as will now be explained in more detail with reference to  FIG. 1 .  FIG. 1  illustrates a portion of a conventional DRAM memory-cell array  100  including a plurality of memory cells  102  arranged in rows and columns, one of which is shown in  FIG. 1 . The memory cell  102  includes an access transistor  104  and a storage capacitor  106  connected in series between a digit line DL and a reference voltage VCC/ 2 . The storage capacitor  106  includes a first conductive plate  107  coupled to the access transistor  104  and a second conductive plate  109  coupled to the reference voltage VCC/ 2 . 
         [0005]    A word line WL activates the access transistor  104  in the memory cell  102 , and also activates the access transistors of all other memory cells (not shown) contained in the same row of the array  100  as the memory cell  102 . To write data into the memory cell  102 , a sense amplifier  108  drives the digit line DL and a complementary digit line DL* to complementary voltage levels corresponding to the data to be stored in the memory cell. The word line WL is then activated, turning ON the access transistor  104  and transferring charge through the access transistor to charge the storage capacitor  106  to the voltage level on the digit line DL corresponding to the data to be stored. The word line WL is thereafter deactivated, turning OFF the access transistor  104  and isolating the storage capacitor  106  from the digit line DL to thereby store the data in the form of a voltage across the storage capacitor. 
         [0006]    To read data from the memory cell  102 , the sense amplifier  108  equilibrates the digit lines DL, DL* to a predetermined voltage level and thereafter activates the word line WL to turn ON the access transistor  104 . In response to the access transistor  104  turning ON, charge is transferred between the storage capacitor  106  and the digit line DL, causing the voltage on the digit line DL to be slightly higher or lower than the voltage on the digit line DL*. The sense amplifier  108  senses the difference between the voltages on the digit lines DL and DL* and drives the voltages on the digit lines to complementary levels in response to the sensed difference. For example, assume a voltage VCC/ 2  corresponding to a binary  1  is stored across the capacitor  106 . In this situation, when the access transistor  104  is activated the equilibrated voltage on the digit line DL will increase slightly relative to the equilibrated voltage on the digit line DL*. As a result, the sense amplifier  108  will drive the voltage on the digit line DL to a supply voltage VCC and will drive the complementary digit line DL* to a reference voltage. The complementary voltages on the digit lines DL, DL* thus correspond to the data stored in the memory cell  102 , and the sense amplifier  108  thereafter applies these signals to other circuitry (not shown) to thereby provide the circuitry with the data stored in the memory cell. 
         [0007]    As previously mentioned, the data stored in the memory cell  102  in the form of the voltage across the capacitor  106  must be periodically refreshed. This is true because once the data is stored in the form of a voltage across the capacitor  106  and the access transistor  104  is deactivated, leakage currents ILK result in this stored voltage changing over time and, if not refreshed, may result in a different binary state of data being stored in the memory cell. These leakage currents ILK arise, for example, from the flow of charge stored on the conductive plate  107  of the capacitor  106  through the access transistor  104  even when the access transistor is turned OFF, and may also arise from the flow of charge from the conductive plates  107 ,  109  to ground, as well as the flow of charge from the plate  107  through a dielectric (not shown) to the plate  109 , as will be appreciated by those skilled in the art. From the above description of the conventional DRAM memory cell  102 , it is seen that each time data is read from the memory cell the storage capacitor  106  is again charged to the proper voltage corresponding to the data stored in the cell. Thus, to refresh memory cells  102 , the memory cells are merely accessed as in a read operation with the sense amplifier  108  driving digit lines DL, DL* to complementary voltages corresponding to the data stored in the memory cell and thereby charging the storage capacitors  106  to the proper voltage. 
         [0008]    The rate at which the data restored in the memory cells  102  must be periodically refreshed is known as the refresh rate of the cells, and is a function of a number of different parameters, including the operating temperature of the DRAM containing the array  100 , the number of rows of memory cells in the array, and the value of the supply voltage VCC applied to the DRAM, as will be appreciated by those skilled in the art. For example, if the array  100  includes N rows of memory cells  102  and each memory cell must be refreshed every M milliseconds, the refresh rate is M/N milliseconds/row, meaning that one row must be accessed every M/N milliseconds in order to properly refresh the memory cells, with every row being accessed at least once every M milliseconds. As the supply voltage VCC decreases, the refresh rate increases due, for example, to a reduced voltage being stored across the storage capacitors  106  and the need to refresh this voltage more frequently to ensure the stored voltage does not decay to an insufficient level due to the leakage currents ILK. The refresh rate also must increase as the supply voltage VCC decreases due to the possibility of restoring incorrect data into the memory cell  102 , as will be appreciated by those skilled in the art. 
         [0009]    When the memory-cell array  100  is contained in a DRAM, a memory controller typically reads data from desired memory cells  102  in response to requests from a microprocessor or other control circuit, each accessed memory cell being automatically refreshed as previously described. The data stored in all the memory cells  102  and not just those accessed by the memory controller, however, must be periodically refreshed. As a result, during normal operation the memory controller will periodically apply a refresh command to the DRAM containing the array  100 , causing control circuitry (not shown) to access each memory cell  102  as previously described and thereby refreshing the memory cells. Even when the memory controller is not accessing the DRAM, the memory cells  102  must still be periodically refreshed. To refresh the memory cells  102  in this situation, the memory controller applies a self-refresh command to the DRAM, placing the DRAM in a self-refresh mode of operation during which circuitry internal to the DRAM (not shown in  FIG. 1 ) refreshes the memory cells  102  periodically, as will be appreciated by those skilled in the art. 
         [0010]    As previously described, in portable and other electronic devices containing DRAM, the supply voltage VCC applied to the DRAM is typically reduced during a low-power mode of operation to reduce power consumption and extend battery life of the device. Notwithstanding the reduced supply voltage VCC, the memory cells in the DRAM must be adequately refreshed to ensure the integrity of the stored data. There is a need for an improved circuit and method for controlling the refresh rate of dynamic data stored in a DRAM or other integrated circuit when the supply voltage is reduced to a very low level during a low-power mode of operation. 
       SUMMARY OF THE INVENTION 
       [0011]    According to one aspect of the present invention, a method and circuit for refreshing dynamic data stored in an integrated circuit are disclosed. The integrated circuit receives a supply voltage and operates in a self-refresh mode of operation to refresh the dynamic data at a refresh time that defines how often the dynamic data is refreshed during the self-refresh mode. The method includes monitoring a magnitude of the supply voltage and adjusting the refresh time as a function of the monitored magnitude of the supply voltage. The integrated circuit may be any type of integrated circuit that stores dynamic data, such as a memory device like a DRAM, double-data rate (DDR) DRAM, SLDRAM, RDRAM, or other type of integrated circuit such as a microprocessor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a diagram illustrating a portion of a memory-cell array in a conventional DRAM. 
           [0013]      FIG. 2  is a functional block diagram of a memory system including a memory controller and a memory device including a self-refresh controller according to one embodiment of the present invention. 
           [0014]      FIGS. 3A and 3B  are diagrams illustrating signals generated by the self-refresh controller of  FIG. 1  in controlling the frequency of a clock signal and thereby controlling a refresh rate of memory cells as a function of a supply voltage according to a first embodiment of the present invention. 
           [0015]      FIG. 4  is a signal diagram illustrating the operation of the self-refresh controller of  FIG. 1  in controlling the frequency of a clock signal and thereby controlling a refresh rate of memory cells as a function of the supply voltage according to a second embodiment of the present invention. 
           [0016]      FIG. 5  is a signal diagram illustrating the operation of the self-refresh controller of  FIG. 1  in controlling the frequency of a clock signal and thereby controlling a refresh rate of memory cells as a function of the supply voltage according to a third embodiment of the present invention. 
           [0017]      FIG. 6  is a functional block diagram illustrating a computer system including the memory device of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]      FIG. 2  is a functional block diagram of a memory system  200  including a memory controller  202  coupled to a memory device  204  that includes a self-refresh controller  206  for adjusting the refresh rate of dynamic data as a function of an applied supply voltage VCC according to one embodiment of the present invention. In operation, the self-refresh controller  206  adjusts a refresh frequency RF of a refresh clock signal RFCLK, which defines a refresh rate of the dynamic data as a function of the supply voltage VCC, to ensure the integrity of data as the supply voltage decreases, as will be explained in more detail below. The memory device  204  in  FIG. 2  is a double-data rate (DDR) synchronous dynamic random access memory (“SDRAM”), although the principles described herein are applicable to any memory device containing memory cells that must be refreshed (i.e., that store dynamic data), such as conventional DRAMs and SDRAMs, as well as packetized memory device like SLDRAMs and RDRAMs, and are equally applicable to any integrated circuit that stores dynamic data. In the following description, certain details are set forth to provide a sufficient understanding of the invention. It will be clear to one skilled in the art, however, that the invention may be practiced without these particular details. In other instances, well-known circuits, control signals, timing protocols, and software operations have not been shown in detail or omitted entirely in order to avoid unnecessarily obscuring the invention. 
         [0019]    Before describing the self-refresh controller  206  in more detail, the various components of the memory device  204  will first be described. The memory controller  202  applies row, column, and bank addresses to an address register  208  over an address bus ADDR. Typically, a row address RA and a bank address BA are initially received by the address register  208  and applied to a row address multiplexer  208  and bank control logic circuit  210 , respectively. The row address multiplexer  208  applies either the row address RA received from the address register  208  or a refresh row address RFRA received from the self-refresh controller  206  to a plurality of row address latch and decoder circuits  214 A-D. The bank control logic  212  activates the row address latch and decoder circuit  214 A-D corresponding to either the received bank address BA or a refresh bank address RFBA from the self-refresh controller  206 , and the activated row address latch and decoder circuit latches and decodes the received row address. In response to the decoded row address, the activated row address latch and decoder  214 A-D applies various signals to a corresponding memory bank or array  216 A-D to thereby activate a row of memory cells corresponding to the decoded row address. The data in the memory cells in the accessed row is stored in sense amplifiers coupled to the array  216 A-D, which also refreshes the accessed memory cells as previously described. The row address multiplexer  210  applies the refresh row address RFRA to the row address latch and decoders  214 A-D and the bank control logic circuit  212  uses the refresh bank address RFBA when the memory device  204  operates in an auto-refresh or self-refresh mode of operation in response to the controller  202  applying an auto- or self-refresh command to the memory device  204 , as will be described in more detail below. 
         [0020]    After the address register  208  memory controller  202  has applied the row and bank addresses RA, BA, the memory controller applies a column address CA on the address bus ADDR. The address register  208  provides the column address CA to a column address counter and latch circuit  218  which, in turn, latches the column address and applies the latched column address to a plurality of column decoders  220 A-D. The bank control logic  212  activates the column decoder  220 A-D corresponding to the received bank address BA, and the activated column decoder decodes the column address CA from the counter and latch circuit  218 . Depending on the operating mode of the memory device  204 , the counter and latch circuit  218  either directly applies the latched column address to the decoders  220 A-D, or applies a sequence of column addresses to the decoders starting at the column address CA provided by the address register  208 . In response to the column address from the counter and latch circuit  218 , the activated column decoder  222 A-D applies decode and control signals to an I/O gating and data masking circuit  222  which, in turn, accesses memory cells corresponding to the decoded column address in the activated row of memory cells in the array  216 A-D being accessed. 
         [0021]    During data read operations, data being read from the activated array  216 A-D is coupled through the I/O gating and data masking circuit  222  to a read latch  224 . The circuit  222  supplies N bits of data to the read latch  224 , which then applies two N/2 bit words to a multiplexer  226 . In the embodiment of  FIG. 3 , the circuit  222  provides 64 bits to the read latch  224  which, in turn, provides two 32 bits words to the multiplexer  226 . A data driver circuit  228  sequentially receives the N/2 bit words from the multiplexer  226  and also receives a data strobe signal DQS from a strobe signal generator  230  and a delayed clock signal CLKDEL from a delay-locked loop (DLL) circuit  232 . The DQS signal has the same frequency as the CLK, CLK* signals, and is used by the controller  202  in latching data from the memory device  204  during read operations, as will be described in more detail below. In response to the delayed clock signal CLKDEL, the data driver circuit  228  sequentially outputs the received N/2 bits words as corresponding data words DQ that are in synchronism with rising and falling edges of the CLK signal, respectively, and also outputs the data strobe signal DQS having rising and falling edges in synchronism with rising and falling edges of the CLK signal, respectively. Each data word DQ and the data strobe signal DQS collectively define a data bus DATA coupled to the controller  202  which, during read operations, latches the each N/2 bit DQ word on the DATA bus responsive to the data strobe signal DQS. As will be appreciated by those skilled in the art, the CLKDEL signal is a delayed version of the CLK signal, and the DLL circuit  232  adjusts the delay of the CLKDEL signal relative to the CLK signal to ensure that the DQS signal and the DQ words are placed on the DATA bus in synchronism with the CLK signal. The DATA bus also includes masking signals DQM 0 -X, which will be described in more detail below with reference to data write operations. 
         [0022]    During data write operations, the memory controller  202  applies N/2 bit data words DQ, the strobe signal DQS, and corresponding data masking signals DM 0 -X on the data bus DATA. A data receiver circuit  234  receives each DQ word and the associated DM 0 -X signals, and applies these to an input register  236  that is clocked by the DQS signal. In response to a rising edge of the DQS signal, the input register  236  latches a first N/2 bit DQ word and the associated DM 0 -X signals, and in response to a falling edge of the DQS signal the input register latches the corresponding N/2 bit DQ word and associated DM 0 -X signals. The input register  236  provides the two latched N/2 bit DQ words as an N-bit word to a write FIFO and driver circuit  238 , which clocks the the applied DQ word and DM 0 -X signals into the write FIFO and driver circuit in response to the DQS signal. The DQ word is clocked out of the write FIFO and driver circuit  238  in response to the CLK signal, and is applied to the I/O gating and masking circuit  222 . The I/O gating and masking circuit  222  transfers the DQ word to the accessed memory cells in the activated array  216 A-D subject to the DM 0 -X signals, which may be used to selectively mask bits or groups of bits in the DQ words (i.e., in the write data) being written to the accessed memory cells. 
         [0023]    A control logic and command decoder circuit  240  receives a plurality of command and clocking signals from the memory controller  202  over a control bus CONT, and generates a plurality of control and timing signals to control the components  206 - 238  during operation of the memory device  204 . The command signals include a chip select signal CS*, a write enable signal WE*, a column address strobe signal CAS*, and a row address strobe signal RAS*, while the clocking signals include a clock enable signal CKE* and complementary clock signals CLK, CLK*, with the “*” designating a signal as being active low. The memory controller  202  drives the command signals CS*, WE*, CAS*, and RAS* to values corresponding to a particular command, such as a read, write, or auto-refresh command. In response to the clock signals CLK, CLK*, the command decoder circuit  240  latches and decodes an applied command, and generates a sequence of control signals that control various components in the memory device to execute the function of the applied command. The clock enable signal CKE enables clocking of the command decoder circuit  240  by the clock signals CLK, CLK*. The command decoder circuit  240  latches command and address signals at positive edges of the CLK, CLK* signals (i.e., the crossing point of CLK going high and CLK* going low), while the input registers  236  and data drivers  228  transfer data into and from, respectively, the memory device  204  in response to both edges of the data strobe signal DQS and thus at double the frequency of the strobe signal and clock signals CLK, CLK*. For this reason the memory device  204  is referred to as a double-data-rate device, with data being transferred to and from the device at double the rate of a conventional SDRAM, which transfers data at a rate corresponding to the frequency of the applied clock signal. The detailed operation of the control logic and command generator circuit  240  in generating the control and timing signals is conventional, and thus, for the sake of brevity, will not be described in more detail. 
         [0024]    As previously mentioned, in battery-powered electronic devices it is desirable to place the memory device  204  in a low-power mode of operation when the memory controller  202  is not accessing data stored in the memory device. In the memory device  204 , this low-power mode of operation is known as a self-refresh mode. To place the memory device  204  in the self-refresh mode of operation, the memory controller  202  applies a self-refresh command to the memory device. In response to the self-refresh command, the command decoder circuit  240  applies control signals to the row address multiplexer  210  and the bank control logic circuit  212  that cause the circuits to utilize the refresh row address RFRA and refresh bank address RFBA from the self-refresh controller  206  to sequentially access each row of memory cells in the memory array  216 A-D to thereby refresh the memory cells. The self-refresh controller  206  controls the refresh rate at which the memory cells in the arrays  216 A-DO are refreshed as a function of a supply voltage VCC applied to the memory device  204 . The operation of the self-refresh controller  206  during the self-refresh mode along with the structure of the self-refresh controller will now be described in more detail. 
         [0025]    The self-refresh controller  206  includes a bias voltage generator  242  that receives the supply voltage VCC and generates a bias voltage VBIAS having a value that is a function of the magnitude of the supply voltage. A self-refresh oscillator  244  receives the bias voltage VBIAS and generates a refresh clock signal RFCLK having a refresh frequency RF that is a function of the bias voltage VBIAS. The self-refresh oscillator  244  applies the refresh clock signal RFCLK to clock a self-refresh row-bank address counter  246  which sequentially generates the refresh row addresses RFRA and bank addresses RFBA in response to the RFCLK signal, and applies the refresh row address to the row address multiplexer  210  and refresh bank address to the bank control logic circuit  212  as previously described. 
         [0026]    In operation, upon receiving a self-refresh command, the control logic and command decoder circuit  240  resets the counter  246  and applies control signals causing the row address multiplexer  210  and bank control logic circuit  212  to utilize the refreshed row address RFRA and refresh bank address RFBA, respectively. The self-refresh oscillator  244  applies the refresh clock signal RFCLK to clock the counter  246  which, in turn, sequentially generates the refresh row addresses RFRA and refresh bank addresses RFBA. The sequentially generated refresh row addresses RFRA are applied through the multiplexer  210  and latched and decoded by the activated row address latch and decoder circuit  214 A-D, with the bank control logic circuit  212  activating the circuit  214 A-D corresponding to the refresh bank address RFBA. The refresh controller  206  generates a given refresh bank address RFBA and then generates refresh row addresses RFRA to sequentially activate all rows in the memory array  216 A-D corresponding to the bank address, and thereafter generates a new bank address and activates each row in the new memory array, and so on for each memory array. In this way, the refresh controller  206  sequentially activates rows of memory cells in the arrays  216 A-D to thereby refresh the memory cells. Although the refresh controller  206  is discussed as generating addresses that refresh the memory cells during the self-refresh mode, one skilled in the art will appreciate that the control logic and command decoder circuit  240  also generates signals to control various components in the memory device  204  during this mode of operation. 
         [0027]    The refresh rate of the memory cells in the arrays  216 A-D is determined by the rate at which the counter  246  sequentially generates the refresh row and bank addresses RFRA, RFBA, which is determined by the frequency RF of the applied refresh clock signal RFCLK, as will be appreciated by those skilled in the art. Thus, the frequency RF of the RFCLK clock signal determines the refresh rate of the memory cells in the arrays  216 A-D. As previously mentioned, the frequency RF of the RFCLK signal is a function of the bias voltage VBIAS from the variable bias voltage generator  242 , and the bias voltage is a function of the magnitude of the supply voltage VCC. The refresh rate of the memory cells in the arrays  216 A-D is therefore a function the magnitude of the supply voltage VCC. In this way, the self refresh controller  206  controls the refresh rate as a function of the supply voltage VCC to ensure the refresh rate is sufficient to reliably maintain the data stored in the arrays  216 A′-D. For example, as the supply voltage VCC decreases during a low-power mode of operation, the self-refresh controller  206  increases the refresh rate of the memory cells in the arrays  216 A-D to ensure data integrity. 
         [0028]    In the self-refresh controller  206 , the variable bias voltage generator  242  controls the bias voltage VBIAS as a function of the magnitude of the supply voltage VCC, and the bias voltage is applied to the self-refresh oscillator  244  to control the frequency RF of the RFCLK signal and thereby control the refresh rate of the memory cells in the arrays  216 A-D as a function of the supply voltage VCC. Accordingly, the precise manner in which the variable bias voltage generator  242  controls the bias voltage VBIAS as a function of the supply voltage VCC and the precise manner in which the self-refresh oscillator  244  controls the frequency RF of the RFCLK signal in response to the bias voltage determine how the self refresh controller  206  controls the refresh rate as the supply voltage varies. 
         [0029]      FIGS. 3A and 3B  are signal diagrams illustrating the operation of the variable bias voltage generator  242  and self-refresh oscillator  244  in combination to control the frequency RF of the RFCLK signal as a function of the supply voltage VCC according to one embodiment of the present invention. In the embodiment of  FIG. 3A , the variable bias voltage generator  242  maintains the bias voltage VBIAS at a relatively constant value VBC when the supply voltage VCC is greater than a minimum value VMIN. As seen in  FIG. 3B , the relatively constant bias voltage VBC when the supply voltage VCC is greater than the voltage VMIN results in the oscillator  244  developing the RFCLK signal have a relatively constant frequency RFN. The supply voltage VCC being greater than the minimum value VMIN corresponds to a normal operating mode of the memory device  204 . When the supply voltage VCC is less than or equal to the minimum value VMIN, the variable bias voltage generator  242  begins increasing the bias voltage VBIAS as the supply voltage decreases, which increases the frequency RF of the RFCLK signal and thereby increases the refresh rate of the memory cells in the arrays  216 A-D. The supply voltage VCC being less than or equal to the minimum value VMIN and greater than a lower limit VL corresponds to a low-power operating mode of the memory device  204 . Thus, in the embodiment of  FIGS. 3A and 3B , the refresh rate is increased as the supply voltage VCC decreases below a minimum value VMIN to ensure data is adequately refresh during a low-power mode of operation. The lower limit VL corresponds to a supply voltage VCC having such a small magnitude that the refresh controller  206  can no longer reliably refresh data stored in the memory arrays  216 A-D. 
         [0030]      FIG. 4  is a signal diagram illustrating the operation of the variable bias voltage generator  242  and the self-refresh oscillator  244  in combination to control the frequency RF of the RFCLK signal as a function of the supply voltage VCC according to a second embodiment of the present invention. In the embodiment of  FIG. 4 , the variable bias voltage generator  242  maintains the bias voltage VBIAS relatively constant when the supply voltage VCC is greater than a minimum value VMIN, resulting in the oscillator  244  maintaining the frequency RF of the RFCLK at a relatively constant value RFN. Once again, when the supply voltage VCC is greater than the minimum value VMIN the memory device  204  operates in a normal operating mode. In this embodiment, when the variable bias voltage generator  242  detects the supply voltage VCC is less than or equal to the minimum value VMIN, the voltage generator outputs the supply voltage as the bias voltage VBIAS. As seen in  FIG. 4 , when the supply voltage VCC is output as the bias voltage VBIAS, the frequency RF of the RFCLK signal increases to a maximum value RFM due to the increased magnitude of the bias voltage, and the refresh rate increases accordingly. The frequency RF and, accordingly, the refresh rate thereafter decrease as the supply voltage VCC and thus the bias voltage VBIAS decrease. The supply voltage VCC being less than or equal to the minimum value VMIN and greater than a lower limit VL once again corresponds to a low-power operating mode of the memory device  204 . In the embodiment of  FIG. 4 , the magnitude of the bias voltage VBIAS is increased due to the greater magnitude of the supply voltage VCC, which is applied as the bias voltage. This increased bias voltage VBIAS increases the frequency RF of the RFCLK signal which, in turn, increases the refresh rate of the memory cells in the arrays  216 A-D. 
         [0031]      FIG. 5  is a signal diagram illustrating the operation of the variable bias voltage generator  242  and the self-refresh oscillator  244  in combination to control the frequency RF of the RFCLK signal as a function of the supply voltage VCC according to a third embodiment of the present invention. In this embodiment, the variable bias voltage generator  242  maintains the bias voltage VBIAS at a relatively constant value VBC when the supply voltage VCC is greater than a minimum value VMIN to thereby cause the oscillator  244  to develop the RFCLK signal have a relatively constant frequency RFN. The supply voltage VCC being greater than the minimum value VMIN once again corresponds to a normal operating mode of the memory device  204 . When the supply voltage VCC is less than or equal to the minimum value VMIN, the variable bias voltage generator  242  begins increasing the bias voltage VBIAS as the supply voltage decreases, which increases the frequency RF of the RFCLK signal and thereby increases the refresh rate of the memory cells in the arrays  216 A-D. The variable bias voltage generator  242  and oscillator  244  operate in this way, which corresponds to the operation previously described with reference to  FIGS. 3A and 3B , to increase the refresh rate as the supply Voltage VCC decreases. 
         [0032]    The generator  242  and oscillator  244  operate in this manner until the variable bias voltage generator  242  detects the supply voltage VCC is less than a first lower limit VF. When the variable bias voltage generator  242  determines the supply voltage VCC is less than or equal to the first lower limit VF, the bias voltage generator operates as previously described with reference to  FIG. 4 , outputting the supply voltage as the bias voltage VBIAS to the oscillator  244 . The increased magnitude of the supply voltage VCC being output as the bias voltage VBIAS causes the frequency RF of the RFCLK signal to increase to a maximum value RFM, and the refresh rate increases accordingly. The frequency RF and, accordingly, the refresh rate thereafter decrease as the supply voltage VCC and thus the bias voltage VBIAS decrease. The supply voltage VCC being less than or equal to the minimum value VMIN and greater than a second lower limit VL once again corresponds to a low-power operating mode of the memory device  204 . It should be noted that in this embodiment, the low-power operating mode includes two sub modes, a first submode corresponding to the operation of the generator  242  and oscillator  244  when the supply voltage VCC is between the minimum value VMIN and the first lower limit VF, and a second sub mode when the supply voltage is between the first lower limit VF and the second lower limit VL. 
         [0033]    Referring back to  FIG. 2 , in another embodiment of the refresh controller  206 , the memory controller  202  monitors the supply voltage VCC. When the memory controller  202  determines the supply voltage VCC is less than a minimum value VMIN, the memory controller applies a refresh rate adjustment command to the memory device  204 . This refresh rate adjustment command may, for example, correspond to a load mode command that loads appropriate information into mode registers contained in the control logic and command decode circuit  240 . In response to receiving the refresh rate adjustment command, the command decode circuit  240  applies control signals to the variable bias voltage generator  242 , causing the voltage generator to operate as previously described for the embodiment of  FIGS. 3A-3B . In this embodiment, the memory controller  202  could also further monitor the supply voltage VCC and send another refresh rate adjustment command to the memory device  204  when the supply voltage becomes less than a first lower limit VF, with the command decode circuit  240  thereafter causing the variable bias voltage generator  242  and oscillator  244  to operate as previously described for the embodiment of  FIG. 5  when the supply voltage is between the first lower limit VF and the second lower limit VL. When the memory controller  202  applies this second refresh rate adjustment command, the refresh controller  202  operates the same as in the embodiment of  FIG. 4 . 
         [0034]    As will be appreciated by those skilled in the art, other embodiments of the refresh controller  206  in which the refresh controller controls the refresh rate in different ways as a function of the magnitude of the supply voltage VCC are well within the scope of the present invention. 
         [0035]      FIG. 6  is a block diagram of a computer system  700  including computer circuitry  702  which includes the memory device  204  of  FIG. 2 , and which may also include of the memory controller  202  of  FIG. 2  as well. Typically, the computer circuitry  702  is coupled to the memory controller  202  through address, data, and control buses to provide for writing data to and reading data from the memory device  204 . The computer circuitry  702  includes circuitry for performing various computing functions, such as executing specific software to perform specific calculations or tasks. In addition, the computer system  700  includes one or more input devices  704 , such as a keyboard or a mouse, coupled to the computer circuitry  702  to allow an operator to interface with the computer system. Typically, the computer system  700  also includes one or more output devices  706  coupled to the computer circuitry  702 , such as output devices typically including a printer and a video terminal. One or more data storage devices  708  are also typically coupled to the computer circuitry  702  to store data or retrieve data from external storage media (not shown). Examples of typical storage devices  708  include hard and floppy disks, tape cassettes, compact disk read-only (CD-ROMs) and compact disk read-write (CD-RW) memories, and digital video disks (DVDs). 
         [0036]    It is to be understood that even though various embodiments and advantages of the present invention have been set forth in the foregoing description, the above disclosure is illustrative only, and changes may be made in detail, and yet remain within the broad principles of the invention. For example, many of the components described above may be implemented using either digital or analog circuitry, or a combination of both, and also, where appropriate, may be realized through software executing on suitable processing circuitry. Therefore, the present invention is to be limited only by the appended claims.