Patent Publication Number: US-10789995-B2

Title: Data storage system and operating method for non-volatile memory

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority of Taiwan Patent Application No. 107116288, filed on May 14, 2018, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to data storage devices and methods for operating non-volatile memory. 
     Description of the Related Art 
     There are various forms of non-volatile memory (NVM) for long-term data retention, such as flash memory, magnetoresistive RAM, ferroelectric RAM, resistive RAM, spin transfer torque-RAM (STT-RAM), and so on. 
     The reliability of non-volatile data may be affected by ambient temperature. Taking vehicle electronics as an example, an in-vehicle computer may be used in weather and temperature conditions that vary drastically. When the air conditioner is not turned on, the temperature may be as low as minus 40 degrees Celsius, or as high as 85 degrees Celsius. Such a large temperature difference will greatly affect the reliability of data storage. For example, data that has been written at a low temperature may not be read correctly (high-temperature reading is particularly prone to errors). 
     In addition to vehicle electronics, non-volatile memory has many other applications that face the problems of environmental temperature differences which may affect the reliability of the data. 
     BRIEF SUMMARY OF THE INVENTION 
     The present disclosure discloses a non-volatile memory preheating technique to deal with the drastic variability of ambient temperatures. 
     A data storage system in accordance with an exemplary embodiment of the disclosure includes a non-volatile memory and a controller. The controller operates the non-volatile memory to preheat the non-volatile memory to a temperature target. The controller avoids writing valid data to the non-volatile memory until preheating the non-volatile memory to the temperature target. 
     In an exemplary embodiment, the controller writes dummy data to the non-volatile memory until preheating the non-volatile memory to the temperature target. In an exemplary embodiment, the controller reads the non-volatile memory and regards read data as dummy data until preheating the non-volatile memory to the temperature target. 
     In an exemplary embodiment, the controller overclocks to accelerate the non-volatile memory and thereby preheats the non-volatile memory to the temperature target. In an exemplary embodiment, the controller increases the transmission rate of the non-volatile memory to accelerate the non-volatile memory and thereby preheats the non-volatile memory to the temperature target. In an exemplary embodiment, the data storage system includes an on-die termination (ODT). The controller turns on the on-die termination and thereby preheats the non-volatile memory to the temperature target. 
     In an exemplary embodiment, the data storage system includes an active heat dissipation device. The controller turns off the active heat dissipation device and thereby preheats the non-volatile memory to the temperature target. 
     In an exemplary embodiment, the data storage system includes a thermometer. The controller gets the current temperature from the thermometer, and has historical programming temperature in record. The historical programming temperature is an average of the programming temperatures of a target block of the non-volatile memory, or the detected temperature of the last programming on the target block. When the data storage system is powered on, the controller determines whether the difference between the current temperature and the historical programming temperature is greater than a threshold. When the difference is greater than the threshold, the controller operates the non-volatile memory to preheat the non-volatile memory to the temperature target. 
     In another exemplary embodiment, the controller gets the current temperature from the thermometer. When the data storage system is powered on, the controller determines whether the current temperature is within a normal range. When the current temperature is not within the normal range, the controller operates the non-volatile memory to preheat the non-volatile memory to the temperature target. 
     In an exemplary embodiment, the controller and the non-volatile memory are packaged in a multiple-chip package (MCP). Due to the MCP design, it is easy to preheat the non-volatile memory by operating the non-volatile memory. 
     In another exemplary embodiment, an operating method for non-volatile memory includes the following steps: operating a non-volatile memory to preheat the non-volatile memory to a temperature target; and avoiding writing valid data to the non-volatile memory until preheating the non-volatile memory to the temperature target. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  illustrates an in-vehicle computer  100  implemented in accordance with an embodiment of the present invention, including a host  102  and a data storage device  104 ; 
         FIG. 2A  is a flowchart of a method for preheating a data storage medium according to an exemplary embodiment of the present invention, wherein the controller  108  may preheat the flash memory  106  according to the data storage medium preheating method; 
         FIG. 2B  is a flowchart of a method for preheating a data storage medium in accordance with another exemplary embodiment of the present invention; 
         FIG. 3  illustrates the temperature variations of the components of an MCP data storage device  104 ; and 
         FIG. 4  shows voltage distribution probability, corresponding to the different temperatures (25 degrees Celsius, minus 40 degrees Celsius, minus 30 degrees Celsius, and minus 20 degrees Celsius), of triple level cells (TLCs) at different digital representations, wherein the threshold voltages (Vth) for identifying the digital representation of TLCs can be obtained from  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description shows exemplary embodiments of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     A non-volatile memory for long-term data retention may be a flash memory, a magnetoresistive RAM, a ferroelectric RAM, a resistive RAM, a spin transfer torque-RAM (STT-RAM) and so on. A non-volatile memory may be used to implement a data storage device or a data center. The following discussion uses flash memory as an example. 
     Flash memory is often used as a storage medium in today&#39;s data storage devices. Flash memory is often implemented in memory cards, USB flash devices, SSDs, and so on. In another exemplary embodiment, a flash memory may be packaged with a controller to form a multiple-chip package named eMMC. 
     A data storage device using a flash memory as a storage medium can be applied in a variety of electronic devices, including a smartphone, a wearable device, a tablet computer, a virtual reality device, etc. A calculation module of an electronic device may be regarded as a host that operates a data storage device equipped on the electronic device to access a flash memory within the data storage device. 
     A data center may be built with data storage devices using flash memories as the storage medium. For example, a server may operate an array of SSDs to form a data center. The server may be regarded as a host that operates the SSDs to access the flash memories within the SSDs. 
     In the following paragraphs, vehicle electronics (or an in-vehicle computer) are discussed especially for the problems due to ambient temperature variations. The present invention is not intended to be limited to vehicle electronics. In today&#39;s extreme climate, data storage devices for various applications may face considerable ambient temperature variations and require the design of the invention. 
       FIG. 1  illustrates an in-vehicle computer  100  implemented in accordance with an embodiment of the present invention, including a host  102  and a data storage device  104 . The data storage device  104  uses a flash memory  106  as a non-volatile storage medium and provides a controller  108  to operate the flash memory  106  in accordance with commands from the host  102 . The illustrated data storage device  104  has a dynamic random access memory (DRAM)  110  which is used by the controller  108  when controlling the flash memory  106 . In other exemplary embodiments, the DRAM  110  can be replaced with other volatile storage media, such as a static random access memory (SRAM). In one exemplary embodiment, the data storage device  104  is a Multi-Chip Package (MCP) with the flash memory  106 , the controller  108 , and the dynamic random access memory (DRAM)  110  being packaged together. The DRAM  110  is not limited to being fabricated inside the multi-chip package and may be externally connected. 
     The controller  108  of the present invention performs a preheating operation on the flash memory  106  to ensure the data reliability of the flash memory  106 . Writing data to the flash memory  106  at an extremely low temperature is avoided. 
     Referring to  FIG. 1 , the flash memory  106  has a thermometer  112  and the controller  108  has a thermometer  114 . Either thermometer  112  or thermometer  114  is used by the controller  108  to implement the preheating operation of the present invention. Based on the temperature detected by the thermometer  112  or  114 , the controller  108  may activate or terminate the preheating of the data storage medium. 
     A variety of dummy operations can be applied to achieve the purpose of preheating, including writing dummy data (rather than valid data) to the flash memory  106 , or meaningless reading of the flash memory  106  (e.g., treating the read data as dummy data or directly discard the read data). In order to accelerate the preheating, the controller  108  may operate at the higher operating frequency. The controller  108  operating at the higher operating frequency may also switch the flash memory  106  to operate at the higher operating frequency. For example, the controller  108  may switch the flash memory  106  from a DDR mode to a DDR2 mode, or switch the flash memory  106  from an SDR mode to a DDR mode. If the data storage device  104  has an active heat dissipation device  118 , such as a fan, the preheating may be achieved by stopping the active heat dissipation device  118 . The flash memory  106  shown in  FIG. 1  includes an on-die termination  116 , which is turned on when the flash memory  106  is switched from the DDR mode to the DDR2 mode to cope with electromagnetic interference (Glitch). When the on-chip termination  116  is turned on, heat may be generated due to power consuming. The controller  108 , therefore, may achieve the preheating by conducting the on-chip termination  116 . 
       FIG. 2A  is a flowchart of a method for preheating a data storage medium according to an exemplary embodiment of the present invention. The controller  108  may preheat the flash memory  106  according to the data storage medium preheating method. 
     When the in-vehicle computer  100  is powered on, the data storage device  104  starts an initialization procedure (step S 202 ) to enable the controller  108 , the flash memory  106 , and the thermometer  112  or  114 . The DRAM  110  may be also enabled during the initialization procedure. 
     In step S 204 , the controller  108  retrieves the historical programming temperature and the current temperature. The historical programming temperature may be the average programming temperature of a target block of the flash memory  106 , or the detected temperature of the last programming on the target block. The average temperature or the final temperature may be recorded at the end of the target block as the EOB (end of block) information. In another exemplary embodiment, the average temperature or the final temperature may be recorded in a spare area of the target block. The current temperature and the historical programming temperature are preferably sensed by the thermometer  112  or  114 . The target block may be the block that the controller  108  currently wants to access. The target block may be allocated for the storage of system information, or programmed in an SLC (single level cell) mode, or having the EOB information stored therein. 
     In step S 206 , the controller  108  determines whether the difference between the historical programming temperature and the current temperature is greater than a threshold. When the historical programming temperature is 80 degrees Celsius, the current temperature is minus 40 degrees Celsius, and the threshold is set to 100, the controller  108  determines that the difference between the historical programming temperature and the current temperature is 120, which is greater than the threshold and, therefore, step S 208  is performed. When the controller  108  determines that the difference between the historical programming temperature and the current temperature is not greater than the threshold, the preheating procedure for the data storage medium ends. 
     In step S 208 , the controller  108  modifies the operating parameters of the flash memory  106 . For example, the operating frequency of the flash memory  106  may be promoted from DDR  200  to DDR  400 . In another exemplary embodiment, the controller  108  may modify its own operating parameters. For example, the operating frequency of the controller  108  may be promoted from 375M Hz to 525M Hz. In another exemplary embodiment, the controller  108  may modify the operating parameters of the DRAM  110 . For example, the operating frequency of the DRAM  110  may be promoted from DDR  200  to DDR  400 . When the flash memory  106  and the peripheral components operate at the higher operating frequencies, more energy is consumed which results in a higher temperature. In another exemplary embodiment, the controller  108  may turn off or otherwise disable the active heat dissipation device  118  to achieve the preheating of the present invention. 
     In step S 210 , the controller  108  performs dummy operations on the data storage medium with the modified operating parameters. The controller  108  may write dummy data to the flash memory  106 , or reads a large amount of data (meaningless reading) from the flash memory  106 , or repeatedly reads data from a target block. By performing the dummy operations, the temperature of the flash memory  106  and the peripheral components (such as the controller  108  and the DRAM  110 ) increases. The increased temperature is detected by the thermometer  112  or  114 . For example, the temperature detected by the thermometer  112  or  114  may be raised from minus 40 degrees Celsius to minus 20 degrees Celsius. The determination of step S 206  is no longer satisfied, and the controller  108  ends the preheating of the data storage medium. The controller  108  restores the operating parameters of the flash memory  106 , the controller  108  and the DRAM  110  to operate the flash memory  106  in a normal (preset) mode for normal operations. 
     When operating the flash memory  106  in the normal mode, the operating parameters adopted by the controller  118  may be specially set to operate the flash memory  106  at a lower access rate (e.g. SDR) for firmware retrieving. After retrieving the firmware, the controller  108  may modify the operating parameters to operate the flash memory  106  at a higher access rate (e.g., DDR). The modified operating parameters may equal to the operating parameters used in step S 208  (e.g. DDR2), or be lower than the operating parameters used in step S 208  (for example, being modified to DDR which is slower than DDR2). 
       FIG. 2B  is a flowchart of a method for preheating a data storage medium in accordance with another exemplary embodiment of the present invention. This exemplary embodiment has some steps the same as those of the previous exemplary embodiment, including steps S 202 , S 208  and S 210 . Only the different steps S 212  and S 214  are described below. 
     In step S 212 , the controller  108  gets the current temperature that is preferably detected by the thermometer  112  or  114 . 
     In step S 214 , the controller  108  determines whether the current temperature is in a normal range. When the current temperature is minus 40 degrees Celsius and the normal range is from minus 20 degrees Celsius to 80 degrees Celsius (considering the general temperature 25 degrees Celsius), it is determined that the current temperature is not in the normal range. Thus, steps S 208  and S 210  are performed for preheating. The thermometer  112  or  114  then detects the increased temperature. 
     The preheating technology of the present invention has a particularly significant effect in the application of Multi-Chip Package (MCP).  FIG. 3  illustrates the temperature variations of the components of an MCP data storage device  104 . At the time of powering-on, the flash memory  106 , the controller  108 , and the dynamic random access memory  110  are at an extremely low temperature, minus 40 degrees Celsius. For example, in a snowy environment, the temperature inside the car may be so low when the vehicle is not started. According to the preheating method for the data storage medium, the flash memory  106 , the controller  108  and the dynamic random access memory  110  all generate thermal energy and thereby the preheating effect is achieved. The preheating can significantly speed up in a multi-chip package. Due to the preheating, the temperature of the components within the data storage device  104  is raised. As shown, the flash memory  106  is heated from minus 40 degrees Celsius to minus 20 degrees Celsius, and the controller  108  is heated from minus 40 degrees Celsius to minus 10 degrees Celsius, and the dynamic random access memory  110  is heated from minus 40 degrees Celsius to minus 20 degrees Celsius. Taking the vehicle electronics in a snowy environment as an example, even if the air conditioner on the vehicle is not fully functioning, the data storage device  104  of the in-vehicle computer  100  can work normally due to the preheating method for the data storage medium. Because of the preheating design of the present invention, the operations of the data storage device  104  do not depend on the air conditioner. 
     Corresponding to the different temperatures (25 degrees Celsius, minus 40 degrees Celsius, minus 30 degrees Celsius, and minus 20 degrees Celsius),  FIG. 4  shows voltage distribution probability of triple level cells (TLCs) at different digital representations. The threshold voltages (Vth) for identifying the digital representation of the TLCs can be obtained from  FIG. 4 . The following description is made corresponding to  FIG. 3 . When power has just been turned on, the flash memory  106  is at minus 40 degrees Celsius. Compared with a distribution probability of 25 degrees Celsius, the TLCs at minus 40 degree Celsius correspond to threshold voltages (Vth) which are obviously shifted to the left, and are in danger of falling into an unrecognizable area. The flash memory  106  is warmed up to minus 30 degrees Celsius by the pre-heating operation. Compared with the distribution probability of minus 40 degrees Celsius, the TLCs at minus 30 degree Celsius correspond to threshold voltages (Vth) which are slightly modified, and the right shift causes fewer chances of falling into an unrecognizable area. After reaching the preheating target (minus 20 degrees Celsius), the threshold voltages (Vth) for the TLCs are shifted to the right again, successfully avoiding falling into an unrecognizable area. The preheating technology of the present invention significantly improves the reliability of the data stored in the flash memory  106 . 
     Any techniques in which the controller  108  performs dummy operations on the flash memory  106  to achieve the purpose of preheating belong to the scope of the present invention. While the invention has been described by way of example and in terms of the preferred embodiments, it should be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.