PowerProtector: Superior Data Protection for NAND Flash

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1 PowerProtector: Superior Data Protection for NAND Flash Introduction The unstable power conditions of outdoor applications such as transportation, telecommunications/networking and embedded systems run the risk of data loss and drive corruption during a sudden power failure. Power failures result in downtime as embedded systems need to be reformatted and operating systems reinstalled. Furthermore, the overall productivity decreases as the cost of ownership increases for industrial grade memory solutions, which are specifically designed for high performance mission critical applications. Industrial grade storage such as embedded modules and SSDs require superior performance and reliability that surpass existing SuperCap designs. ATP s Power Protector Technology ensures data integrity during a sudden power down under all conditions. The Four Levels of Power Protection During the data write onto a page of NAND flash, the entire procedure must be completed to ensure that the data string is stored properly. The risk of data

2 loss is high during the writing procedure when a power failure occurs in the NAND flash memory. Risk of data loss is much higher in a Multiple Level Cell (MLC) NAND flash storage device, since the MLC product stores two bits of data, as opposed to one bit of data in a Single Level Cell (SLC) product. A power failure during the write procedure to one of the two bits causes destruction of previously stored data. NAND flash memory is exposed to the risk of data loss in the following scenarios during a system power outage: a. Damaged Firmware Protection (Level I): During a sudden power loss, firmware in some solid-state drives can cause corruption. A power protection feature can prevent this type of malfunction from occurring. b. Linking Table Damage Protection (Level II): A NAND flash device creates a linking table to keep track of the physical flash pages with respect to the logical block addresses from the host end. The linking table must be constantly updated so that the host can access the most current data via the two mapping addresses recorded on the flash device and the host. However, the linking table is also cached first and re-written to the flash chips from time to time, so frequent updates to the index do not disrupt the ongoing writing performance. If the power supply is disrupted while some mapping information is still in the flash cache, the mapping index will become damaged, along with the data in the flash mapped by the respective linking table. The worst case scenario results in a total destruction of the index table, causing a complete data loss. The controller with the power protection design backs up multiple copies of the linking table for future restore situations when the linking table is distorted, or lost during a sudden power loss. Thus, the risk of complete data

3 loss as a result of losing the linking table is eliminated. c. Page/Block Protection (Level III): When writing data onto flash chips, the data is divided into segments of mini-second (ms) program operations. These subdivided operations are written onto the flash chips one operation at a time. Figure 1 illustrates a blackout occurring during multiple segments of mini-second (ms) program operations. A device without advanced power protection technology can provide limited data protection and can result in data loss in the eighth mini-second programming operation. The data, written by the previous program operation 7 and prior operations is retained, as the data has already been written onto the flash chips during programming operations 1 to 7. However, after a reboot, if the host tries to access the data written by program operation 8, a data error will occur. Figure 1: A Blackout during a Mini-Second Programming Operation d. Last Write Page/Block Protection (Level IV): An advanced power protection feature provides sufficient power to allow the last mini-second program operation to complete. For instance, in Figure 1, enough power is stored for the flash product to complete program operation 8 during a power failure so that the data contained in the program 8 operation is safely written onto the flash chip. The following table (Table 1) summarizes the various levels of power

4 protection against data loss: Level More Advanced IV III II Description of Power Protection Levels Last page/block protection (for data being written onto the flash) Page/Block protection Index Table protection Less Advanced I F/W protection for the next boot up Table 1: Different Power Protection Levels Losing data in diverse blackout situations is a serious threat to industrial applications, as system downtime and the consequent system restore can result in significant financial loss. In order to reduce the risks caused by power outages, an industrial grade NAND flash device must incorporate a back up power protection solution. The Working Concept of PowerProtector A stand alone hardware design is the ideal configuration for power backup. This ensures a sufficient amount of reserve power during any power abnormality and minimizes the consequent host re-designs for the addition new features. The reliable back up power circuit provides sufficient power for a complete finish of the write operation within the NAND flash product during a power failure. A power loss detection circuit activates the power protection mechanism during a sudden power failure. The device then draws power from the power

5 protection reservoir. The reserve power allows enough time for the flash device to conclude the last writing command without losing any data. Figure 2: The Working Concept of PowerProtector The diagrams below further explain how the ATP PowerProtector technology prolongs the functionality of NAND flash in two scenarios, essentially allowing the completion of the last command or data given by the controller: Figure 3: The Power Vs Time Diagram for PowerProtector during a Power Glitch

6 Figure 4: The Power Vs Time Diagram for PowerProtector during a Permanent Power Outage Figure 3 illustrates how PowerProtector supplies enough power during a power glitch for the flash product to complete an operation and continue the next operation once power resumes. Figure 4 demonstrates how PowerProtector allows the storage device to finish the last operation and stops the controller from sending further data to the flash chip. Proper operational shut-down can prevent data errors from occurring when a power cut-off occurs in the middle of a power program/erase operation. Table 2: PowerProtector Operating Times Table 2 illustrates the typical and maximum PowerProtector operating times for completing various commands such as read, program and erase during a

7 power failure. In a NAND product, the read and program commands are executed one page at a time and can take up to 0.5ms and 3ms respectively. On the other hand, the erase command is conducted one block at a time and can take up to 10ms. The ATP PowerProtector stores enough power to operate more than 10ms during a power down to complete the last command. Available Power Back Up Solutions: a. Battery based solution: Battery based solutions are not ideal for embedded applications due to their need for replacement over time and high maintenance costs. In addition, legislation in some countries does not allow for the use of battery solutions due to safety concerns. b. SuperCap based solution: SuperCap is the most the common backup power solution on the market. However, due to certain limitations it is not suitable for industrial applications. SuperCaps are well known for their sensitivity to temperature change and their tendency of losing capacitance and functionality at extreme temperatures that are frequently found in mission critical and industrial environments. Bulky Supercaps, like batteries, also cannot be implemented in small form factor embedded modules with restricted space requirements. Moreover, SuperCap requires specific controllers, customized firmware and complicated host system re-layouts. A SuperCap based solution has an average life span of less than two years; the capacitance degrades over time and eventually fails to perform. This is solution is not suitable given that the typical life span requirement for industrial applications is over five years.

8 c. Solid state (Tantalum) capacitor based solution- The ATP PowerProtector A solid-state capacitor based power back up solution is a stand-alone hardware design and requires no specific controllers or customized firmware, providing greater flexibility during the design of a new product or a customized form factor. The design can support multiple form factors (eusb, CF, SSD) with a wide temperature range of up to 85 C and has a longer life span. A series of tantalum capacitors is a good choice for solid sate capacitors, since it has a relatively thin dielectric, allowing high capacitance values. Dry tantalum capacitors result in longer life times and present very low current leakage during extended usage. The physical size of tantalum solid-state capacitors is much smaller, which is another advantage for small form factor flash product designs. The smaller size of tantalum capacitors also requires a lower voltage rating. Table 3 summarizes the three different backup solutions designs on the market, and highlights the Tantalum Cap (ATP PowerProtector) advantages. Table 3: A Comparison Table of Back up Solutions ATP Power Cycling Test Technology

9 Comparable products provide similar PowerProtector features on their NAND flash products, but few can validate their designs. ATP has combined its knowledge of NAND flash internal working principles and the continuous enhancement of NAND flash products to introduce ATP s own power cycling test technology. The ATP Power Cycling Test demonstrates the functionality of a product with ATP PowerProtector under sudden power-downs situations. Conversely, a product without the ATP PowerProtector technology will usually fail. At the beginning of each cycle, the ATP power cycling test first writes a segment of data pattern into a testing device and removes power at the exact micro second while the device is programing data into NAND blocks. The ATP power-cycling test can create a sudden power failure at the precise moment on the P08 Stage as seen in Figure 1. After certain power cycles, a product without the PowerProtector feature results in data corruption at this stage and shows a Fail. This is due to contrasting data in the post- test check conducted after each power cycle. On the contrary, a product with ATP PowerProtector will show a Pass even after thousands of power cycles without any data loss (Figure 5).

10 Fig ure 5: The Screen Shots of ATP Power Cycling Tester During the Power Cycling Test, a device without ATP PowerProtector provides a timeframe of few-hundreds-micro-seconds when data loss is most likely to occur during the occurrence of a power glitch. This finding is critical for product development teams for fine-tuning the programming of future NAND flash products with their host systems. The apparent different testing results validate the effectiveness of the ATP Power Protector feature in a NAND flash product. Conclusion Power back up solution offer an advanced level of protection ensuring data integrity is not compromised during a power failure scenario and preserves data in mission critical applications. An advanced power back up solution

11 requires the following features: a stand-alone design, solid-state capacitors and long life span. ATP PowerProtector offers these features, which exceed the performance and reliability offered by other existing backup solutions. Moreover, the ATP engineering team has developed the ATP Power Cycling Test, which removes power at the exact microsecond during the programing of the NAND flash product, validating the effectiveness of the PowerProtector technology. About ATP ATP Electronics is a leading manufacturer of high performance, high quality and durable NAND flash memory solutions and DRAM memory modules. With over twenty years of experience in the design, manufacturing and support of memory products, ATP continues to focus on mission critical applications such as industrial, telecom, medical and enterprise computing where high levels of technical support, performance consistency and wide operating temperature ranges are required. As a true manufacturer, ATP offers in house design, testing and product tuning. ATP also offers extensive supply chain support with controlled/fixed BOM s and long product life cycles. For more information on ATP products please visit