Using nvSRAMs for power-fail reliability in enterprise SSDs

Use nvSRAM--Page 3.

Nonvolatile SRAM (nvSRAM)

nvSRAM combines two workhorse CMOS technologies into one device - SRAM and SONOS non-volatile technology. During normal power-on system condition, the nvSRAM behaves just like a conventional SRAM. The SRAM part of the IC performs random access read/write at speeds up to 20 ns access times, using standard Asynchronous SRAM signals and timing. But on a power glitch or fail, the intelligence in the chip detects the threat and automatically saves a copy of the SRAM data into non-volatile memory where it can stay unchanged for over 20 years. On power-up RECALL, the IC returns the data copy back to the SRAM and system operation can continue precisely from where it was left last giving you a fast SRAM that never loses data. The latest high density (16Mb) nvSRAMs are also available in high bandwidth DDR NAND Flash (ONFI 3.0/Toggle 2.0) interfaces as well.

The transfer between SRAM and non-volatile array inside is completely parallel (cell for cell) allowing the STORE operation to complete in 8 ms or less, without user knowledge. The IC family also provides user controlled software STORE and RECALL initiation commands, as well as a user controlled hardware STORE initiation command in most versions.

nvSRAM is a highly reliable product, manufactured in a high-volume, proven CMOS + SONOS process. In addition, it has more than 20 years of history in military, commercial, storage, medical, and industrial applications.

Figure 3 shows the concept of nvSRAM, which combines a fast SRAM element and a nonvolatile element into a single cell. Figure 4 shows the cell structure of the nvSRAM.

 

Figure 3: nvSRAM Concept

Figure 4: nvSRAM Cell

Nonvolatile SRAM – Asynchronous Solutions for Enterprise SSD

Figure 5 shows an asynchronous nvSRAM used as the nonvolatile buffer for Enterprise SSD data streams and metadata that needs to be backed up on power loss. The V_CAP capacitor shown in Figure 5 powers the STORE cycle that moves data from SRAM to nonvolatile cells. V_CAP is a standard capacitor of approximately 50 µF (see datasheets for details).

Figure 5: Enterprise SSD Asynchronous nvSRAM Solution

For new designs, asynchronous nvSRAM densities are available today from 256-kbit to 8-Mbit, with 16-Mbit devices being introduced in 2012.

Nonvolatile SRAM – Synchronous Solutions for Enterprise SSD

Figure 6 shows nonvolatile SRAM devices for Enterprise SSDs, based on the new Synchronous High bandwidth (up to 12.8Gbps) NAND interface nvSRAM. These devices will be available at 16-Mbit densities and are currently being sampled, with production in the first quarter of 2013.

Figure 6: Enterprise SSD Synchronous nvSRAM Solution

As explained earlier, a Supercapacitor or discrete bank of tantalum capacitors is used as a secondary voltage source to provide energy required for data transfer from SDRAM to NAND flash on power down.

This transfer from a fast volatile memory to a nonvolatile memory at power down uses the same concept that the Cypress nvSRAM invented 20 years ago. The difference is that the Cypress nvSRAM includes the power detect, data transfer management, fast volatile memory, and nonvolatile memory all in a monolithic IC. The data transfer is completed inside all memory cells at the same time using little power and taking only a few milliseconds. Meanwhile, the SDRAM to flash transfer is done at the system level using high-power I/O connections that can quickly drain a large capacitor and take a much longer time to complete.

The SSD controller in the Enterprise SSD architecture also supports the high-speed synchronous NAND interfaces to NAND flash (ONFI 3.0, Toggle DDR 2.0) devices. High-speed synchronous NAND interface is now supported in time-proven nvSRAM core technology, with industry standard ONFI 3.0/Toggle 2.0 interfaces, to provide Enterprise SSD makers a high performance synchronous nonvolatile memory solution. This new nvSRAM can sit directly on the NAND flash bus and become the active memory space for the critical nonvolatile data (see Figure 6). The new nvSRAM interfaces are being designed to the open standards, and will use standard commands and standard signal timing. This approach eliminates or minimizes the super capacitor/bank of tantalum capacitors and the data transfer logic, and gives the Enterprise SSD system a much shorter backup. Also eliminated are the reliability issues associated with the capacitor back-up solutions.

Enterprise SSDs require reliable and fast backup of critical data streams and metadata during power loss. The current capacitor back-up solutions have serious reliability issues. This article discussed the asynchronous nvSRAM solution and introduced a synchronous nvSRAM that can sit on the NAND flash bus. The nvSRAM provides reliable and fast back-up of critical Enterprise SSD data, eliminating the reliability concerns of super capacitors and the bank of tantalum capacitors in the process.

 

About the Author

Pramodh Tumkur Prakash is Senior Product Marketing Engineer at Cypress.  He has an M.S. in microelectronics with 7 years experience in Application/System Development.  He can be reached at tup@cypress.com.