NONVOLATILE SOLID-STATE STORAGE DRAM by its nature is volatile, but nonvolatile storage is always part of the system solution. Nonvolatile solid-state storage has seen dramatic change over the years with rising capacities and falling costs. A range of technologies is now in general use, from flash to MRAM and FRAM.
Read-only memory (ROM) is a well-known nonvolatile storage technology that’s showing more traction in standard microcontrollers. It has always been a factor in custom chips because it’s the most efficient nonvolatile storage technology. Unfortunately, ROM can’t be changed like the other nonvolatile storage technologies covered here.
One example of ROM use involves Luminary Micro’s LM3S9000 microcontroller, which has runtime libraries that provide StellarisWare Library services. This is in contrast to typical custom ROM-based microcontrollers that contain the entire application. In Luminary Micro’s case, the main application that uses the ROM code is stored in a device using another nonvolatile memory. The ROM may have boot code allowing the main application to come from a range of sources, including via a network connection.
Flash memory covers a wide range of solutions. FRAM and MRAM, which hold lots of promise and are currently used for important yet niche applications, have similar characteristics.
These nonvolatile memories effectively replace SRAM, operating at SRAM speeds. However, they don’t have the write lifetime issues of flash memory. This allows them to be used for primary and secondary storage. Capacities are growing and costs are dropping, but they still trail both SRAM and flash. This leads to some interesting combinations, like the RAID controller that was mentioned earlier.
The 8051-based, VRS51L3xxx microcontroller family from FRAM vendor Ramtron combines 64 kbytes of flash memory, 4 kbytes of SRAM, and up to 8 kbytes of FRAM (Fig. 3). The flash memory is used for program storage and long-term, slowchanging data, while the SRAM and FRAM are used for read/ write data, with FRAM handling nonvolatile chores.
FRAM and MRAM also show up in plug-compatible versions that can replace SRAM and flash parts. Everspin’s MR2Axx MRAM line is pin-compatible with standard 8- and 16-bit SRAM parts. These parts are also available in ball-grid array (BGA) packages with 35-ns read/write times and extended industrial temp versions. Up to 512 kbytes of Everspin’s MRAM parts are used in Emerson Network Power’s Freescale MPC864xD-based MVME7100 single-board computer (Fig. 4). Look for 16-Mbit parts later this year as well as automotive-compatible parts.
Coming soon is phase change memory (PCM) from Numonyx. As with Z-RAM, it will have to challenge entrenched technologies, but its performance and scalability promises to push it past the competition once it becomes established. It’s still a couple of years away, but keep an eye on this technology.
FLASH IT The established technology of the day is flash memory, encompassing a range of implementations. Flash memory found in most standalone flash products exhibits a higher density than that incorporated in microcontrollers. That’s because it must be implemented using the same process as the logic circuits.
Standalone flash memory comes in a range of formats, too, from chips to removable device formats such as Compact Flash, SD/XD, MiniSD, MicroSD, Memory Stick, and, of course, USB flash drives. Many of these are employed in embedded applications as well, leading to more rugged, industrial versions like WinSystems’ 16-Gbyte industrial-grade Compact Flash (Fig. 5). Its dual-channel operation supports sustained read transfers up to 40 Mbytes/s and writes using interleaving up to 30 Mbytes/s.
For embedded applications, even more options are available. Modules that plug into integrated drive electronics (IDE) headers are common replacements for hard drives. Initially, the capacity of these flash drives was low. However, it has grown significantly, allowing these devices to move from boot chores to a complete replacement of hard drives in many applications.
Western Digital Solid State Storage, formally Silicon Systems, is one source of flash drives that utilize the Small Form Factor (SFF) SIG Silicon Blade form factor. The Silicon Drive Blade is a latching, rugged alternative to the 10-pin module also available from Western Digital (Fig. 6). Available from a number of sources, it plugs into the 10-pin header found on most PC motherboards.
Form-factor decisions tend to pale against other technology choices when it comes to flash memory. NAND versus NOR and SLC versus MLC technologies introduce a host of tradeoffs that designers must consider. No one approach satisfies all application requirements. In fact, a mix of technologies is appearing in some more demanding applications.
Some general specs from Toshiba provide some insight into these tradeoffs. For example, NAND erase speeds are 2 ms while NOR is 900 ms. On the other hand, NOR capacities are four times that of NAND, reaching 256 Mbits and growing. NOR’s read speeds, which clock at 103 Mbytes/s, are at least four times faster than NAND. NOR’s write speed, though, is on the order of 0.5 Mbytes/s versus 8 Mbytes/s for SLC NAND.
The SLC versus MLC tradeoff is similar. MLC offers higher density, but at a significant loss of write lifetime. All flash technologies have the limit, which make alternatives like MRAM and FRAM desirable. If these technologies could approach or exceed flash capacity for a similar price, then there would be a major change in the memory landscape. Unfortunately, that’s unlikely in the near term.
This means that wear-leveling techniques are becoming more important, especially given MLC’s limitations in this area and its significantly higher capacity. The target for hard-drive replacement is a five-year lifetime. Though this is sufficient for enterprise solutions, it may not necessarily suit embedded applications that have a longer lifetime. This means designers must pay closer attention to a wider range of specifications than in the past.
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