Cell-phone system designers have more choices today in picking a flash memory solution. Identifying and selecting the right solution that meets their design objectives is very important to ensure success. Of course, each memory solution has its pros and cons.
NOR memory is the oldest and the most widely used solution. In these designs, NOR is used for both code and data storage. The code is executed directly (execute-in-place) from the NOR memory. The volatile RAM stores the run-time variables and data structures. The NOR can be designed into a system easily because it interfaces directly with the host processor, reducing time-to-market.
If an application requires more code and data storage, NOR becomes very expensive because it's pricey and only available in densities up to 1 Gbit. Because of these issues, NOR is mainly used in low-end cell phones. NAND is replacing NOR to store music, photo, video, and movie files in high-end phones.
NOR Plus NAND Memory
The NOR plus NAND memory uses NOR to store the BIOS code, while the NAND is used for both code (operating system and applications) and data storage. The volatile RAM is used for storing run-time variables and data structures and for code execution. This memory solution uses either code shadowing or demand paging for executing the OS and applications stored in the NAND.
With code shadowing, the entire OS and all the applications are shadowed to the volatile RAM and are then executed from the volatile RAM. This approach requires a bigger RAM, which increases the overall system cost and power consumption.
With demand paging, the system does not shadow the entire OS and applications to the volatile RAM. Only the required components of the OS are copied to the volatile RAM. The remaining components of the OS and required applications are copied only on demand. This approach uses smaller RAM, reducing the overall system cost and power consumption. However, demand paging requires a more advanced OS on the host, increasing system complexity.
The NOR plus NAND solution suits applications that require large data storage because NAND is available in high densities (mono die up to 32 Gbit). It offers all the advantages of NOR for code storage and lower-cost NAND for data storage. It's still expensive, though, as it uses a pricey NOR just for system boot-up and BIOS code execution.
Unlike NOR, NAND can not directly interface with the processor, nor does it support execute-in-place and random access. Also, since NAND memory is less reliable, it requires error detection and correction, defect management, and wear-leveling implemented either in a separate controller or in an embedded controller in the host.
This solution is very similar to NOR plus NAND, except it eliminates the pricey boot NOR. The NAND controller or an embedded boot ROM in the host can provide the boot function. The NAND memory solution has similar disadvantages as the NOR plus NAND solution, yet it reduces cost by eliminating the pricey NOR.
Hybrid memory solutions like Samsung OneNAND and Sandisk MDOC H3 use SRAM and NAND. They interface to the host using a NOR-type bus interface and offer faster read performance than NAND and faster write performance than NOR. Due to faster write performance, these devices are more suitable for storing music, photo, video, and movie files in high-end phones.
Hybrid solutions are available in high densities because they use NAND for nonvolatile storage. These solutions can boot directly from the NAND, eliminating the pricey boot NOR in high-end cell phones and reducing overall system cost. They also reduce component count and save board space. However, these hybrid solutions have longer boot time. They are complex and difficult to integrate, and they require an advanced OS on the host that supports demand paging.
On the other hand, the Spansion ORNAND combines up to 1 Gbit of MirrorBit NOR with a NAND interface and offers faster write than its previous NOR. But the maximum density of ORNAND is far behind the maximum density of NAND in the market today. It is very clear that the high-density NOR manufacturers have a lot of catching up to do to remain competitive.
SST's All-in-OneMemory takes several major steps beyond the other solutions in the market. This completely managed memory subsystem includes execute-in-place (XIP) code storage, data storage, and system RAM memory components in a single package and on a single bus. Also, this unified architecture blends key benefits of NOR (fast read), NAND (lower cost and higher density), and RAM (simple bus operation) memory. It offers multi-gigabytes of XIP code storage and satisfies the growing data storage needs of today’s multimedia cell-phone applications.
All-in-OneMemory consists of a memory controller with built-in boot NOR, NAND, and volatile RAM, all in a single package. The RAM block is divided into two direct host-accessible user-configured sections: a cache partition for pseudo-NOR (PNOR) and a system RAM partition for the host. The NAND block is used as nonvolatile storage for the PNOR area and memory-mapped ATA NAND disk area. The expandable PNOR block replaces the pricey high-density NOR in a conventional memory subsystem by emulating high-density NOR using RAM and NAND.
Additionally, All-in-OneMemory offers instant secure boot, memory demand paging, NAND flash management, and the industry-standard ATA data-storage protocol on the RAM bus in a small-footprint package. By managing the system's key memory components in a single package, All-in-OneMemory simplifies the host interface, reduces system complexity, shortens design time, reduces overall system cost, and improves quality and reliability.
Added benefits include a large and expandable XIP PNOR area, programmable memory area size, robust hardware error detection and correction for MLC and SLC NAND, scalability, and configurability. Also, because All-in-OneMemory doesn't require any complicated software and hardware development, it offers ease-of-integration and accelerates time-to-market.