Smaller, faster, cheaper – This is the tenet of most companies designing consumer devices for a few decades now. Every component being designed-in is scrutinized to keep the device form factor small without compromising performance and cost. To push the envelope around overall system performance, memories need to be fast, dense and tiny too.
As the performance metrics of serial flash memory interfaces match or exceeds that of parallel interfaces, we are seeing more applications switching to serial flash memories as they have a smaller footprint on the board. We witnessed this with NOR flash – shipments of Serial NOR flash devices overtook Parallel NOR in 2014-15. Serial interface for NAND is gaining traction now. The total TAM for SPI NAND devices is expected to grow and the number of unit shipments for low to mid densities (1Gb to 8Gb) is expected to increase as well. The TAM and unit shipments for Parallel NAND are still very significant and expected to remain big and taper off only slowly going forward (Gartner 2015).
The number of active signals in a serial interface is orders of magnitude lower which results in simplified board designs and smaller form factor memory chips. The below snapshot demonstrates the extent of simplification in routing that serial interfaces introduce, saving on board space:
Source: Forward Insights SLC NAND Market report: Report No. FI-NFL-SLC-1014
In the past, most embedded applications relied only on SPI NOR flash devices for code shadowing, storing BIOS, firmware and some data. With the rise in storage requirements, Serial NAND memories are becoming more attractive – it is available in greater densities (starting at 1Gb); it offers a lower cost-per-megabit than any serial flash solution, capable of higher write speeds and features on-chip ECC.
These features are extremely attractive for applications such as wearable devices, digital televisions, set top boxes and routers/gateways.
The tradeoffs are that Serial NAND cannot read data as quickly as Parallel NAND solutions, but this is a problem shared by Serial NOR solutions as well. Even with this minor tradeoff, Serial NAND can be an ideal choice for many applications.
At the system level, there are two design options to pick from –
- Use a combination of SPI NOR (for secure boot-up) and SPI NAND (for high density data storage).
- Replace SPI NOR with SPI NAND device to handle boot up and data storage. However, boot-up times are slower in SPI NAND (by 4x), and may be ideal for applications that do not require very quick boot-up.
Both choices direct designers to strongly consider using SPI NAND as a low cost, high density storage solution alternative based on the application needs.
Leading the pack, Micron was the first memory supplier to introduce SPI NAND (in 2008). With an update in the technology node, the second generation of SPI NAND flash – SLC NAND Flash (for both IT-grade and auto-grade) in 25nm technology was announced recently. These devices will be supported by Micron’s Product Longevity Program (PLP) to ensure long term product availability and support. SLC NAND Flash has also been enabled on key chip sets. It is available in attractive 8-pin, 16-pin and 24-ball package options.
Summarized below are a few key features of SLC NAND Flash devices –
- 25nm Single Level Cell (SLC) technology
- Standard and extended SPI-compatible serial bus interface
- Temperature range supported - -40ºC to +105ºC
- IT-grade and Auto-grade with longevity support (PLP)
- Operating Voltage (Max Clock): 3.3V (133 MHz) and 1.8V (83 MHz)
- Drop in compatible with SPI NOR flash; can be configured to boot up the device
- High clock frequency support and optimal Read/Write performance
- End to end system level validation with total software solution
- On-chip ECC provides data corruption protection using 8-bit internal ECC per sector
- Secures data stored with key security features –
- Volatile Block protect – protects blocks from program/erase operation
- Hardware Write protection – locks Block Lock Register bits
- Device Lock Tight – Prevents block protection state to change
- Ten pages one-time programmable (OTP) NAND Flash Memory area
- SPI NAND software is available in Linux/Uboot/LLD