Discussions about flash data storage typically focus on NAND technologies, but NOR is another type of flash memory that, in some circumstances, can be a better fit, especially as an embedded device.
NOR supports faster random read operations than NAND. It's also more reliable and consumes less power. But NOR flash memory is slower for sequential reads as well as write and erase operations, and it's more expensive, which is why it's suited only to specific use cases.
NOR flash memory basics
NOR is a non-volatile memory technology that stores data in memory cells, similar to a NAND memory chip. A NOR cell is similar to a metal-oxide-semiconductor field-effect transistor (MOSFET), in which voltage determines the electrical conductivity. The main difference between a NOR cell and a regular MOSFET is that the NOR cell contains two gates instead of one: the floating gate and the control gate.
The gates determine how electrical charges are stored in the cell. An insulating layer separates the two gates, and a tunnel oxide layer separates the floating gate from the underlying substrate. The tunnel oxide layer is thin enough to allow electrons to pass in and out of the floating gate, thus providing the mechanism for storing data. One end of the cell is connected to the source line, or ground, and the other end to the bit line, which is used to program the cells.
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Today's NOR memory is available in two types: parallel and serial. Parallel NOR, the larger of the two, has been around longer and has a much higher pin count. It uses a parallel address and data bus, similar to static RAM, and delivers higher system throughput than serial NOR, leading to faster system boot times.
Serial NOR is smaller than parallel NOR, uses less power and typically costs less, while still delivering good performance. Many serial NOR modules now use the serial peripheral interface bus, which is why they're often referred to as SPI NOR.
NOR vs. NAND flash memory
NOR flash performs random read operations faster than NAND. NOR also tends to be more reliable and consumes less power. However, NOR is slower at serial reads and erasing and writing data, and it's more expensive to produce than NAND, resulting in a higher cost per bit. A NAND storage device can deliver greater data storage capacities, whereas a NOR module typically tops out at 2 Gb.
Unlike NAND flash, NOR uses no shared connections, provides direct connectivity to individual memory cells and has enough address and data lines to map the entire memory region. As a result, NOR can deliver faster random access to any location in the memory array. With NAND flash, memory cells are strung together to increase density, resulting in more complex read operations and longer random-access times.
NAND is faster at reading data sequentially and at programming and erasing data. It can also write data in smaller chunks than NOR, and the mechanisms for erasing data are more efficient. In addition, NAND memory cells are much smaller, leading to greater densities and a lower cost per bit. But unlike NOR flash, NAND is more susceptible to bit failure, which is why NAND must incorporate such techniques as error correction code.
NOR flash memory use cases
NAND flash is typically used for data storage and NOR flash is deployed as an embedded device for code execution. NOR flash is used for industrial robotics, medical devices, scientific instruments, IoT devices or portable consumer products such as cameras, wearables or mobile phones. NOR might also be used alongside NAND in devices such as smartphones and tablets.
NOR devices can also be embedded in network components such as routers, switches or gateways. The automotive industry has taken a keen interest in NOR embedded devices, not only because of their reliability, but also because NOR can withstand a greater range of temperatures, making it suitable for components under the hood and on the dashboard. For example, NOR is suitable for touchscreens, infotainment components or advanced driver-assistance systems.
As an embedded device, NOR can be used as code storage for any read-only applications that require fast random access and high reliability, such as boot, OS or execute-in-place code. NOR offers quick initialization times that can benefit systems requiring fast and reliable boot processes. In addition, programs stored on a NOR memory chip can be executed directly, without needing to be copied into RAM, an operation much more difficult with NAND.
Today's NOR flash market
Several vendors offer NOR flash modules, including Cypress Semiconductor Corp., Macronix International Co. Ltd., Micron Technology Inc. and Winbond Electronics Corp. For example, Cypress sells a 256 Mb serial NOR flash module that qualifies for automotive use, with a temperature range of -40 C to 105 C. The device is based on the Quad SPI interface and has an operating voltage of 3 volts. It costs between $4.12 and $7.08, depending on the number of units purchased.
Although it can be difficult to predict NOR's future, the flash technology is proving to be one of the best options for embedding memory into systems such as consumer mobile products, automobile components, IoT devices and other systems. The fast random access reads and long-term reliability make it suitable for many use cases, especially as mobility and IoT continue to expand. NOR might be invisible to most consumers, at least on the surface, but it will likely play a vital role for some time to come.