NAND vs NOR Flash: Differences and Uses
Both are non-volatile flash memories, but they are wired differently. NOR flash connects its cells in parallel, so it offers fast random reads and can run code in place, but it is expensive per bit. NAND flash connects its cells in series, so it packs far more bits into the same area and is cheap, but it reads and writes in pages and needs error correction. That is why NAND stores your data and NOR stores firmware.
| Aspect | NAND flash | NOR flash |
|---|---|---|
| Cell connection | Cells are connected in series in strings, with select transistors at both ends | Cells are connected in parallel to the bit line, so each cell can be accessed directly |
| Read access | Read in pages (several KB at a time). The first access takes tens of microseconds, so random reads are slow | Fast random access at the byte or word level, in the range of tens of nanoseconds |
| Write (program) | Programmed one page at a time; high throughput for sequential writes | Programmed in small units; slower for large amounts of data |
| Erase unit | Block (a few MB or more) | Sector (typically 64 to 128 KB) |
| Programming mechanism | Fowler-Nordheim tunneling for both program and erase | Channel hot-electron injection to program, tunneling to erase |
| Density and cost per bit | High density and low cost; 3D NAND stacks hundreds of layers, and TLC/QLC store 3 to 4 bits per cell | Lower density (usually up to a few gigabits per chip) and higher cost per bit |
| Execute in place (XIP) | No. Code must be copied to RAM first | Yes. A processor can run code directly from the chip |
| Reliability | Bad blocks and bit errors are normal, so strong ECC, bad-block management and wear leveling are required | Few bit errors, so only light error correction is needed |
| Endurance | From about 100,000 program/erase cycles (SLC) down to roughly 1,000 or fewer (QLC) | Typically around 100,000 program/erase cycles |
| Typical uses | SSDs, USB drives, memory cards, smartphone storage (eMMC, UFS) | Boot code and firmware: BIOS/UEFI chips, microcontroller code, automotive and industrial devices |
Figures are typical orders of magnitude and vary by process generation and product.