Embedded Design Handbook

ID 683689
Date 8/28/2023
Public
Document Table of Contents

3.5.4. Flash Memory

Flash memory is a non-volatile memory type used frequently in embedded systems. In FPGA-based embedded systems, flash memory is always external because FPGAs do not contain flash memory. Because flash memory retains its contents after power is removed, it is commonly used to hold microprocessor boot code as well as any data which needs to be preserved in the case of a power failure. Flash memories are available with either a parallel or a serial interface. The fundamental storage technology for parallel and serial flash devices is the same.

Unlike SRAM, flash memory cannot be updated with a simple write transaction. Every write to a flash device uses a write command consisting of a fixed sequence of consecutive read and write transactions. Before flash memory can be written, it must be erased. All flash devices are divided into some number of erase blocks, or sectors, which vary in size, depending on the flash vendor and device size. Entire sections of flash must be erased as a unit; individual words cannot be erased. These requirements sometimes make flash devices difficult to use.