Flash Device Options

All flash devices are structured as a number of sectors. On some devices, the sectors are all the same size ; on others the sector sizes vary. Some have features that allow the firmware to lock a sector so that it cannot be erased unless a physical connection is inserted onto the hardware. Some devices have reset input lines, and others do not. Densities vary from 64KB to 8MB in a single flash device.

Flash Locking Facilities

Erasing or writing flash memory involves a special, nontrivial algorithm. Thus, it is relatively safe to assume that this algorithm will not be executed accidentally . However, it is still nice to have the option to protect certain sectors from misbehaving code. Many of the available flash devices have the ability to protect one or more sectors from write operations. Some of these devices allow you to protect a specified sector or group of sectors by placing the device in an external programming device and applying a high voltage to one of the pins. Others have a more flexible configuration that uses an external write protect pin and a lock sequence. In this latter type of device, a sector is write-protected or locked by executing a specific command sequence. Once locked, a sector can only be modified after first being unlocked. This process makes it even more difficult to corrupt a sector accidentally. Locking can be used to assure that some very basic boot code is always available for the CPU regardless of what happens to the programming of the other sectors.

If this safeguard is still not enough, an alternate technique can prevent a sector from being modified until a power cycle occurs. This method is typically accomplished by enabling the write-protect pin and then initiating the lock sequence to a particular device sector. Because there is no unlock sequence that works when the write-protect pin is enabled and the write protect pin can not change state until the next hard reset, the sector is protected from all erroneous writes except those that happen immediately upon boot. If you need more protection than this method provides, use EPROM!

Bottom-Boot and Top-Boot Flash Devices

Some devices are organized as a few small sectors at the bottom address space, followed by large sectors that fill the remaining space. Some devices have a few small sectors at the top of the devices address space and use large sectors in the lower address space. Since boot code is typically placed in small sectors, flash memory is some times described as bottom-boot or top-boot, depending on where the smaller sectors are located.

Ultimately, one sector contains the memory space that the CPU accesses as a result of a power cycle or reset (boot). This sector is usually referred to as the boot sector . Because some CPUs have a reset vector that is at the top of memory space and others have a reset vector at the bottom of memory space, the flash devices come in bottom-boot and top-boot flavors. A processor that boots to the top of memory space would probably use a top-boot device, and a processor that boots to the bottom of its memory space would be more suited to a bottom-boot device.

When the boot sector of the flash device exists in the boot-time address space of the CPU, it can be protected by making the boot sectors unmodifiable. Since only a small amount of code is typically needed to provide a basic boot, there is little wasted space if you dedicate a small sector to an unmodifiable boot routine. This supports the ability to make as much of the flash space in-system reprogrammable without losing the security of knowing that if all of the reprogrammable flash was accidentally corrupted, the system would still be able to boot through the small amount of code in the unmodifiable boot sector.