Page 56 - Embedded Microprocessor Systems Real World Design

Flash memory is a type of electrically erasable PROM (EEPROM). For general
use in program storage, devices designated as EEPROM mostly have been replaced
by flash memory. However, in some applications, specialized EEPROMs are very
common. We’ll address these later.
When an EPROM needs to be erased and reprogrammed, you just pull it out of
the socket and take it to a PROM eraser and then to a programmer. As flash mem-
ories have grown in density, this becomes impractical since they no longer fit in a
dual inline package (DIP). Early flash parts were available in PLCC (plastic leaded
chip carrier) packages, which could be socketed, but many newer parts are only
available in packages such as TSOP (thin small outline package) or BGA (Ball grid
array) that are difficult or impossible to put in a socket. The parts are soldered on
the board. The result is that in many designs, the only way to program the flash
memory is in-circuit, using the microprocessor itself. This is fine if you want to
program a flash memory to update an existing program. But how does the program
get into the memory in the first place?
Flash memories still can be programmed by a programmer, using a special
socket, before they are installed on the board. But what happens if the vendor (or
your own manufacturing department) inadvertently skips the programming step?
Or what if you get a batch of boards with the wrong program in the flash? Do you
scrap the entire lot?
Some designs are intended to have the flash memory programmed by an in-
circuit programmer when the boards are tested. This is common in high-volume
designs. In a design that does not use this technique, it is a good idea to provide
a means to program the flash using an external fixture. To do this, the micro-
processor must tristate its address and data lines so the external circuit can get to
the flash. If the microprocessor supports DMA (see the DMA section later in this
chapter), it can be put in a hold state. Many processors will tristate their buses if
they are held in reset. If buffers or latches are used for the signals, or if the signals
pass through a PLD before they reach the flash, they can be tristated there.
Once the processor has released the bus, some means must be provided to access
the flash memory. This can be accomplished with a connector that brings out the
addreddata buses and control signals. If there is no room for that, a matrix of
pads on the PCB, accessed with spring-loaded test pins, can be used instead.
Finally, an alternative to directly programming the flash is to provide a means,
such as a header, to install a daughterboard containing a small flash memory
that replaces the system flash. By mapping part of the system flash to a different
location in memory when the daughterboard is installed, the boot portion of
the system flash can be reprogrammed. Such remapping can be accomplished
with a jumper on the main board, or it could be automatically activated when the
daughterboard is installed. The boot portion of the system flash, of course, would
be programmed with code that permits the remaining flash memory to be
programmed.

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