Page 55 - Embedded Microprocessor Systems Real World Design

Programming flash memories typically requires a specified sequence of writes to
specific locations. The Atmel AT49F080 uses the following sequence to initiate the
erase cycle:
Ad& Data
5555 AA
2AAA 55
5555 80
5555 AA
2AAA 55
5555 10
Once erase is started, the memorywill complete the operation itself, timing it inter-
nally. Similar command sequences are used to protect and unprotect the boot block,
to request the manufacturer’s ID fiom the device, and to program a byte in the memory.
Most flash devices use -DATA polling when programming. This allows the
processor to poll the device by attempting to read the location just programmed.
The flash memory returns the complement of the data that was written until the
internally timed programming cycle is complete.
Not all block-organized flash memories have a single small boot block and a
larger main block. Some have multiple boot blocks, and some divide the memory
into a few large blocks.
Most modern flash devices can be programmed using only the normal supply
voltage (5V, 2.7V, or 3V). Internal charge pumps generate the higher voltage
needed for programming (typically 12V). Flash devices also can be programmed
in a PROM programmer, which usually allows the boot block erase lockout to be
overridden. Some microcontrollers with internal flash memory require an exter-
nal programming voltage.
While programming flash memory is different from programming an EPROM,
reading a flash memory is exactly like reading an EPROM. The flash memory will
have an additional input that controls writing of the memory array and which is
inactive during reading.
Flash memory devices also have a means to read the device manufacturer and
ID code. This is useful for device programmers, but it also is often needed for
in-circuit programming. Different manufacturers have different algorithms for
erasing and programming flash memory. If you want to have multiple sources
for the flash memory in your design, your software will need to read the flash to
determine which device is installed so it can determine which programming algo-
rithm to use. You also will need to retain multiple programming algorithms in
memory, one for each type of device you can substitute into the system. This was
no problem with EPROM-all 27256 EPROMs work the same when reading. Pro-
gramming differences were taken care of by the device programmer. On the other
hand, EPROMs cannot be reprogrammed in-circuit.

40 Embedded Microprocessor Systems