Re: Hardware Project

[david@uow.edu.au (David Wilson)]

dempson@actrix.gen.nz (David Empson) writes:
>Yet another variant is Flash ROM.  I've never been entirely clear on the
>difference between Flash ROM and EEPROM, as they seem to overlap in
>features.  Flash is generally characterised by being able to be erased
>and reprogrammed in sectors, while EEPROM is more often bulk erased and
>reprogrammed one byte at a time.

I had the same query re the difference between Flash and EEPROM.

Here is one explanation from http://webopedia.internet.com/TERM/E/EEPROM.html:

	EEPROM is similar to flash memory
	(sometimes called flash EEPROM).
	The principal difference is that
	EEPROM requires data to be written
	or erased one byte at a time whereas
	flash memory allows data to be
	written or erased in blocks. This
	makes flash memory faster. 

I find this explanation unconvincing but reading a few data sheets gives some
credence to it. For example, the MC68HC912B32 has both EEPROM and Flash EEPROM
and is described as follows:

	The chip is the first microcontroller to include both byte-erasable
	EEPROM and Flash EEPROM on the same device.

The Flash chips I have been using lately in another project are 4Mb (512KByte)
and can be erased in 64KB blocks and are rewritten one byte at a time needing
only 5v supply (the 1Mb parts we had before this needed 12v to program).

The MC68HC12A4 has internal 16 bit wide EEPROM which can be erased either by
the byte, aligned word, aligned row (32 bytes) or bulk (4KB) but can only
be written by the byte or aligned word.

Based on that, Flash seems less flexible than EEPROM. Having to erase 64KB
to rewrite a sector would prevent its use as a standard disk but would still
be useful for a read mostly disk.

EEPROM is typically good for 10,000 erase/write cycles while flash is 100,000
erase/write cycles (based on a sample of two datasheets).
--
David Wilson  School of IT & CS, Uni of Wollongong, Australia  david@uow.edu.au