On Sun, Jun 28, 2009 at 15:25, Frieder Ferlemannfrieder.ferlemann@web.de wrote:
Hi Urja,
I've just seen that 128MBit serial flash memories with SPI interface (like M25P128) seem to exist. I don't know if/when devices of this type will find their way into motherboards, but these devices would already today reach the limits of serial-flash-protocol.txt (24 bit address and length fields).
Could you add a reserved (and for the foreseeable future ignored) byte to these fields or directly use 32 bit?
I've tried to keep the protocol overhead as minimal as possible, this made me select 3-byte addresses and lengths. The protocol is extensible as in that it's possible to add read n extended (or the like) operation(s) later on, if this is needed. As you might have noticed, if i understood correctly, the M25P128 also reached the limit of the standard motherboard SPI flash memory size (24 bits, that's part of the reason why i chose 24 bits, no motherboard can address more).
I'd like to have some more input on this (somebody else), as I'd hate to slow down the protocol just in case somebody makes 256-Mbit SPI flash chips in the future (oh, somebody will, i know). My current view is that when this is needed, the protocol can extend by having the 6 commands currently using 24-bit values get an _EXT version for 32-bit values, and document how the 24-bit addresses map with 32-bit addresses. (i'd say that the 24-bit address space is top mapped in the 32-bit space, but not sure)