[LinuxBIOS] rom-madness: CFI vs FWH vs SPI, 5V, 3.3, ...

Adam Talbot talbotx at comcast.net
Tue Apr 10 10:16:01 CEST 2007

Wow, that is a great history.  Any chance to put that on the wiki?
-Adam Talbot

Peter Stuge wrote:
> On Tue, Apr 10, 2007 at 03:54:22AM +0200, Luc Verhaegen wrote:
>> I asked about the 3.3V vs 5V roms, and i only got further confusion...
> I'll go chronologically.
> First, there's parallel flash. Parallel flash on PC mainboards has an
> address bus which is between 17 and 19 bits wide and a data bus which
> is 8 bits wide. Naturally many flash chips can be combined with glue
> logic if a larger address bus is required but I haven't seen that
> since 286:s.
> Parallel flash typically comes in 32 pin DIP (rectangular, pins along
> both longer sides), PLCC (nearly square, pins on all four sides) or
> 32, 40 or 44 pin TSOP (about 1mm thick, rectangular, pins along both
> shorter sides) packages.
> Parallel flash found in PCs uses 5V signalling. As Segher pointed
> out, early flash chips required a special programming voltage around
> 12V for erasing and programming, but after a while reprogramming
> became possible with a single voltage supply.
> Then, there's the LPC bus, Low Pin Count Interface. LPC is a five pin
> bus, four combined address/data lines and one clock. There's also a
> handful optional sync and control signals for better reliability.
> LPC does not mandate a single electrical signalling environment,
> instead it refers to the PCI signalling used in the system. If system
> PCI uses 5V then LPC devices must handle 5V. If system PCI uses 3.3V
> then LPC devices must work with 3.3V. LPC chips come in 32-pin PLCC
> packages. 3.3V LPC devices may break if put in a 5V parallel socket.
> 5V LPC flash is not common.
> FWH, Firmware Hub, is an LPC flash ROM with some extra functionality
> like a couple of GPIO signals and also a few pins for arbitration
> between several FWH chips on the same LPC bus.
> So far, flash has been programmed the same way, using a protocol
> standardized by JEDEC, short for the Joint Electronic Device
> Engineering Council.
> Any flash chip data sheet documents the commands. One thing to note
> is that in order to do anything but read data from the memory (ie.
> identify the particular chip, erase it or program it) writes need to
> be made to the chip. A data protection scheme requires certain data
> to be written to certain addresses in different patterns depending on
> the desired operation.
> SPI, Serial Peripheral Interface, is a four-wire bus (chip select,
> serial in, serial out and clock) that is becoming popular because
> memory devices can be made smaller, with fewer pins, which allows
> more space for other chips on the board. SPI chips are usually in
> 8-pin SOIC (about 1x1 cm, a couple of mm high, pins on two opposite
> sides) packages and those found on PC mainboards so far have been
> using 3.3V signals, but SPI also does not mandate a single electrical
> signalling environment.
> SPI chips use a different command set, but the principles are still
> the same, there are commands for block erase, chip erase, byte
> program and identification. SPI being serial also has a stream
> program mode where only one command needs to be sent to the device
> followed by a large stream of data that should be written to memory.
> All flash needs to be erased before programmed. Erased flash has all
> bits set. Programming flash entails flipping bits to 0 at appropriate
> places. Bits can not be flipped back to 1 individually, only by erase
> commands.
> Flash has different page sizes. Flash can have boot block protection
> requiring special action to be taken in order to erase the top or
> bottom pages.
> These reprogramming parameters depend on the device ID, which as
> mentioned earlier can only be read from the device when writes to the
> chip are working.
> CFI is the Common Flash Interface as developed by Intel and friends
> and published by Intel.
> CFI amounts to storing the parameters themselves in the flash chip,
> rather than just a device ID. This way, software written for CFI will
> be future proof and not need upgrading any time a new device ID is
> put on the market.
> CFI covers both 8-bit wide and 16-bit wide flash.
> //Peter

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