How Do You Write a Single Variable to Flash?

The method for erasing and programming flash depends on the particular device. You really have to check the data sheet. Typically, you have to present a specific sequence of values on the address and data busses, followed by the address and value you want to write. For AMD-style parts, which is pretty much anyone but Intel, the sequence is usually something like:

Addr Data
---- -----
AAAA 55
5555 AA
SA A0 ; SA is any address in the sector
addr data ; Addr is the actual address to program,
;data is the value you want

Flash memory is usually divided into "sectors" of some number of words, with the size depending on the device.

Flash sectors must be erased before writing. An entire sector must be erased as a unit. Erasure sets all bits in the sector to '1'. Programming (writing) flash can only set a 1 to a 0. You can program flash multiple times without erasing, and you can even program the same location multiple times if it happens that you never need to change a 0 to a 1.

Many micros with built-in flash also have a built-in code library in their boot ROM with routines to do the programming that you can call. See your data sheet.

While flash does have a limited number of write cycles, this number is generally pretty high if you're talking about configuration data. Typical lifetimes these days are 1,000,000 erase cycles for generic parts. Sometimes you see 100,000. Flash intended only for program updates -- usually built into a micro -- sometimes guarantee only 10,000 erase cycles. But even that is enough to reprogram the chip twice a day, every day, for ten years. You can't use flash for operating variables as though it were RAM, but for holding code or configuration parameters, and even for a lot of logging applications, the lifetime really isn't much of a concern in practice.

Is everyone else eagerly awaiting MRAM?

The method for erasing and programming flash depends on the particular device. You really have to check the data sheet. Typically, you have to present a specific sequence of values on the address and data busses, followed by the address and value you want to write. For AMD-style parts, which is pretty much anyone but Intel, the sequence is usually something like:

Addr Data
---- -----
AAAA 55
5555 AA
SA A0 ; SA is any address in the sector
addr data ; Addr is the actual address to program,
;data is the value you want

Flash memory is usually divided into "sectors" of some number of words, with the size depending on the device.

Flash sectors must be erased before writing. An entire sector must be erased as a unit. Erasure sets all bits in the sector to '1'. Programming (writing) flash can only set a 1 to a 0. You can program flash multiple times without erasing, and you can even program the same location multiple times if it happens that you never need to change a 0 to a 1.

Many micros with built-in flash also have a built-in code library in their boot ROM with routines to do the programming that you can call. See your data sheet.

While flash does have a limited number of write cycles, this number is generally pretty high if you're talking about configuration data. Typical lifetimes these days are 1,000,000 erase cycles for generic parts. Sometimes you see 100,000. Flash intended only for program updates -- usually built into a micro -- sometimes guarantee only 10,000 erase cycles. But even that is enough to reprogram the chip twice a day, every day, for ten years. You can't use flash for operating variables as though it were RAM, but for holding code or configuration parameters, and even for a lot of logging applications, the lifetime really isn't much of a concern in practice.

Is everyone else eagerly awaiting MRAM?

Is everyone else eagerly awaiting MRAM?
Awaiting? I am using it.

Erik

Interesting. What's the application? And who's the vendor? How does the cost stack up against flash?

Interesting. What's the application? And who's the vendor? How does the cost stack up against flash?
Frequently (every 20min updated data that must be kept during powerdown.
Ramtron
about flash *3

Erik

"RAMTRON"

That's FRAM - not MRAM.

Motorola is working on MRAM - I saw mention of samples due in "late 2003"...

sorry, FerroMagnetic there you go.

Erik

"sorry, FerroMagnetic there you go."

Well, there's yer problem, mate: it's Ferroelectric RAM, innit?

http://www.ramtron.com/aboutfram/framfacts.htm