Why Serial Flash has SPI, but Serial EEPROM - I2C ?

Dear,

I'd like to understand why ALL Serial Flash chips always come with SPI vs Serial EEPROM chips that always come with I2C ? Why not vice versa for example? For my current project I'd like to add a small external memory, let's say 16 kBytes, as a backup for some case. Writing to EEPROM is too slow, 5 msec for 8 bytes, wheres Flash is good, but it comes with SPI = 5 PINS: SCK, MISO,MOSI, CS, and in addition I have to switch its VCC off by MCU, since standby current is not desirable, so I need yet another pin. I'd like to have performance of Flash, but with I2C. May be somebody here is aware of such a component ?

Thanks, E.L.

Reply to
elil
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This is just an educated guess. In the general case,

(a) Flash chips have larger capacities than EEPROM's. (b) SPI can transfer data at higher rates than I2C.

It makes sense to match the faster transfer mechanism with the devices having the largest storage capacity. This does not preclude, of course, having a combination the other way around. And no, I am not aware of such a component.

-- Roberto Waltman

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Reply to
Roberto Waltman

Roberto Waltman forgot to write:

Even if it is more expensive (pin-count wise). For lower capacity devices, where transfer time is less important, a lower pin count allows using smaller packages, with cost and easy of routing advantages, etc.

-- Roberto Waltman

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Reply to
Roberto Waltman

That can't be understood, because it simply isn't true. SPI EEPROMs exist. Microwire (which is really just a subset of SPI) EEPROMs exist.

Possibly because Flash by design has a massive asymmetry of read vs. write speeds. To accommodate that speed in a useful fashion, the interface has to be _fast_. I2C usually just isn't fast enough.

Reply to
Hans-Bernhard Bröker

For read cycles the interface matters, when it comes to flash or EEPROM writes cycles even slow I2C (much less than 100kHz) is often TOO fast for multibyte write cycles. let alone times for page erases.

Newer serial devices for I2C support even the 3.4MHz I2C spec that has been around for a long time, I2C has never been limited to 400kHz, just by some controller designs.

I tend to try where possible to use devices like Ramtron FRAM devices for these types of applications if the capacity can cope with it as both SPi and I2C variants are available. Easy interfacing and less software overhead.

You are looking for 128kbit device Ramtron have 128kbit and 512kbit as SPI and I2C, use what matches your requirements.

Write cycles are faster than serial bus speeds, writes can be done without delays.

NO erase cycles, very low standby current.

Can be erased more times than most EEPROM and a lot of flash. data retention period is speced longer as well.

Generally pin compatible with serial Flas/eeprom devices (especially

8 pin varieties).

Other manufacturers exist.

--
Paul Carpenter          | paul@pcserviceselectronics.co.uk
    PC Services
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Paul

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