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

 
 
elil
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      08-08-2011, 07:24 PM
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.
 
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Roberto Waltman
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      08-08-2011, 08:17 PM
elil wrote:
>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?
> ... May be somebody here is aware of such a component ?


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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Roberto Waltman
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      08-08-2011, 08:23 PM
Roberto Waltman forgot to write:

>elil wrote:
>>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?
>> ... May be somebody here is aware of such a component ?

>
>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.


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.

>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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Hans-Bernhard Bröker
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      08-08-2011, 11:06 PM
On 08.08.2011 21:24, elil wrote:

> I'd like to understand why ALL Serial Flash chips always come with SPI
> vs Serial EEPROM chips that always come with I2C ?


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.

> Why not vice versa for example?


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.

 
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Paul
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      08-09-2011, 12:05 AM
In article <(E-Mail Removed)>, (E-Mail Removed) says...
>
> On 08.08.2011 21:24, elil wrote:
>
> > I'd like to understand why ALL Serial Flash chips always come with SPI
> > vs Serial EEPROM chips that always come with I2C ?

>
> 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.
>
> > Why not vice versa for example?

>
> 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.


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.

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