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'Why serial RAM ???'
1997\07\22@104742 by Ben Roothooft

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With all this talk lately about the serial RAM's...

Why use a SERIAL ram in the first place ?
They're much slower than the traditional parrallel
memories and RAMTRON has parrallel FRAM's as
well... so why bother ?

Ben.
(maybe a stoopid question... just curious)

1997\07\22@114415 by James Musselman

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One reason is that it uses much fewer precious I/O lines and
also fewer I/O= less board space too.
James

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1997\07\22@122917 by Harold Hallikainen

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On Tue, 22 Jul 1997 16:33:48 +0200 Ben Roothooft
<Ben.RoothooftspamKILLspamESAT.KULEUVEN.AC.BE> writes:
>With all this talk lately about the serial RAM's...
>
>Why use a SERIAL ram in the first place ?
>They're much slower than the traditional parrallel
>memories and RAMTRON has parrallel FRAM's as
>well... so why bother ?


       Mostly just to save I/O pins to do something useful (like I/O).

Harold

1997\07\22@194814 by John Payson

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> With all this talk lately about the serial RAM's...
>
> Why use a SERIAL ram in the first place ?
> They're much slower than the traditional parrallel
> memories and RAMTRON has parrallel FRAM's as
> well... so why bother ?

Let's look at some characteristics of common devices
[the "device" column is at the end of the table rather than the start to
maximize the likelihood that everyone's mailreader will handle the tabs
well.]

I/O     BOARD   READ    READ    WRITE   WRITE   GLITCH  DEVICE
PINS    SPACE   SPEED   ENDUR.  SPEED   ENDUR.  RESIST  TYPE
-----   -----   -----   -----   -----   -----   ------  --------------
None    None    Fast    Inf.    V.Slow  M's     V.Good  Int. EEPROM (16x84)
Many    Lots    Fast    Inf.    Fast    Inf.    Vol.    Parallel RAM
Many    Lots    Fast    Inf.    *1      K's     V.Good  Parallel flash
Many    Lots    Fast    Inf.    Slow    M's     V.Good  Parallel EEPROM
Few     Little  Medium  Inf.    Medium  Inf.    Vol.    Serial RAM
Few     Little  Medium  10G     Medium  10G     Fair    Serial FRAM
Few     Little  Medium  Inf.    Slow    M's     V.Good  Serial EEPROM
Few     Little  Medium  Inf.    *1      K's     V.Good  Serial flash

I/O Pins: Many==16-24 or more; Few=2-4
Board space: Lots==28 or 32-pin package; Little==8-pin package
Read/write speed: Fast==under 10us; Medium==10-1000us; Slow==over 1ms
 V.Slow==over 5ms; *1==medium except for page-erase cycle (over 100ms
 on some devices; over 2 seconds on others)
Read/write endurance: K's==10,000-1M (typ.); M's==1M-20M (typically);
 10G==10^10.
Glitch-resist: V.Good==power glitches and such are unlikely to cause any
 damage to device contents outside of current byte being written.
 Vol.==power glitches (or loss of power) may trash entire device contents.
 Fair==memory contents non-volatile but stray signals may cause memory loss
   (even while trying to READ the device)

Looking at the above table, serial RAMs are the only class of device which
combines the features (Few I/O; infinite read endurance; medium-speed write)
or (Few I/O; infinite write endurance).  For applications which demand either
of these feature combinations (or any of the others that serial RAMs uniquely
provide) serial RAMs are probably the best choice... if they can be found
anywhere...

1997\07\22@203403 by William Chops Westfield

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   Few     Little  Medium  Inf.    Medium  Inf.    Vol.    Serial RAM
   Few     Little  Medium  Inf.    Slow    M's     V.Good  Serial EEPROM

I'd rate the serial devices as "slow" with eeprom/flash writes being "very
slow", unless your microcontroller has native HW support for running the
serial protocol.  Anything that takes software to read and write memory
is slower than "medium", which I'd use for the difference between on-board
(zero-wait-state) memory and off-board slowery memory (with wait states.)
fast: 0 additional time
medium:  <10 clocks
slow: > 10 instructions
very slow: > 100 instructions

1997\07\22@213638 by John Payson

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>     Few     Little  Medium  Inf.    Medium  Inf.    Vol.    Serial RAM
>     Few     Little  Medium  Inf.    Slow    M's     V.Good  Serial EEPROM
>
> I'd rate the serial devices as "slow" with eeprom/flash writes being "very
> slow", unless your microcontroller has native HW support for running the
> serial protocol.  Anything that takes software to read and write memory
> is slower than "medium", which I'd use for the difference between on-board
> (zero-wait-state) memory and off-board slowery memory (with wait states.)
> fast: 0 additional time
> medium:  <10 clocks
> slow: > 10 instructions
> very slow: > 100 instructions

I gave definitions at the bottom:
|Read/write speed: Fast==under 10us; Medium==10-1000us; Slow==over 1ms
|  V.Slow==over 5ms; *1==medium except for page-erase cycle (over 100ms
|  on some devices; over 2 seconds on others)

Under your proposed definition, NOTHING would qualify as "fast" other than
the PIC's internal registers (even the internal EEPROM would be "medium"
speed).  While the I2C stuff can't be read as fast as the internal EEPROM,
it's no real slouch either, especially compared with the things I call "slow".
Even without hardware serial support, there's a huge difference between the
speed at which an I2C or SPI chip can send/receive data and the speed at
which it most serial EEPROMs can write.  At 4MHz, for example, reading data
from an I2C device will take about 50us-80us per byte (depending upon what
speed/space tradeoffs you make).  On a 10MHz device this time may be reduced
to about 25us per byte.  While parallel devices are an order of magnitude
faster, 50us per byte is still much faster than even a fast EEPROM write
cycle (e.g. even a 16-bit page write that takes 5ms is 6 times as slow per
byte as a 50us byte read).

Given that serial memory reads are almost an order of magnitude faster than
even page-write EEPROMs and two orders of magnitude faster than the 16C84's
internal EEPROM data writes, my terms "Fast", "Medium", "Slow", and "V.Slow"
are about an order of magnitude apart each.  Would you have preferred that
I use "Medium", "Slow", "Molasses", and "Waiting for Godot"?

1997\07\23@075336 by Andy Kunz

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>I/O     BOARD   READ    READ    WRITE   WRITE   GLITCH  DEVICE
>PINS    SPACE   SPEED   ENDUR.  SPEED   ENDUR.  RESIST  TYPE
>-----   -----   -----   -----   -----   -----   ------  --------------
>None    None    Fast    Inf.    V.Slow  M's     V.Good  Int. EEPROM (16x84)
>Many    Lots    Fast    Inf.    Fast    Inf.    Vol.    Parallel RAM
>Many    Lots    Fast    Inf.    *1      K's     V.Good  Parallel flash
>Many    Lots    Fast    Inf.    Slow    M's     V.Good  Parallel EEPROM
>Few     Little  Medium  Inf.    Medium  Inf.    Vol.    Serial RAM
>Few     Little  Medium  10G     Medium  10G     Fair    Serial FRAM
>Few     Little  Medium  Inf.    Slow    M's     V.Good  Serial EEPROM
>Few     Little  Medium  Inf.    *1      K's     V.Good  Serial flash

John,

You forgot FRAM!  It's pin compatible (8-pin) with SEEPROM, software
compatible (I2C or SPI), and DOES NOT have a 10mS write delay, can also
write "blocks" up to the size of the device in one fell swoop.

Also is non-volatile and has more write cycles than SEEPROM.

Andy

==================================================================
Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
         Hardware & Software for Industry & R/C Hobbies
       "Go fast, turn right, and keep the wet side down!"
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