At 22:47 29/05/97 -0624, you wrote:
>I am considering a data logging project that would sample a number of sensors
>and record the data for later playback into a PC host for analysis. Minimally
>it would need to record one or two hundred bytes per second for several
>minutes, ideally quite a bit more. So something like 32K to 256K of memory
>would need to be written and then later read back. There is almost no
>requirement to randomly access this memory, just need to write it as data
>comes in and read it for upload to the PC.
>
>I know that I could build a data and address bus out of I/O pins, but that
>seems like overkill and would probably use too many pins anyhow.
>
>I have seen serial EEPROMS and they would almost be perfect except for the
>slow programing and power consumption. Is there a serial SRAM available?
>or a simple way to use commodity SRAM chips?
>
>Any other thoughts?

I've been thinking along similar lines. So here goes,

EEPROM has several important features:

    Relatively low power (I can turn it off when I'm not using it)
    Not affected by power glitches (except when writing)
    SLOW (1ms write time)
    Only one supplier of 128k x 8 chips (that I've found)
    Expensive (10 x cost of PIC)

SRAM has some important features too:

    Eats power like it's going out of fashion
    Wiped by power glitches
    Damn FAST
    Lots of suppliers
    Fairly cheap (2 x cost of PIC)

DRAM entered briefly into my considerations:

    Pretty low power
    Wiped by some power glitches
    Pretty fast
    Needs refresh electronics (real killer)

For the sort of data rates you are talking about EEPROM should be quite
feasible. If you need to write more than 500 bytes/second you might have to
work a bit harder.

Rather than have a proper address bus how about using a counter? You are
only going to want to read/write sequentially anyway.

What I need now is a couple of months to design the board and write some
(really quite simple) code for it. Sadly there are lots of other projects
mounting up on my desk at the moment, so let me know how you get on.

Keith.



==========================================================
Keith Dowsett         "Variables won't; constants aren't."

E-mail: kdowsett@rpms.ac.uk
   WWW: http://kd.rpms.ac.uk/index.htm