David Gould <PICLIST@MITVMA.MIT.EDU> 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.
> ....
> 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?

David:

You could rig up an SRAM with a counter chip on its address lines,
then just clock the counter with the PIC and present data 8 bits at
a time to the SRAM's data bus... But I'd probably recommend using
the serial EEPROMs, instead.

Microchip makes large (64K) EEPROMs, and they can be powered from a
pin on the PIC (so you can power them down when you're not actually
accessing them).

As you've noticed, the write time IS relatively slow (10 milliseconds
max), but you can get around that with just a little bit of
cleverness:

    1.  The large EEPROMs allow "block writes" of 8 or 16 bytes at a
        time.  Writing a block takes the same amount of time as
        writing a single byte, so using this feature drops the write
        time to 1.25 ms/byte, max.

    2.  If you use multiple EEPROMs, you can interleave writes to
        them.  That is, you can write a block of 8 bytes to one
        chip, then (while that chip is occupied in its 10-ms write
        cycle), you can write the next block of 8 bytes to another
        chip.  If you use two EEPROM chips, this cuts the write time
        in half, to 0.625 ms/byte.

At 0.625 milliseconds per byte, you can store 1600 samples per
second.

-Andy

=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering, Vista, California
=== http://www.geocities.com/SiliconValley/2499