Barry, I do something similar in my weather station. I use a
Dallas Semi 512KByte NVSRAM with an RTCC, crystal, and battery
built in. At the top of RAM space, I have large tables to ease the
calculation of Dew Point, Heat Index, and Wind Chill. I added a
jumper option to make that section of SRAM look like ROM. I'm
using a 16C74A (moving it to a 16C77), and I depend on the fact
that TRISE (!CS, !WR, !RD) is an input after powerup/Reset. I
have pull-ups on those lines. Since I'm using the PIC as a host
rather than a Parallel Slave Port, I have to disable the A/D and
change the lines to outputs. This requires loading PORTE with
1's before it becomes an output. The bottom line is; pay careful
attention to the data book to ensure these lines (or whatever one
uses) will be in a known state.

   - Tom

At 08:45 AM 1/22/99 -0500, Barry King wrote:
>Eric,
>
>I missed the fact that you need such a large and fast memory.  All
>the EEPROM and Flash techs take a comparatively long time to write.
>
>My application is in data loggers with very long time bases
>(months, not seconds) so write is speed is *NOT* an issue for me (I
>only write data every 10 minutes or every hour).  So I wanted
>non-volatile, dense and easy to connect, so I went with I2C EEPROMs.
>
>But I think you are probably right to stay with the the SRAMs for
>your application.  Sounds like you've got it about sussed.
>
>Experience is process by which we learn from our mistakes.  Education
>is the process by which we learn from others mistakes.  <g>
>So... For what its worth, here's my experience, I hope its
>educational:
>
>In a former design we used a block of SRAM.  Note carefully which pin
>sets the RAM to the "Safe", i.e. no-write state, and protect it
>against glitches and low power supply.  The commercial NVRAM RAMs
>have this mostly done, just take noise glitch precautions.  For
>example: what is the power up reset state of te PIC pin which is
>connected to RAM -WE?  Floating, right?  Better pull it up with an
>external resistor.  I had to add a pull up resistor arrangement and
>power-fail detect to that old system to really protect the RAM (8051
>based, BTW, not PIC).  Since you probably don't need to power off the
>PIC and hold RAM, it will be easier to protect.
>
>Also, note which pin deselects the chip and puts in micropower
>standby mode.  Use that one to choose which chip is active.  Then the
>idle chip(s) really go to power down.
>
>Cheers,
>---------------------------------------------------
>Barry King
>Engineering Manager
>NRG Systems "Measuring the Wind's Energy"
>barry@nrgsystems.com
>"The witty saying has been deleted due to limited EPROM space"