This is essentially a "brain pick" for experience interfacing SD cards to
PICs. However I will start with a precis of the project, and appreciate any
brain storming.

I have had an idea for a time which has a handful of small "workstations"
which have operators doing events simultaneously, but need to be kept in
step with each other. I had thought of having each workstation use a small
LCD character display for instructions, with a handful of buttons to allow
scrolling the instructions when necessary, and to say operation complete.
When all stations get the operation complete, then one which is the master
one (there is that "naughty" term again) issues the next set of instructions
to be displayed on each workstation. The communication link proposed is
based around using the UART, although I2C is another possibility, but by
using the UART I can link into a network which may already run past the
workstation sites.

To load the instructions, which could change between work sessions, I
started thinking in terms of a CF card. Interface is well documented, and
circuits available. However after purchasing a PDA which uses an SD card, I
was considering using that instead. Instruction data would be formed up
using a spreadsheet, and probably exported in CSV format, so that each row
is a workstation step, and each column a workstation, to make entering and
editing of instructions easy. Download CSV to PDA, onto SD card and move
card to master workstation.

Now I envisage that the card is to be accessed by a PIC, using SPI. Reading
the card looks to be easy, from the Docs, as I can do it in chunks smaller
than 512 bytes. It would appear from the SD Society Docs that the FAT file
system is probably used, but there is no guarantee that the PDA does use
this.

Does anyone have any experience with doing this sort of access to the SD
cards? It all looks as though it should be straight forward, except it would
seem that the SD card is max 3.6V, so I need some sort of level shift. The
easiest way here may be a separate PIC running at 3.6V for the master,
within the "master" workstation (i.e. separate slave PIC as well to handle
the display side), and handle the level shift in the interconnecting
network, which will need 5V at the slaves to handle the LCD displays. The
FAT code should be straight forward.

at this stage I envisage using 18F252 chips throughout, although it could be
16F876s. The PDA is a Palm Tungsten, for what difference that may make.
Already have some 2 line x 20 char 44780 type displays that would suit, and
a couple of SD cards that came with the PDA, which is why the switch in
ideas from using CF.

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