Re: gEDA-user: mcu to sdram interface project

[DJ Delorie <dj@xxxxxxxxxxx>]

> Nice design - should be handy for giant RAM on small MCUs.

That's kinda the whole point ;-)

> It's interesting to compare the S3A with older parts. Nice that it can 
> get by without the 2.5V VCCAUX

It does need a separate 1.2v VCCcore though.

> I'm a bit curious what happens if you forget to set the 3.3V 
> configuration bit - does it damage the part?

I hope I never find out ;-)

> Xilinx docs have all sorts of dire warnings about proper decoupling of 
> the various supplies.

I put a bunch of caps on the back of the board, but I also suspect
it's CYA.

> For FPGA configuration I used slave serial mode and drove the DIN/CCLK 
> direct from a SPI port on my MCU - saved having to use Xilinx's special 
> configuration flash parts. Does mean that the start-up process is a bit 
> more complex though.

I thought of doing that, but the advantage of using a separate eeprom
is that I can use the mcu to download the bitstream to it faster than
I can reprogram the MCU to have a built-in bitstream.  Plus I can
learn something new in the process ;-)

I also thought about adding a serial port, ethernet chip, and sdcard
slot to the test board, but then I decided "one step at a time".  If I
were using a 32 bit mcu I'd reconsider - big RAM is a key to uclinux
support.  The m32c is only a 16 bit MCU.  Maybe I'll add them anyway,
though, just for fun.


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