2009/9/10 Wolfgang Spraul <wolfgang@qi-hardware.com>
Ignacio,

> I would say that for a portable device the MOST important feature, by far,
> is the power consumption, and even the latest low power devices have
> comparatively (to ASICs) high power requirements.

What are the lowest power consumption FPGAs you know about? How much power
do they consume?

The lowest power I know are actualy CPLDs (coolrunner by Xilinx). That means they have non-volatile configuration and have less logic resources as compared to FPGAs.
 

I think one idea was to just leave an unpopulated place for an FPGA on the
board, so we could still sell the device to regular users, but it would
have an easily accessible FPGA option for hacking projects.

Do you really have so much spare real state on the PCB ?.

Note also that you need room not only for the FPGA itself but also for:

1- Connector (huge if you want to make all those I/O pins useful).
2- A CPLD to connect the FPGA programming interface to the SoC.
3- Power regulators (for example, Spartan 6 use 1.2V core voltage), and possibly power management circuitry too to save power when no in use.

And no matter what FPGA/connector you choose you won't fullfill the needs of all the potential users, which I believe will be anyway a minority in the whole user base.
 
Or we offer two versions, one with and one without FPGA?

In order to make that practical you'd need to share same form factors, enclosure, etc, and thus you'd be making the device possible larger than needed.

Seriously, I don't think the potential user base for tinkering justifies the inclusion of an FPGA in the design. If you want to provide extensive I/O capability, just add a host USB port. Supply of external power IS A MUST, but 100mA would be more than enough (and should be switchable from the SoC). And it must be a separate USB port, different from the device port used to connect to the PC. I you want to go a bit more far, add an special connector with I2C, SPI and power.

That is more than enough for the average hacker. If your project is so complex or needs so much bandwidth that hostUSB+I2C+SPI is not enough, neither the FPGA would have been (assuming and low-end, cheap and non-power-hungry FPGA).

Regards.

P.S: got the Ingenic docs. Just haven't had time to properly review them.