[rescue] Loading FCode via RARPD/TFTP

[Romain Dolbeau]

Le jeu. 24 juin 2021 C  18:11, Malte Dehling <mdehling at gmail.com> a C)crit :
> I'm pretty happy with the results :-)

As you should be! Nice results, nice write-up and nice setup.
I never made it past the 'hack it into working for myself' stage; I
didn't really expect anyone else to be interested...

> Is there any documentation on the cg14 at all?  Just curious, I would
> like to understand better how it works.

IIIRC, Mouse on this list has done some work on the cg14
(there's some evidence here <http://ftp.rodents-montreal.org/mouse/Sun-fbs/>)
He might be able to help.

... google found this:
<http://www.sunhelp.org/pipermail/rescue/2008-August/124772.html>

> What makes it expensive?

Low volume for the carrier... Once you go above 5 or 10 the per-unit
cost drops drastically for such a small, not-too-complex board.
Also I was never skilled at soldering and failing eyesight made it
worse, so I had the board fully populated by the manufacturer
(SeeedStudio) as well, adding to the costs.
I ended up paying ~300b, for 5 PCBs with two fully populated. I didn't
need more, and wasn't sure anything would work (never designed a
meaningful PCB before), so I minimized the total cost rather than the
per-unit cost.
Of course, as everything seems fine except perhaps the micro-sd
connector, I now regret I didn't get all 5 populated...

> I've never done any FPGA work so this is probably a naive question: why
> did you choose this particular board if it's so expensive?

It's not super-expensive, far from it (150b, with VAT), but there's
cheaper boards with bigger FPGAs of the same family. And usually with
tons of extra features that made them unsuitable for my use case...

Basically, it's the only one I found that ticked all the boxes:
a) fitting in the SBus form factor (easy)
b) expansion connector that can be easily used to plug on the carrier
(harder than it sounds, high-density connector are common and I wasn't
sure I could deal with those, 2.54mm/0.1" headers are easy)
b) modern FPGA (trivial), I preferred Xilinx because there's more online help
c) enough user pins routed to the connectors (SBus has 82 signals to
deal with, and nearly all of them are needed; you can only skip parity
and some of the interrupts) (not-so-easy as many dev boards adds extra
peripherals that consume pins)
d) onboard flash for self-configuration at power-up that could be
configured with an external power supply (not always obvious)
e) the ability to be powered up by a 5V source on some of the
expansion pins (hard, as the documentation doesn't always make it
obvious; it's usually from a specific connector; and 5V isn't very
high, e.g. the 2.13 I use is fine with it but the 2.14 requires
6V...).
f) some memory I could use (easy to find but somewhat optional as DRAM
is not a trivial thing, though my recent move to Migen/Litex makes it
more likely I could do something with that).

As a bonus, the one that I chose
(<https://www.ztex.de/usb-fpga-2/usb-fpga-2.13.e.html>) has multiple
variants, so I could buy the small FPGA one for testing and move to a
bigger FPGA one if my project didn't fit. Turns out even the small one
is largely sufficient for now.

Cordially,

--
Romain Dolbeau