[libre-riscv-dev] chinese sponsor, looking to design an ECP5-based dev board

[Samuel Falvo II]

This is entirely out of scope for the project at hand, I'm sure; feel
free to summarily dismiss if it is.  But, if we're aiming to sell
these separately from the larger libre-riscv project, I feel this
might enhance the appeal to hackers over all.

Since this card is expected to be approximately the size of a credit
card, I'm wondering if it would be possible to build the board with a
set of edge connector pins along one edge, each (non-power) finger
routed to an I/O on one of the FPGAs?  I'm thinking hackers can then
build their own backplane solutions around these fingers should they
want to use more than one board in a project.

Another, perhaps more flexible, option is to use unpopulated,
through-hole SIL or DIL pads.  For example, if I multiplex address and
data bus pins, I can implement a 16-bit off-chip Wishbone derivative
(with 24-bit address bus) in about 40 pins, so figure 54 pins if you
obey the rule of "1 ground/power pin per 3 data pins", but I'm willing
to bet 50 pins would be more than adequate for quite respectable
throughputs.  A 2x25 DIL connector would be 2.5 inches long, which
seems to be just about the size of the long edge of a Raspberry Pi.

I'm somewhat motivated by my own selfish desires, but also (if you see
the attached image) by the presence of similar kinds of pads on small
boards like my icoBoard Gamma (see the red arrow).  This is a 17x4
matrix of pads offering 64 bits of raw I/O and 4 power.  In a perfect
world, having at least two PMOD ports would be nice as well (in my
Kestrel project, I use one PMOD for two UARTs, one for PS/2 keyboard,
one for PS/2 mouse, one for diagnostics, and I was planning on using
the 17x4 pin matrix for things like audio and VGA breakout, so you can
see how heavily I depend on PMODs!).

There are also high-density connectors (one of four of which are
circled in green) which, IIRC, provides a 32-bit I/O path.

I just wanted to bring this up because it was the icoBoard's sheer I/O
accessibility that attracted me to that board.  I could be an anomaly
though.  :)


On Mon, May 6, 2019 at 5:25 PM Jacob Lifshay <programmerjake at gmail.com> wrote:
>
> I was thinking of having the two SO-DIMM sockets stacked vertically,
> like in: http://web.archive.org/web/20190507001733/https://avadirect-freedomusainc1.netdna-ssl.com/Pictures/500/5003581_6.png
>
> This will save more PCB area than having them next to each other.
>
> On Mon, May 6, 2019 at 5:14 PM Luke Kenneth Casson Leighton
> <lkcl at lkcl.net> wrote:
> > On Tue, May 7, 2019 at 12:43 AM Jacob Lifshay <programmerjake at gmail.com> wrote:
> > > Note that there are plenty of capable Chinese contractors that can do
> > > high-frequency PCB design (not that they will be cheaper).
> >
> >  finding reputable ones has taken me... literally years.  remember,
> > jacob, i've been doing component sourcing, PCB and 3D design for 7
> > years now.
> Ok.
>
> Jacob
>
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--
Samuel A. Falvo II
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