Bluespec uses active-low reset signals, whereas the ECP5 primitives
use active-high. So this was holding the EBRs in reset after the rest
of the design was running. Oops.
With this you can feed a stream of bytes in and get multi-byte structs
out, or vice versa. Handy for hooking up stuff like debuggers to
narrower serial busses.
In practice the flow control is unusable on ULX3S dev boards because
the CTS line isn't hooked up (it's instead wired to JTAG_TDO, to enable
the USB<>UART chip to serve a dual purpose as a bitbanged JTAG programmer)
Still, support for flow control is nice, for the future. And the UART
itself also works regardless of flow control, which is of course nice.
It doesn't matter hugely, but by default nextpnr synthesizes for 12MHz,
which doesn't force it to work too hard on the placement. By requesting
100MHz, it needs to try a bit harder on timing and gives results that
are a bit closer to the fully constrained outcomes.
The numeric types vs numeric value thing sucks, but there's a mild
workaround where you just recurse through numeric types until you
find one that matches the value you wanted. It's icky, but it ensures
registers are exactly the correct width instead of relying on later
synthesis to find and execute the width reduction.
I was mostly using a separate interface to be able to mark the methods
always_enabled and always_ready, but you can attach those annotations
to the module constructor instead.
VRAMs are powers of two, so if memory wiring is wrong and we end up
with ram blocks mirrored at several points in the address space, we
want a write pattern that doesn't repeat cleanly on power of two
blocks. That way, a mirrored memory block cannot contain values that
are valid for all its locations.
To implement the mux tree that feeds into RAM ports, we need to know the
port index of the grantee to be able to wire it up. In theory we could
dispense with the per-port grant signal, but keeping it around allows
each client to deal with local concerns separate from the port routing.
Rather than hardcode one architecture for GARY, the arbiters
are now split and can be allocated per-port. The arbiter interface
includes plumbing so that one arbiter can propagate a write conflict
to another, so it can still implement multi-port arbitration as long
as every client is statically allocated to one port.
To help divvy up the sources better, the build now makes a libpath
consisting of all directories that have bsv files in them, with a few
exceptions: hardware subdirs are target-specific so only get used if
they're the current build target. Experiments are random crap so get
the same treatment. And the 'sim' dir is only test helpers, so they
only get pulled in by tests.
Yosys doesn't understand Verilog-2001 port aliases. Unfortunately
bsc uses those to represent inout ports because it's the only way
to represent a particular kind of shared bus in Verilog source code.
Thankfully, a kind soul at Bluespec Inc made a perl script that
transforms the port alias construct into regular verilog-1995, which
works fine in cases like mine where the only user of the inout port
is a TriState module which tears it apart into separate
input/output/enable signals for the rest of bsc to work with.
David Anderson