Restore CUPCDN with a different bit pattern

[aerkiaga]

This should suffice to get CUPCDN working. Closes #300.

[pepijndevos]

Thank you!

Ps if you have ideas how to use the alu In new modes I'm all ears. Gowin doesn't even use most of their own modes and yosys only uses addsub. But it's a fun exercise trying to use the hardware for something else.

[aerkiaga]

@pepijndevos I was indeed trying some experiments with alus! For the most part, I see using them manually doesn't save a huge deal of space, but MUL does (e.g., 33 vs 128 slices for a 5 x 5 multiplier).

[pepijndevos]

Another aspect is speed. The carry chain is much faster than "soft" logic. But yes if we can get yosys to synthesis MUL that would be fantastic. (but how to trade off vs DSP...)

[aerkiaga]

I'll order a new device soon, would love to lend a hand improving synthesis.

[pepijndevos]

I have some spare Tang Nano 1K I think, but you might want something more capable.