WIP: Add family footnote to 20k-boards

[TheZoq2]

The footnote mentions "C-devices" but I can't quite figure out what that means

Edit: Oh, I found examples/himbaechel. I'll repoint the footnote to that since it seems to be the big new thing

~~I also now realized that there is no example for this board so I attempted to create one. I'm a bit surprised to see nextpnr-gowin in the examples since I thought that was deprecated and replaced by himabechel. Since I don't have -gowin installed, I tried to at least create the example using -himbaechel for porting later.

However, with this cst file

IO_LOC "clk" 4;
IO_LOC "led[0]" 15;
IO_LOC "led[1]" 16;
IO_LOC "led[2]" 17;
IO_LOC "led[3]" 18;
IO_LOC "led[4]" 19;
IO_LOC "led[5]" 20;

I get

nextpnr-himbaechel --json blinky-tangnano20k-synth.json --write blinky-tangnano20k.json --device GW2AR-LV18QN88C8/I7 --vopt family=GW2A-18C --vopt cst:tangnano20k.cst
Info: Using uarch 'gowin' for device 'GW2AR-LV18QN88C8/I7'

Info: Create constant nets...
Info: Modify LUTs...
Info: Pack IOBs...
ERROR: Unconstrained IO:led_OBUF_O_5

not sure what is going wrong there...

Edit: Oh, I now found examples/himbaechel with a working 20k configuration~~

[yrabbit]

Unconstrained IO:led_OBUF_O_5

Gowin IDE allows you to use (input, output, inout) without specifying their specific location in the .CST file.

However, we do not allow this - each port must be specifically assigned to a pin - because there is little point in having ports if you don't know where they will go.

[TheZoq2]

Yeah, I absolutely agree with that, i've spent far too much time on that problem on old xilinx devices 😓

In this case I thought I specified them correctly though 🤔 Either way, it doesn't make much difference since I didn't even have to create an example in the first place, just point to examples/himbaechel

[pepijndevos]

We should really make himbaechel the top example folder and put the old stuff in a subfolder until we actually delete it

[yrabbit]

Definitely. Although I guess doing anything with examples without a working CI is not very convenient, let's wait?
YosysHQ/yosys#4771