Are you making your own softcores? Do you have a recommendation for getting started with that? I've recently gotten interested in making CPUs from logic gates, but I have no idea how to transfer that knowledge to FPGAs.
I know Verilog isn't the best option around, and I'm planning on moving onto something better but for now it was easy to get started, and from what I could see many of those better tools end up generating Verilog so would be nice to be able to debug that.
If you know how to think in logic-gate-terms then FPGAs won't be that difficult I suspect. Essentially either you have plain logic gates where the output is purely a function of the inputs, or you have edge-triggered logic thanks to flip-flops.
Just think of it as writing code that specifies how to wire up set of logic gates, muliplexers and flip-flops.
As a brief example, here's an excerpt from my very first ALU:
wire [8:0] res_and;
wire [8:0] res_xor;
wire [8:0] res_add;
assign res_and = (bus_a & bus_b);
assign res_xor = (bus_a ^ bus_b);
assign res_add = (bus_a + bus_b);
always @(posedge clk)
case (op_sel)
OP_AND: result <= res_and;
OP_XOR: result <= res_xor;
OP_ADD: result <= res_add;
default: result <= 0;
endcase
The wires don't hold state, while registers can. The <= means to latch the right side into the left side, in this case on a positive edge of clk. The case statement is just a multiplexer.
At least to me, it feels very much like "writing a circuit".
[1]: https://www.latticesemi.com/en/Products/DevelopmentBoardsAnd...
He designs a Z-80 from scratch, giving Verilog hints / good practice along the way. The design isn't 'complete' -- you have to learn what he's saying and add in the other parts.
It's not a 'trivial' book but if you want to Get Good -- recommended.
Would you recommend one of those FPGAs over another? I know iCE40 from this amazing project (https://github.com/nickmqb/fpga_craft) so maybe I'll just get that!
This is historical, but a 'must read' if you're new to this stuff.
It shows the logical view, the allocated blocks and the routing in Lattice Radiant IDE.
I haven't done much with the Cyclon IV yet, but I found the iCE40 very easy to get rolling with and the freely available tools have been great so far.
It's not a huge FPGA though so if you want to do large projects maybe consider something else. But as a starter I think it's been great.
Like the subc compiler I mentioned, the tradeoffs working with an older FPGA like that are ok to me.
GL! p.s. I mostly used the TinyFPGA BX to learn about the iCE40, but it doesn't have enough pins to make even a semi-decent full machine (with wide memory bus) so I use the Lattice HX 8K dev board. It's also easy to use.
Typically you're doing so either because you're curious, you're debugging a timing problem, or you're trying to make sure the toolchains inferred a particular primitive you are trying to use on purpose. It's not terrifically informative otherwise.