yosys

# Verilog to a bitstream, with the open-source flow Six tools, one command, and a strict subset of one language. Everything below is what actually passes through Icarus Verilog, Yosys, nextpnr and icepack — not what the standard permits. ## The flow ```sh "$YOSYS_FLOW" blink # the whole thing, for top module `blink` ``` | step | tool | in | out | |---|---|---|---| | sim | `iverilog -g2012` + `vvp` | `rtl/*.v` + `tb/blink_tb.v` | `out/sim.vcd`, PASS/FAIL on stdout | | waves | `vcd2json.py` | `out/sim.vcd` | `out/waves.json` — a signal summary for the verdict (the pane reads the VCD itself) | | synth | `yosys synth_ice40` | `rtl/*.v` | `out/blink.json` — the iCE40 netlist, and cell counts | | schematic | `yosys prep` | `rtl/*.v` | `out/blink_schematic.json` — technology-independent | | svg | `netlistsvg` | that JSON | `out/blink.svg` — the top module drawn (the pane draws every module of the hierarchy) | | pnr | `nextpnr-ice40 --up5k --package sg48` | netlist + `constraints/blink.pcf` | `out/blink.asc`, `out/blink_pnr.json` (utilisation, Fmax, critical paths), `out/blink_routed.json` (placement and routing — the pane's floorplan) | | pack | `icepack` | `.asc` | **`out/blink.bin`** — the bitstream | Each step's full output is at `out/logs/<step>.log`, with `<step>.start`, `<step>.time` and `<step>.exit` beside it and `out/logs/run.json` for the run as a whole — the pane reads those to show which step is running. The flow does not stop the world on a failure: synthesis still runs when simulation fails, so you see every problem at once. Single steps, when you are iterating on one thing: ```sh iverilog -g2012 -o out/sim.vvp rtl/*.v tb/blink_tb.v && vvp out/sim.vvp # just the simulation yosys -p "read_verilog rtl/*.v; synth_ice40 -top blink; stat" # just the cell count ``` Then always finish with the full `"$YOSYS_FLOW" <top>` so the pane and the verdict are current. ## The synthesisable subset Yosys turns a description of hardware into gates. Anything that does not describe hardware is either rejected or, worse, quietly turned into something you did not mean. **Always** ```verilog `default_nettype none // first line of every file: an undeclared name is now an error, // not a silent 1-bit wire. This catches every typo'd port. module counter #( parameter integer WIDTH = 8 // parameters, not `define — they are per-instance ) ( input wire clk, input wire rst, input wire en, output reg [WIDTH-1:0] count // a port assigned in an always block is `reg` ); always @(posedge clk) begin // ONE clock, ONE edge, no other signal in the list if (rst) count <= {WIDTH{1'b0}}; else if (en) count <= count + 1'b1; end endmodule `default_nettype wire // last line: put it back, so other files are not surprised ``` Rules that are not negotiable: - **`<=` in `always @(posedge clk)`, `=` in `always @(*)`.** Non-blocking for flip-flops, blocking for combinational logic. Mixing them is the single most common source of "it simulates but does not synthesise the same". - **A signal is driven from exactly one `always` block** (or one `assign`), never two. Two drivers is an error in synthesis and an `x` in simulation. - **Every `always @(*)` assigns every output on every path.** An `if` without an `else`, or a `case` without a `default`, infers a *latch* — see the pitfalls below. - **Sized literals everywhere**: `8'd0`, `4'b1010`, `16'hBEEF`, `{WIDTH{1'b0}}`. A bare `0` is 32 bits and will silently widen an expression. - **`$clog2(N)`** for a counter's width. It is Verilog-2005 and Yosys supports it. - No `initial` blocks for logic (initial *values* on a `reg` are fine and do synthesise on iCE40 — the bitstream sets the flip-flops). No `#delays`. No `fork`/`join`. No `while`/`forever`. No `real`. `for` loops only with constant bounds — they unroll into copies of hardware. - Multiplication by a constant power of two is a shift and is free; `*` by a variable costs a lot of LUTs (the UP5K has 8 DSP blocks, `synth_ice40 -dsp` maps to them). Division is not free and usually means you want a different algorithm. **State machines** — two blocks, always: ```verilog localparam [1:0] IDLE = 2'd0, RUN = 2'd1, DONE = 2'd2; reg [1:0] state, next; always @(posedge clk) // the register if (rst) state <= IDLE; else state <= next; always @(*) begin // the transition, fully assigned next = state; // <-- the default that prevents a latch case (state) IDLE: if (start) next = RUN; RUN: if (done) next = DONE; DONE: next = IDLE; default: next = IDLE; endcase end ``` ## The testbench One per top module, at `tb/<top>_tb.v`. The shape matters: the pane draws its waves from the VCD, and the verdict reads the word `FAIL`. ```verilog `timescale 1ns / 1ps `default_nettype none module counter_tb; reg clk = 1'b0, rst = 1'b1, en = 1'b0; wire [7:0] count; integer errors = 0; counter #(.WIDTH(8)) dut (.clk(clk), .rst(rst), .en(en), .count(count)); always #5 clk = ~clk; // a 100 MHz clock: 10 ns period task check(input condition, input [8*40-1:0] what); // NOT `expect` — reserved in -g2012 begin if (condition) $display(" ok %0s", what); else begin errors = errors + 1; $display(" FAIL %0s (at %0t)", what, $time); end end endtask initial begin $dumpfile("out/sim.vcd"); // exactly this path — the flow reads it $dumpvars(0, counter_tb); // 0 = this scope and everything under it repeat (2) @(posedge clk); rst = 1'b0; en = 1'b1; repeat (5) @(posedge clk); #1; check(count == 8'd5, "counts five clocks"); if (errors == 0) $display("PASS counter: %0d checks", 3); else $display("FAIL counter: %0d check(s) failed", errors); $finish; // ALWAYS: without it vvp runs forever end endmodule ``` - **`#1` after `@(posedge clk)` before checking.** At the edge itself the non-blocking update has not landed yet; one time unit later it has. - **Simulate in shrunken time.** A 1 Hz blink off a 12 MHz clock is 12 million cycles. Parameterise the design (`CLK_HZ`) and override it in the testbench (`.CLK_HZ(16)`), so the same RTL runs in a hundred clocks. Never change the RTL to make the test fast. - `$dumpvars(0, tb)` dumps everything including the DUT's internals — that is what you want: the pane's Waves tab browses every scope, shows parameters with their values, and opens on the DUT's ports and registers. A line named `tx`/`rx` is decoded as UART (baud measured off the line); an 8-bit bus named `data`/`byte`/`char` starts in ASCII. Keep a dump under a few tens of millions of changes — `$dumpoff` around a long quiet stretch, as `hello_uart`-style testbenches do. - The word `FAIL` anywhere in the output fails the verdict. Do not print it in passing messages. ## The board: iCEBreaker, iCE40UP5K-SG48 5280 logic cells, 30 × 4 kbit block RAMs, 4 × 16 kB single-port RAMs, 8 DSP blocks, 1 PLL. Pin numbers are the board's, from [the iCEBreaker project's own constraints file](https://codeberg.org/icebreaker-fpga/icebreaker-verilog-examples/src/branch/main/icebreaker/icebreaker.pcf). ``` set_io -nowarn clk 35 # 12 MHz oscillator set_frequency clk 12 # nextpnr's extension: this is what Fmax is measured against set_io -nowarn btn_n 10 # on-board button — ACTIVE LOW (0 = pressed) set_io -nowarn ledr_n 11 # red LED — ACTIVE LOW (0 = lit) set_io -nowarn ledg_n 37 # green LED — ACTIVE LOW (0 = lit) set_io -nowarn rx 6 # UART from the on-board FTDI (FPGA's point of view) set_io -nowarn tx 9 # UART to the FTDI ``` The full board — RGB LED (39/40/41), SPI flash, PMOD 1A/1B/2, and the snap-off section's five **active-high** LEDs and three buttons — is commented out in `constraints/blink.pcf`; uncomment what you use. `-nowarn` lets one PCF carry pins the current design does not have. Every port of the top module needs a `set_io` line, or nextpnr fails with "unconstrained IO". Nothing else in the design does; internal signals are routed automatically. Another board: change `constraints/<top>.pcf` and set `YOSYS_DEVICE` / `YOSYS_PACKAGE` (e.g. `YOSYS_DEVICE=--hx8k YOSYS_PACKAGE=ct256` for the HX8K breakout). ## Reading the report `out/<top>.report.json` is what the pane draws; read it when you want the numbers in words. - **`synthesis.byType`** — what Yosys mapped the design to. `SB_LUT4` is a 4-input lookup table, `SB_DFFSR`/`SB_DFFE` are flip-flops, `SB_CARRY` is the fast carry chain an adder uses, `SB_RAM40_4K` is block RAM. Roughly: a logic cell is one LUT4 + one flip-flop, so `ICESTORM_LC` ≈ max(LUTs, FFs) after packing, not their sum. - **`pnr.utilization`** — used / available per resource, with a percentage. Under 70 % is comfortable; over 90 % and nextpnr starts to struggle to route. - **`pnr.clocks[].achievedMHz` vs `constraintMHz`** — the design closes at the first, the PCF's `set_frequency` asks for the second. `pass: false` means the critical path is too long: the fix is to break it with a pipeline register, not to lower the clock, unless lowering it is honest. The path itself, hop by hop with the RTL line of each net, is `critical_paths` in `out/<top>_pnr.json` — and drawn on the floorplan in the pane's Chip tab. - **`bitstream.path`** — `out/<top>.bin`, and `iceprog out/<top>.bin` flashes a board over USB (install `icestorm`'s `iceprog`; the user needs the board plugged in). ## Pitfalls that cost a day **Inferred latch.** A combinational block that does not assign an output on every path becomes a level-sensitive latch — which on an FPGA is built out of a LUT feeding itself, is not timed, and glitches. Yosys says `Warning: ... latch` and the verdict raises it. ```verilog always @(*) if (sel) y = a; // BAD: what is y when sel is 0? A latch. always @(*) begin y = 1'b0; if (sel) y = a; end // GOOD: a default first. ``` **Multi-driven net.** Two `always` blocks (or an `always` and an `assign`) writing the same signal. Simulation shows `x`, synthesis errors with "conflicting drivers". One signal, one driver. **Blocking assignment in sequential logic.** `always @(posedge clk) begin a = b; c = a; end` makes *one* flip-flop and a wire; with `<=` it makes two flip-flops in a shift register. Simulation and synthesis can disagree about which you meant. Use `<=`. **An asynchronous input sampled directly.** A button or an incoming UART line is not synchronous to your clock; sampling it straight into logic causes metastability. Two flip-flops first, always: ```verilog reg [1:0] sync; always @(posedge clk) sync <= {sync[0], btn_n}; wire btn_safe = sync[1]; ``` **Reset that is not thought about.** On iCE40 a `reg x = 1'b0;` initial value *is* honoured — the bitstream loads it — so a global reset is often unnecessary. If you do use one, use it synchronously (`if (rst)` inside `@(posedge clk)`) and on every register in the block. **A counter one bit too narrow.** `reg [7:0] c; if (c == 300)` never fires. Size from the constant: `reg [$clog2(LIMIT)-1:0]`. **Width mismatch.** `wire [7:0] a = b + c;` where `b`,`c` are 8-bit silently drops the carry. Widen first: `{1'b0, b} + {1'b0, c}`. **`$finish` missing.** `vvp` runs forever and the flow hangs. Every testbench ends with `$finish`. ## Blocks you will need **Clock divider / strobe** — one cycle high every N clocks, which is how you make anything slow: ```verilog localparam integer DIV = CLK_HZ / RATE_HZ; reg [$clog2(DIV)-1:0] div = 0; reg tick = 1'b0; always @(posedge clk) begin tick <= 1'b0; if (div == DIV - 1) begin div <= 0; tick <= 1'b1; end else div <= div + 1'b1; end ``` **PWM** — `duty` out of 2^BITS, no multiplier, one adder: ```verilog module pwm #(parameter integer BITS = 8) ( input wire clk, input wire [BITS-1:0] duty, output wire out ); reg [BITS-1:0] acc = 0; always @(posedge clk) acc <= acc + 1'b1; assign out = (acc < duty); endmodule ``` (For an LED, gamma matters: perceived brightness goes as roughly the square of `duty`.) **Button debounce** — hold the input steady for a few milliseconds before believing it: ```verilog module debounce #(parameter integer COUNT = 12_000) ( // 1 ms at 12 MHz input wire clk, input wire in, output reg out = 1'b0 ); reg [1:0] sync = 2'b00; reg [$clog2(COUNT)-1:0] n = 0; always @(posedge clk) begin sync <= {sync[0], in}; if (sync[1] == out) n <= 0; else if (n == COUNT - 1) begin out <= sync[1]; n <= 0; end else n <= n + 1'b1; end endmodule ``` **UART transmitter** — 8N1, at `CLK_HZ / BAUD` clocks per bit (12 MHz / 115200 = 104): ```verilog module uart_tx #(parameter integer CLK_HZ = 12_000_000, parameter integer BAUD = 115_200) ( input wire clk, input wire send, // pulse high for one clock input wire [7:0] data, output reg tx = 1'b1, // idles high output wire busy ); localparam integer DIV = CLK_HZ / BAUD; reg [$clog2(DIV)-1:0] cnt = 0; reg [3:0] bit_i = 4'd0; // 0 = idle, 1 = start, 2..9 = data, 10 = stop reg [7:0] shift = 8'd0; assign busy = (bit_i != 4'd0); always @(posedge clk) begin if (!busy) begin