GF-PNR -- Place & Route Skill

Supported Targets

FPGA Family	Tool	Command
Lattice iCE40	nextpnr-ice40	nextpnr-ice40 --up5k --package sg48 --json synth.json --pcf constraints.pcf --asc output.asc
Lattice ECP5	nextpnr-ecp5	nextpnr-ecp5 --85k --package CABGA381 --json synth.json --lpf constraints.lpf --textcfg output.config
Gowin	nextpnr-gowin	nextpnr-gowin --device GW1NR-LV9QN88PC6/I5 --json synth.json --cst constraints.cst

NOT Supported: Xilinx (use Vivado), Intel (use Quartus). For these, GateFlow generates TCL scripts instead.

Tool Detection

which nextpnr-ice40 || which nextpnr-ecp5 || which nextpnr-gowin

If not found:

---GATEFLOW-RESULT---
STATUS: ERROR
DETAILS: nextpnr not installed. Install for place & route.
  macOS: brew install nextpnr
  Linux: see https://github.com/YosysHQ/nextpnr
---END-GATEFLOW-RESULT---

Workflow

1. Check project target (board.yaml -> pnr_target)
2. Verify synthesis output exists (synth.json from Yosys)
3. Verify constraint file exists
4. Run nextpnr with correct flags
5. Report timing and utilization
6. Generate bitstream if P&R succeeds

Bitstream Generation

After P&R:

• iCE40: icepack output.asc output.bin
• ECP5: ecppack output.config output.bit
• Gowin: built into nextpnr-gowin output

Result Format

---GATEFLOW-RESULT---
STATUS: PASS | FAIL | ERROR
TIMING:
  Fmax: 125.3 MHz
  Slack: +2.1 ns
UTILIZATION:
  LUTs: 142 / 5280 (2.7%)
  FFs: 87 / 5280 (1.6%)
FILES: [output files]
DETAILS: [summary or timing violations]
---END-GATEFLOW-RESULT---

Common Flags

Flag	Purpose
--freq <mhz>	Target frequency
--seed <n> / -r	RNG seed / randomize
--opt-timing	Post-placement timing opt
--report <file>	JSON timing/utilization
--timing-allow-fail	Proceed despite violations
--router2-heatmap <prefix>	Congestion heatmaps
--placer heap	Heap placer (default)

Target Details

iCE40

Devices: --lp1k, --lp8k, --hx1k, --hx8k, --up5k Output: --asc <file> | Constraints: --pcf <file> Full pipeline: yosys -p "synth_ice40 -json out.json" design.v && nextpnr-ice40 --hx8k --package ct256 --json out.json --pcf pins.pcf --asc out.asc && icepack out.asc out.bin

ECP5

Devices: --12k, --25k, --45k, --85k Output: --textcfg <file> | Constraints: --lpf <file>, --sdc <file> Full pipeline: yosys -p "synth_ecp5 -json out.json" design.v && nextpnr-ecp5 --25k --package CABGA256 --json out.json --lpf pins.lpf --textcfg out.config && ecppack out.config out.bit

Gowin

Device: --device <part> (e.g., GW1NR-LV9QN88PC6/I5) Family: --vopt family=<fam> (for C-silicon, e.g., GW1N-9C) Constraints: --vopt cst=<file> (mandatory) Full pipeline: yosys -p "synth_gowin -json out.json" design.v && nextpnr-himbaechel --device GW1NR-LV9QN88PC6/I5 --vopt family=GW1N-9C --vopt cst=pins.cst --json out.json --write routed.json && gowin_pack -d GW1N-9C -o out.fs routed.json

Timing Analysis

Fmax

• Maximum frequency from worst-case critical path
• Use --freq 0 for auto-determine, --freq N for target

Slack

• Positive = timing met | Negative = violation | Zero = exact
• Critical path = path with worst slack

Improving Fmax

1. Pipeline long combinational paths
2. --opt-timing for post-placement optimization
3. Different seeds (-r) for better placement
4. Increase --placer-heap-timingweight (20-50)
5. --router2-tmg-ripup for timing-driven rerouting
6. Yosys: -nowidelut for ECP5 (big Fmax improvement)

Utilization Thresholds

Usage	Status
< 50%	Comfortable
50-70%	Normal
70-80%	Tight
> 80%	Warning: congestion likely
> 95%	May fail to route

Common Failures

Failure	Fix
Routing congestion	Larger device, optimize synth, different seed
Timing violations	Pipeline, --opt-timing, different seed
Placement failure	Check constraints, try --placer sa
Combinatorial loops	Break with register, or --ignore-loops

Constraint Formats

PCF (iCE40)

set_io clk 21
set_io led[0] 99
set_frequency clk 48

LPF (ECP5)

LOCATE COMP "clk" SITE "P6";
IOBUF PORT "clk" IO_TYPE=LVCMOS33;
FREQUENCY PORT "clk" 48000000.0 HZ;

CST (Gowin)

IO_LOC "clk" 52;
IO_PORT "clk" IO_TYPE=LVCMOS33;