codejunkie99 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
- Check project target (board.yaml -> pnr_target)
- Verify synthesis output exists (synth.json from Yosys)
- Verify constraint file exists
- Run nextpnr with correct flags
- Report timing and utilization
- 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 0for auto-determine,--freq Nfor target
Slack
- Positive = timing met | Negative = violation | Zero = exact
- Critical path = path with worst slack
Improving Fmax
- Pipeline long combinational paths
--opt-timingfor post-placement optimization- Different seeds (
-r) for better placement - Increase
--placer-heap-timingweight(20-50) --router2-tmg-ripupfor timing-driven rerouting- Yosys:
-nowidelutfor 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;