Build and load bitstreams (open toolchain)

Source: the ULX3S MANUAL (emard/ulx3s @ 6a92cec, read 2026-09-27) and the programmers catalogued here (openFPGALoader review).

Build (ECP5)

yosys synth_ecp5 → nextpnr-ecp5 → ecppack (Project Trellis):

yosys -p "synth_ecp5 -top top -json top.json" top.v
nextpnr-ecp5 --85k --package CABGA381 --lpf ulx3s_v20.lpf --json top.json --textcfg top.config
ecppack top.config top.bit

Pick --12k, --25k, --45k or --85k for your chip; the ULX3S package is always CABGA381.
Use ulx3s_v20.lpf for boards v2.x/v3.0.x and ulx3s_v316.lpf for v3.1.6/v3.1.7 (see the ULX3S board page and the LPF catalogue).
A common trick: build with --25k and pass --idcode 0x21111043 to ecppack to load the bitstream on a 12F (same die).
Toolchain bundle recommended by the manual: YosysHQ oss-cad-suite-build (also ships fujprog and openFPGALoader). Lattice Diamond is only needed for Diamond-only historical projects and .vme files.

Build (iCE40)

yosys synth_ice40 → nextpnr-ice40 (--up5k --package sg48, --hx8k --package ct256, HX4K boards: --hx8k --package tq144:4k) → icepack. Older designs use arachne-pnr instead of nextpnr-ice40.

Load (ULX3S)

Tool	SRAM	Flash	Notes
openFPGALoader	`openFPGALoader -b ulx3s top.bit`	`openFPGALoader -b ulx3s -f top.bit`	Recommended; accepts `.bit.gz`. `-b ulx3s_dfu` uses the US2 DFU bootloader
fujprog	`fujprog top.bit`	`fujprog -j flash top.bit`	Successor of ujprog; FT231X on US1
OpenOCD	`ft232r` driver (slow)		Or an external FT2232 on the J4 JTAG header
ESP32	emard/esp32ecp5 (MicroPython, FTP/web)	yes	Can also write the SD card
dfu-util		`dfu-util -a0 -D top.bit`	Needs the DFU bootloader; user image at flash 0x200000

The LPF line SYSCONFIG ... MASTER_SPI_PORT=ENABLE lets JTAG write the flash at any time; set MASTER_SPI_PORT=DISABLE if the user bitstream must drive the flash pins.
Linux udev: FT231X is 0403:6015 (group dialout).
The ESP32 needs the passthru bitstream in FPGA flash (emard/ulx3s-passthru) to be flashed from US1. A bitstream that holds wifi_en asserted plus ESP32 firmware that grabs JTAG can lock JTAG out; jumper J3 disables the ESP32.