Dual-Purpose Pins with Gowin FPGAs and OSS Tools

The Gowin series of FPGA support configuration pin reuse on some special dual-purpose pins. Those pins are used during programming of the FPGA, e.g. for JTAG or as READY and DONE signals. When enabling pin reuse, they can be used as normal GPIO pins by the FPGA bitstream after configuration. The OSS toolchain supports setting these flags and here’s how.

The pin reuse feature is documented in Gowin UG290E, section 4.1.2 Configuration Pin Reuse. Whereas the documentation explains how to configure this feature using the vendor toolchain, there’s little documentation on how to do it with the OSS tools. The fact that the vendor GUI places the pin reuse configuration in the Place & Route section provides some useful hint though: For Gowin FPGAs, this configuration is not part of some dedicated, static configuration on the FPGA chip. It can therefore not be programmed independently. These flags are part of the normal bitstream instead and therefore need to be inserted in the bitstream packing phase.

It turns out that gowin_pack indeed does support encoding these configuration bits. Simply use the following flags as needed when calling gowin_pack to pack your final bitstream:

--jtag_as_gpio
--sspi_as_gpio
--mspi_as_gpio
--ready_as_gpio
--done_as_gpio
--reconfign_as_gpio