open-source-silicon.dev

are the inputs and outputs registered? I am finding conflicting information about this in the docs. If I wanted to an IO pin as a clock (&lt;40MHz), is there anything stopping me from doing that?

You mean the GPIO cells?
They are not registered. Source: <https://antmicro-skywater-pdk-docs.readthedocs.io/en/latest/contents/libraries/sky130_fd_io/docs/user_guide.html#sky130-fd-io-gpiov2-additional-features|https://antmicro-skywater-pdk-docs.readthedocs.io/en/latest/contents/libraries/sky130_fd_io/docs/user_guide.html#sky130-fd-io-gpiov2-additional-features>

The maximum fmax limit on the GPIO is placed by its switching speed.

The datasheet specifies 33MHz output speed.

I also made my own gpio cells, but they are not yet ASIC verified and I have no confidence on ESD protection I made. Here: <https://github.com/armleo/armleo_gpio|https://github.com/armleo/armleo_gpio>

Maximum configurable up to 300MHz, but obviously you will hut other limitations before you can hit that frequency