Software control over GPIO mode

[matty0005]

In the vast majority of microcontrollers you can control the tristate/mode of the pin so that you can either set it to be an input or output. Currently the neorv32 does not have such control in software. There are a few cases when you may wish to change the state on the fly (eg bitbanging an interface that you don't want to create a seperate hardware module for).

To not break the stuff mentioned in #538, you could prehaps add an output (from the gpio module, eg gpio_z) that allows you to control the tristate in a top level design. Eg,

GPIO_TRISTATE: for i in 0 to 7 generate
    gpio_io(i) <= gpio_o(i) when gpio_z(i) = '1' else 'Z';
end generate; 
gpio_i <= gpio_io;

Then in the software side of things, you could do the following:

NEORV32_GPIO->MODE_HI = 0x01; // set GPIO 1 as output, rest as inputs. 

I can draft a PR for this, but I just wanted to see what everyone thought before hand. My only concern is that it would requrie another set of registers to be added to the GPIO module which may result in changing the addresses of the other peripherals.

Anyway, its food for thought.

[stnolting]

Currently the neorv32 does not have such control in software.

That's right. From my point of view such a "programmable" input/output selection does not make much sense for a FPGA platform. For FPGAs you have full control which physical pins are mapped to GPIO inputs or outputs anyway. 🤔

If you require a tri-state functionality, then I would recommend to add this inside the top's "IO ring".
For example, you could use the upper 32-bits of the GPIO output ports to control the tri-state logic for the lower 32-bits of the GPIO inputs and outputs. Thus, NEORV32_GPIO->OUTPUT_HI would reflect the "data direction" configuration register.