This repo contains the source code for the RP2040 based port-expander known as 'rp2040-expander'. Some demonstration modules, which make use of the rp2040-expander's API, are also included in this repo.

rp2040-expander is a minimal software environment which utilizes a Raspberry Pi RP2040 microcontroller as a port-expander peripheral for a host processor.

The RP2040 has no attached SPI Flash, may not execute any software and is unlikely to require a quartz crystal (subject to the intended use, please see: LINK whitepaper here).

The rp2040-expander is controlled by the host using the RP2040's 'Serial Wire Debug' interface over which 'Debug Access Port' transactions are used to control the internal RP2040 peripherals or to download a program to RP2040 RAM. Please see the LINK whitepaper here for RAM downloaded program details.

Generic source code is supplied in the /host directory to provide the necessary SWD, DAP and higher level control primitives and to provide the functional API to the host system.

The /port_xxx directories contain host specific code that provide per-platform support, i.e. GPIO manipulations to implement the host side SWD interface.

The /demo directory contains the modules used to exercise some of the rp2040-expander's API.

The SWD interface is 'bit-bashed' and requires a few supporting functions to perform the GPIO manipulations required to provide the transfers over SWD.

A port of this software requires that the user provide these functions for their intended host platform.

Two programmable GPIOs are required on the host, one is used only as an output from the host, and drives the SWCLK signal, the other GPIO must be bidirectional and is used for the SWDIO signal.

The Debug Access Ports within the RP2040 provide access to internal debug resources and using them it is possible read and write to the RP2040's memory and internal registers such as GPIO control registers.

In this application only a small subset of the RP2040 DAP functionality is required and the code contained here supports that subset and no more.

Support for two Host platforms are provided within this repo:

• An RP2040 hosted demo; that is running the demo software on an RP2040 which in turn controls a second RP2040 which performs the expander function.

Any available GPIO ports can be used on the host RP2040, the source here uses GPIO2 and GPIO3.

You can connect any suitable RP2040 boards, most obviously two Pico boards. (INSERT interconnect diagram here)

• A Raspberry Pi computer hosted demo; that is running the demo on a Pi computer (e.g. Pi 3, Pi 4 ) controlling an RP2040 which performs the expander function.

Any available GPIO ports can be used; the source here uses GPIO17 and GPIO27. (INSERT interconnect diagram here)