RP2040 access point

Cornell University ECE4760
Access point and web server as Control Panel
Pi Pico RP2040

Access point and web server
The access point example code in the WIFI section of the picoW examples runs as-is, but I hacked it out of the examples build-structure to compile alone, then added a VGA driver. The code brings up a stand-alone WIFI access point, with DHCP server, DNS server and web server. The supplied code serves one page, with one clickable link that toggles the picoW on-board LED. I added a bunch of comments, generalized the query-string parser (query-string is the part of URL after the ? character), and adding ability to serve several pages of html. Since there is no file system, and no outside connection to the internet, the page request processing had to be quite simple, but supports run-time modifiable pages, text, html forms, hyperlinks, and javascript. The javascript, of course, runs on the browser.

This example is intended to show how to set up simple web pages for process control, not to produce a general web server. Web pages are written in raw HTML. Simple text has the usual formatting tags and can be modifed on the fly by the server for text-based output. The example has input interactors in the form of text input, graphic sliders to set numerical values, and drop down menus. Simple SVG graphics can be embedded as can javascript.

As a first test, I wanted to see if a cell phone could act a a reasonable user interface to code running on the picoW. In this case the application code was just a video thread running on core1 which displayed the status of the server and browser variables. LWIP is running on core0. My Android phone connected to the picoW with no complaints. The code runs without a serial monitor or VGA attached, but obviously then you can only interact via browser.
Demo functionality:

After the system boots you should be able to point your phone WIFI to:
access_point_ssid = "picow_test"
password = "password" using WPA2
The IP address should appear as 192.168.4.1/ledtest
At least two phones can attach to the page at the same time.
Each browser page is written as a string to be used as a format string for an sprintf that inserts the runtime
variables, one for each '%' format designation. The scheme is quite powerful. It can dynamically change the name of displayed controls, set control options (e.g. selected parameter in the LED control), and display user input/output.
LEDtest page (html)
- The /ledtest page has a link to a second page.
- The browser can set a select drop-down list to toggle the on-board LED. An SVG filled circle changes dynamically to show the LED status (green if on, black if off). LED status is also echoed back to the browser text, and displayed on the VGA. When the drop-down list option is touched, the selection is auto-submitted back to the server.
- The browser can set a text box to any integer to be displayed on VGA when the submit button is pressed.
- The brower can set a graphical slider control to a range of integer values. The current value is changed in realtime as the slider is moved via a one-line javascript attached to the oninput event and also displayed on the VGA when the slider is released using auto_submit onchange. The one-line javascript is hacked slightly from
  https://stackoverflow.com/questions/74320571/inserting-oninput-to-an-input-field
- The server serial console thread accepts a command to set an integer which appears on the VGA, and in the browser on the next page load.The server serial console thread accepts a command to set an integer which appears on the VGA, and in the browser on the next page load.
The second page (html)
- The /newpage page has a link back to /ledtest
- There is a slider intereactor to set the 8-bit color for a rectangle on the VGA screen attached to the rp2040. The oninput event is used to update the input number as you slide your finger. The onchange event auto_submits the number to the server which signals the graphics thread.
- A small SVG rectangle approximates the 8-bit VGA color set, but is extended to standard 24-bit web colors and is therefore slightly different.

Three screens are shown below refecting different views of the program state. First is a browser window running on my cellphone and after pressing the submit buttion. Notice that there is no internet connection and that the IP I chose for the server is 192.168.4.1. The page is ledtest. The second image shows page 2 of the browser interface. The third image is the server-attached VGA screen showing the status of all the browser and console variables.

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CODE, ZIP of project
Note that you will need to delete the build directory and specify a new one.

VGA driver and LWIP considerations
Both the VGA driver written by Hunter Adams (and converted to 256 colors by Bruce) and the LWIP supplied as part of the C SDK use significant hardware to accelerate operations. The VGA driver uses 4 DMA channels and all 4 state machines in one PIO. The LWIP uses 2 DMA channels and two PIO state machines. LWIP defaults to PIO1 (see cyw43_bus_pio_spi.c), so the VGA must use PIO0. The LWIP uses the claim mechanism to grab DMA channels. This means that any unused channel may be grabbed. The VGA driver was modified to use the claim system so that it does not step on the channels used by LWIP. Once this small change was made, the two libraries played nicely together.