Pico display slave on a Waveshare 2.9inch Touch e-Paper Module (2023)
Raspberry Pi Pico (with the Waveshare 2.9 inch Touch e-Paper module as a hat) is USB connected to (and powered by) a Raspberry Pi host
which runs a nodejs server for communication using a local etcd3
server for inter component communication. The system is a proprietary
hot tub controller and this is the outside wall interface (there is also a http interface). ePaper was chosen
for the high contrast, not for power saving. The unit is mounted in a 2 gang faceplate matching that of the
build.
Install Pico SDK in ~/third_party directory and the rest
should be straight forward CMake process (with make install using picotool to automatically upload the image).
The host system components are in the system directory, with as test.js as a test interface,
and main.js as the production interface. Run the test interface as while [ true ]; do node -d -c config.ini test.js; done
as the /dev/pico device will disappear once picotool puts into BOOTSEL mode. This has the benefit
of reloading test.js for any host side changes.
Note the LED is wired to GPIO 14 to provide a visible blinking indication that
a warning message needs to be read, which is done so by tapping
the centre of the screen. The design is such that the screen is laid out with area for four buttons. On the main screen
these control the setpoint (up, down) and the operating mode (running / standby). The third button on the
main screen is hidden but renders a control menu, which is a list of
configurable actions (uploaded from host) and access to a diagnostic
dump from the etcd3 server. Changes to any variables are only
confirmed once reflected back from the host, thus modes and settings
and controls are enforced in the host. The Pico has an internal
temperature sensor which is also sampled and downloaded to the host.
This particular Waveshare device was chosen because it has a touch interface, is an approprtiate size, has a protective screen layer, is built for the Pico, and has a fast partial refresh rate (3 seconds) with low ghosting. The physical buttons are not used (there are four: three are usable, the fourth is tied to reset).
See https://www.waveshare.com/Pico-CapTouch-ePaper-2.9.htm and https://www.waveshare.com/wiki/Pico-CapTouch-ePaper-2.9.