Typie-Walkie with LoRa and E-Paper Screen

This project is a variant of the Typie-Walkie E-Paper Screen. The radio of the initial project, WiFi, does not match with the low-power features of the e-paper screen.

Looking for another radio, LoRa came up for short messages over long distances. The boards are the iLabs Challenger RP2040 LoRa: they combine a RFM95W-SX1276 sub-1 GHz radio with an RP2040 MCU.

The touch e-paper screens were sampled by Pervasive Displays. The Challenger boards were bought at The Pi Hut.

Hardware

The main difference is the board, here the Challenger RP2040 LoRa with a sub-1 GHz radio.

The RP2040 offers two SPI buses: one is used for the e-paper panel; the other for the radio. The touch controller is driven through I²C.

The boards feature a native connector for a LiPo battery.

Software

The application is identical for the two devices.

The libraries for the screen remain unchanged. The Pervasive Displays Library Suite manages all the elements of the interface:

The input keyboard is used to type in the messages (TX);
The Send button sends the messages (TX); and
The output text-box displays the messages received (RX).

The keyboard contains three sets of keys: capital letters, lowercase and symbols.

Additionally, the § key changes the layout, and the « key acts as backspace.

The management of the LoRa radio was supposed to be easy thanks to the off-the-shelves Arduino LoRa library. Alas, it wasn't the case.

Too many messages were lost, whatever the options used with or without interrupts and callbacks. By luck, the RP2040 features two cores: dedicating each one to a specific task solved the problem.

The first core manages the touch and screen.
The second core manages the LoRa radio.

Both cores communicate through two hardware FIFOs: one FIFO provides write access to the first core and read access to the second; the other FIFO does the opposite.

The SDK also brings useful tools like mutexes to avoid collisions on the serial port. Those tools were encapsulated as objects to match the syntax of the Galaxia library.

Conclusion

Compared to other sub-1 GHz solutions (like the Texas Instruments CC1310, CC1350, CC1312, CC1352), the iLabs Challenger RP2040 LoRa doesn't provide the same level of integration. One core of the RP2040 is dedicated to the radio to avoid losing messages.

Compared to MQTT, LoRa doesn't provide the application layer: it needs to be developed with proprietary solutions.

The LoRa radio is consistent with the e-paper technology to bring a truly low-power solution.