Pico OpenTherm RX

OpenTherm is a protocol for central heating boilers and controllers to communicate using 32-bit frames (see OpenTherm 2.2 spec).

This is a project for the RPi Pico (RP2040) to send and receive OpenTherm frames using PIO state machines: one to encode/transmit and the other to receive/decode. Both SMs are connected to the same GPIO (loopback test).

The repository is structured as a VS Code project with CMake. It is configured to manage the target device via a Picoprobe USB/SWD bridge.

NOTE: this code is less tested and less frequently updated than my pico-opentherm-boiler. You almost certainly want to be using that instead.

Notes

OpenTherm encodes bits with Manchester / Bi-phase-L (rising edge '0', falling edge '1'). Frames consist of a leading '1', 32 data bits and a final '1'. Frames are sent LSB first at 1kb/s with the first data bit being an (even) parity bit.

The transmit and receive processes communicate with the CPU via the PIO state machine FIFO queues at one 32-bit word per frame.

DevLog

01/04/2025

• create project on GitHub from my newly fixed pico-pio-template
• update readme.md (this file)

02/04/2025

• set project name and update filenames and CMakeLists

07/04/2025

• add PIO code to receive and decode a frame (compiles OK)
• add OT_tx_library with PIO code to transmit a frame for loopback testing (compiles OK)

09/04/2025

• test loopback code, fix error in OT_rx_library
• link to 'pico/rand' to randomise frame content

10/04/2025

• define data type for frame bitfields
• display binary frame fields
• add parity check
• update documentation

26/04/2025

• back-port new receiver PIO code from pico-opentherm-boiler
• fix bit ordering in transmit code: OpenTherm frames are send MSB first