Open RTT

Open RTT is a small open-source RAM transport for embedded targets and debugger-side host tools. It gives firmware a lightweight text channel without consuming a UART, USB CDC endpoint, or extra board wiring. The target exposes a fixed control block plus two ring buffers in RAM; the host reads and writes that RAM through the active debug probe.

The current repository contains:

• firmware/rtt: portable target-side C library
• host/dss: TI DSS host backend
• examples/f28p65x: minimal C2000 integration example
• tests: native tests and a basic toolchain check script

Open RTT is intentionally simple:

• one control block in RAM
• one up ring (target -> host)
• one down ring (host -> target)
• ASCII payloads stored in 16-bit cells
• line-oriented command path from host to target

It is inspired by the RTT style of debugger-assisted I/O, but the protocol and implementation in this repository are intentionally explicit and easy to reimplement for other debuggers and MCU families.

Why use it

• early bring-up when no serial path is available yet
• interactive commands over JTAG or SWD
• debug logs without dedicating board pins
• a common transport shape for TI C2000, MSPM0, MSP430, Cortex-M, Cortex-R, and other targets

Repository layout

• firmware/rtt/include/rtt.h: public target API
• firmware/rtt/include/rtt_protocol.h: wire-visible memory layout
• firmware/rtt/src: target implementation
• host/dss/rtt_host.js: debugger-side polling host for CCS DSS
• examples/f28p65x/linker_snippet.cmd: linker section example
• examples/f28p65x/rtt_demo.c: minimal echo application
• tests/native/test_rtt.c: native behavior tests

Quick start

1. Add these target files to your firmware build:
   • firmware/rtt/src/rtt.c
   • firmware/rtt/src/rtt_ringbuf.c
   • firmware/rtt/src/rtt_port.c
   • include path firmware/rtt/include
2. Reserve RAM for .rtt_block and .rtt_buf.
3. Call rtt_init() during system startup.
4. Use rtt_write_line() or rtt_write_char() to publish output.
5. Use rtt_read_line() or rtt_read_char() to consume host input.
6. Start the host backend from DSS:

dss.sh host/dss/rtt_host.js \
  --ccxml /path/to/target_xds110.ccxml \
  --symbols /path/to/app.out \
  --part F28P65

If the symbol table is unavailable, use --cb-addr <native-address> instead of --symbols.

Documentation

• docs/README.md: documentation index
• docs/protocol_spec.md: control block and ring format
• docs/FIRMWARE_INTEGRATION.md: target-side integration guide
• docs/DEBUGGER_BACKEND_PORTING.md: host/backend design and porting notes
• docs/USAGE.md: end-to-end usage and troubleshooting
• docs/CONTRIBUTING.md: contribution workflow

Current capabilities

• fixed protocol version 0x00020000
• 92-byte control block
• 16-bit transport cells so C28x word-addressed targets are handled cleanly
• host-side target profiles for several TI MCU families in the DSS backend
• whole-line admission on the down ring so partial commands are not injected into firmware

Current limitations

• single channel only
• ASCII payloads only
• no binary framing yet
• no timestamps or message metadata
• DSS backend is the only host backend currently checked in

Open source contributions

This project is intended to be maintained as an open-source transport library. Contributions are welcome for:

• new debugger backends
• new MCU integration examples
• protocol review and hardening
• tests and CI improvements
• documentation fixes and translations

If you want to contribute code, examples, or target support, start with docs/CONTRIBUTING.md. Backend ports for ST-Link, OpenOCD, pyOCD, J-Link, ESP-Prog, and similar tools are especially useful.