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raspi-pico-aprs-tnc

A TX-only TNC (Terminal Network Controller) to generate the AFSK audio tones for APRS (Automatic Packet Reporting System) messages using a Raspberry Pi Pico microcontroller board.

An analog line-out audio signal will be generated at GPIO-pin 'GP0'. You can observe it by using a scope, listen to it by using an audio amp, or connect it to any RF transceiver to send it on the air (ham radio license required).

Image: Pico's output signal probed by a DSO. We clearly see the 1200 Hz and 2200 Hz tones of the 1200 Bd 2-AFSK.

Basically, the data/signal flow is as follows:

APRS (text msg + geo-coordinates + meta-data) -> AX.25 -> PCM -> PWM -> Band-Pass filtering -> AFSK audio signal

Both a static library libaprs_pico.a and an example application will be generated by the build.

Preliminaries

Your host platform for cross-compilation is assumed to be LINUX.

Install the Pico-SDK following the instructions given in the Raspberry Pi 'Getting Started' Guide (pdf)
Set the PICO_SDK_PATH environment variable to point to your Pico-SDK installation directory

Hardware

We just need a simple band-pass filter to extract the AFSK-signal from the PWM signal:

Build the library and the example application

(cd into the cloned dir)
cmake -S . -B build
cmake --build build

build/lib/libaprs_pico.a and build/aprs_pico_example.xxx will be created.

Run the example application

cd build
(flash 'aprs_pico_example.uf2' or 'aprs_pico_example.elf' to the Pico board as usual)

The GPIO-pin 'GP0' is the line-out for the analog AFSK-signal. You can observe it by using a scope, listen to it by using an audio amp, or connect it to any RF transceiver to send it on the air (ham radio license required).

TODO (Aug 2021)

Thorough evaluation, in general
Send the APRS message on the console (USB or UART) rather than hard-coding
Show how to physically connect to a Baofeng HT
PTT control for RF transceivers

Ingredients / Acknowledgements

For APRS => AX.25 => PCM conversion I'm using my modified version of fsphil's ax25beacon
For PCM => PWM conversion I'm using the audio lib from pico-extras (WARNING: maturity seems to be alpha/beta)

2.8 KiB Czysty Wina Historia