2.6 KiB
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 produced 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).
Basically, this is the data/signal flow:
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 is assumed to be LINUX.
If you have already installed the Pico-SDK, set the PICO_SDK_PATH environment variable accordingly to avoid installing the SDK twice.
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 connect to a Baofeng HT
- PTT control for RF tranceivers
Acknowledgements
- For APRS => AX.25 conversion I'm using a 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)