# raspi-pico-aprs-tnc A TX-only [TNC](https://en.wikipedia.org/wiki/Terminal_node_controller) (Terminal Node Controller) to generate the [AFSK](https://en.wikipedia.org/wiki/Frequency-shift_keying#Audio_FSK) (Audio Frequency-Shift Keying) audio tones for [APRS](https://en.wikipedia.org/wiki/Automatic_Packet_Reporting_System) (Automatic Packet Reporting System) messages using a [RP2040](https://en.wikipedia.org/wiki/RP2040) microcontroller on a [Raspberry Pi Pico](https://en.wikipedia.org/wiki/Raspberry_Pi) board.  Block diagram showing the signal flow An analog line-out audio signal will be generated by a band-pass filter connected to GPIO-pin 'GP0' which provides the binary PWM signal. You can probe it by 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: Line-out signal (see [below](#Hardware)) probed by a DSO. We clearly see the 1200 Hz and 2200 Hz tones of the 1200 Bd 2-AFSK. Both a static library `libaprs_pico.a` and an example application will be generated by the build. ## Prerequisites Your host platform for cross-compilation is assumed to be LINUX. 1. Install the Pico-SDK following the instructions given in the [Raspberry Pi 'Getting Started' Guide](https://datasheets.raspberrypi.org/pico/getting-started-with-pico.pdf) (pdf) 1. Set the `PICO_SDK_PATH` environment variable to point to your Pico-SDK installation directory 1. Install the [pico-extras](https://github.com/raspberrypi/pico-extras) libraries: ``` cd $PICO_SDK_PATH/.. git clone -b master https://github.com/raspberrypi/pico-extras.git cd pico-extras git submodule update --init ``` ## Hardware We just need a simple band-pass filter to extract the analog AFSK-signal from the binary PWM signal:  The line-out voltage can be as high as 2.7 V_pp (~1 V_rms) (at full-scale volume setting in the software and high-impedance load). ## Build the library and the example application ``` git clone https://github.com/eleccoder/raspi-pico-aprs-tnc.git cd raspi-pico-aprs-tnc cmake -S . -B build cmake --build build ``` `build/lib/libaprs_pico.a` and `build/aprs_pico_example.uf2|.elf|.xxx` will be generated. ## Run the example application ``` cd build Flash 'aprs_pico_example[.uf2|.elf|.bin]' to the Pico board as usual ``` The analog AFSK audio signal will be available at the filter's line-out. You can probe it by 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 (Sept. 2021) - [x] Thorough evaluation, in general - [x] Send the APRS message on the console (USB or UART) rather than hard-coding - [x] Show how to physically connect to a Baofeng HT - [x] PTT control for RF transceivers ## Ingredients / Acknowledgements - For APRS => AX.25 => PCM conversion I'm using [my modified version](https://github.com/eleccoder/ax25-aprs-lib) of [fsphil's ax25beacon](https://github.com/fsphil/ax25beacon) - For PCM => PWM conversion I'm using the `pico_audio_pwm` library from [pico-extras](https://github.com/raspberrypi/pico-extras) (NOTE: ATTOW, maturity seems to be rather alpha/beta)