Beta release

README.md

@@ -1,4 +1,46 @@
 # raspi-pico-aprs-tnc
-A TX-only TNC to send APRS messages using a Raspberry Pi Pico
+A TX-only [TNC](https://en.wikipedia.org/wiki/Terminal_node_controller) (Terminal Network Controller) to generate the AFSK audio tones for [APRS](https://en.wikipedia.org/wiki/Automatic_Packet_Reporting_System) (Automatic Packet Reporting System) messages using a Raspberry Pi Pico microcontroller board.
 
-## :construction_worker_man: This is work in progress (Aug, 2021)
+Basically, this is the data/signal flow:
+
+```
+APRS (text msg + geo-coordinates + meta-data) -> AX.25 -> PCM -> PWM -> Low-Pass filtering -> AFSK signal @ GPIO 'GP0'
+```
+
+## 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 it twice.
+
+## Hardware
+
+We just need a simple low-pass filter to extract the AFSK-signal from the PWM signal. Just connect GPIO-pin 'GP0' to a 1k resistor and the latter to a 47 nF cap.
+The junction between the resistor and the cap is our signal output.
+
+## Build the application
+```
+(cd into the clone dir)
+cmake -S . -B build
+cmake --build build
+```
+
+## Run the application
+```
+cd build
+(copy any of the flash files 'aprs_pico.uf2' or 'aprs_pico.elf' to the Pico board)
+```
+
+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 an HT to bring it on the air (ham radio license required).
+
+## Modify the application
+
+To send an APRS message of your choice, you have to modify the *main()* function in `src/aprs_pico.c`.
+
+## TODO (Aug 2021)
+
+- [x] Send the APRS message on the console (USB or UART) rather than hard-coding
+- [x] Code documentation
+- [x] Show how to connect to a Baofeng HT
+- [x] PTT control for HTs
+- [x] Scope screenshots
+- [x] Schematic & Fritzings