Resource added

README.md

@@ -3,18 +3,16 @@
 # 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.
 
+![Signal Flow](https://github.com/eleccoder/raspi-pico-aprs-tnc/blob/main/doc/img/signal_flow.png)
+
+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).
 
 ![AFSK scope screenshot](https://github.com/eleccoder/raspi-pico-aprs-tnc/blob/main/doc/img/afsk_scope.png)
 
 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.
 
-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.
 
 
@@ -29,6 +27,8 @@ Your host platform for cross-compilation is assumed to be LINUX.
 ```
   cd $PICO_SDK_PATH/..
   git clone -b master https://github.com/raspberrypi/pico-extras.git
+  cd pico-extras
+  git submodule update --init
 ```
 
 ## Hardware