project: write extra readme text

firmware/README.md

@@ -0,0 +1,35 @@
+
+# Building the Firmware
+
+In order to build the firmware, [platformio](https://platformio.org/install/cli) must be installed. 
+
+To build the firmware, navigate to this folder using the terminal of your choice and type
+
+```bash
+platformio run
+```
+
+## Flashing the firmware 
+
+If you happen to have an stlink compatible interface (e.g the [programmer attached the nucleo-boards](https://jeelabs.org/book/1547a/])), 
+then (after connecting said interface to the PC and the PCB), 
+the firmware can be flashed with 
+
+```bash
+platformio run -t upload
+```
+
+# serial control Syntax
+Upon connecting the stm32 to a PC using USB it will register itself as a new Serial Port. The following text commands can be issued via said Serial connection (the baudrate is irrelevant): 
+
+- `C[number]\n` will change the input to the specified channel. The stm32 echo the command as it was understood. e.g: When Sending `C0\n` it will respond `C0\r\n` to acknowledge the channel switched to **Input 1**. When issuing a `C10` there will be no response, since 10 is out of range. *NOTE:* when using the buttons on the PCB to switch the input, the stm will also issue a `C[number]\r\n` over the serial port if it is connected to notify the PCB of the external input change. 
+
+- `G[number]\n` will activate an input gain of [number] in dB. It will also respond with the closest gain it can do. e.g: `G10` will result in the stm responding `G12\r\n` which means it will add a 12db input boost, which was the closest matching _valid_ input gain value. You can check the datasheet of the BD3491 to find the possible gain values if this peaks your interest :) 
+
+- `L[number]\n` or `R[number]\n` will activate an attenuator in either the left or the right channel. [number] is once again in dB. It works much like the `G`-command. Setting [number] to 0 will deactivate the Attenuation.
+
+- `B[number]\n` or `T[number]\n` adds **B**ass or **T**reble gain to the output
+
+- `M[0/1]\n` will either mute or unmute the output
+
+- `S[0/1]\n` will connect all inputs to the output