readme: Rewrite and extend the readme [CI SKIP]

Explain how things are.

README.md

@@ -1,21 +1,83 @@
 # Clock Generator [![status-badge](https://ci.onders.org/api/badges/finga/clock_generator/status.svg)](https://ci.onders.org/finga/clock_generator)
 
-A simple board to control an Si5351 clock generator board with the
+The "Clock Generator" generates up to three different clock signals
-TWI. Though, this is not limited to the Si5351 board as it is designed
+between 15kHz and 162MHz.
-to be universal usable.
+
+Though, the board is not limited to this, it is designed to also be
+usable for other applications.
 
 ## Board
-The board is populated with an ATmega328p, a display and its
+The board is populated with an ATmega328p, an LCD and its backlight
-backlight driver, a switchable rotary encoder and powered by a mini
+driver, a rotary encoder which is also a push button and is powered
-USB port. All unneeded MCU pins are accessible via pin headers.
+via mini USB. The TWI port of the ATmega interfaces the Si5351 which
+is also driven from VCC and GND, a pin breakout for all unused pins
+and some more VCC and GND pins as well as an ICSP header.
 
-## Firmware
+The general supply voltage behind the LDO regulator is 3.3V
-Currently there are two different firmwares. The older one which is
+
-written in C and is not finished, and the newer one which is in an
+Note that in the PCB layout the display and the rotary encoder are
-early but functioning state and written in Rust.
+placed on the backside of the PCB.
+
+### ATmega328p
+The Atmega328p is programmable via the ICSP header and is the center
+piece of the board. All free pins can be used with J3 and the TWI with
+J4.
+
+#### Connectors
+##### Breakout (J3)
+| Pin left | ATmega328p | ATmega328p | Pin right |
+|----------|------------|------------|-----------|
+| 1        | +3.3V      | +3.3V      | 2         |
+| 3        | PC1        | PD2        | 4         |
+| 5        | PC2        | PD3        | 6         |
+| 7        | PC3        | PD4        | 8         |
+| 9        | PD0        | PD6        | 10        |
+| 11       | PD1        | PD7        | 12        |
+| 13       | GND        | GND        | 14        |
+
+##### TWI (J4)
+| Pin | Signal |
+|-----|--------|
+| 1   | +3.3V  |
+| 2   | GND    |
+| 3   | SDA    |
+| 4   | SCL    |
+
+### Si5351
+The Si5351 is capable of generating three 8kHz to 160MHz clock
+signals.
+
+Internally, the Si5351 consists out of two phase locked loops which
+feed three parallel and independently configurable clock synthesizers.
+
+### Display
+The DOGS102 graphic LCD module is controlled by SPI (Serial Peripheral
+Interface) and the RST (reset) and CD (control data/display data)
+pins. The contrast can be configured on the setup screen and its value
+is stored in the EEPROM (Electrically Erasable Programmable Read-Only
+Memory) of the ATmega.
+
+#### Backlight
+To drive the backlight, the Timer/Counter0 from the ATmega are used to
+generate a PWM (Pulse Width Modulation) signal. This enables the
+backlight to be variably bright. The brightness can also be configured
+on the setup screen, and is stored inside the EEPROM.
+
+### Rotary Encoder
+To control the Clock Generator a single rotary encoder which is also a
+push button is used.
+
+## Firmware/Programming
+Currently, there are two different implementations of the
+firmware. The older one which is written in C and is not finished, and
+the newer one which is in an early but functioning state and written
+in Rust.
+
+Note that mostly due to the display, the supply voltage after the LDO
+regulator is 3.3V.
 
 ### The Rust Firmware
 To flash the firmware, connect the ICSP pins of the board to the
 programmer and inside the `firmware/rust/` directory run `cargo make
-all`. This burns the fuses, writes the the default values to the
+all`. This burns the fuses, writes the default configuration values to
-eeprom and flashes the firmware.
+the EEPROM and flashes the firmware.