add some usage notes to docs

docs/src/SUMMARY.md

@@ -24,3 +24,4 @@
 - [Usage Guide](./usage.md)
   - [Loop Station](./loopstation.md)
   - [Drum Machine](./drummachine.md)
+  - [MIDI](./midi.md)

docs/src/loopstation.md

@@ -1,3 +1,12 @@
 # Loop Station
 
-**TODO**: work in progress
+In this mode the first three buttons play each corresponding output channel.
+
+The fourth button, with the LED always lit, is the record button.
+Hold it down for a while and release it to set the length of the loop.
+
+Notes played while the record button is held will be looped.
+
+The top-right button below the encoder clears the recorded loop.
+
+You can modify the loop length using the encoder menu.

docs/src/midi.md

@@ -0,0 +1,10 @@
+# MIDI
+
+The MIDI mode is only available when a computer is connected via USB to the Raspberry Pi Pico.
+
+You only have two settings in this mode, the receive and transmit MIDI channels, ranging from 1 to 16.
+
+Notes received on the selected Rx channel will be played on the output channels.
+Velocity is ignored.
+
+Any pressed note buttons are sent on the selected Tx channel.

docs/src/pcb2.md

@@ -8,6 +8,10 @@ It is planned to be used without a case.
 
 You can [download the Gerber files to order PCBs online](./plot/fab_pcb2.zip).
 
+**Warning:**
+Revision 1 is missing diodes on the button matrix.
+So when holding down three or more buttons simultaneously you get ghosting on other buttons.
+
 ## Bill of Materials
 
 These are all the parts required to assemble the V2 PCB.

docs/src/usage.md

@@ -1,3 +1,18 @@
 # Usage Guide
 
-**TODO**: work in progress
+Turn on LARS using the power switch on the PCB.
+
+In normal operation, the bottom of the display always shows the battery level in percent, and the voltage.
+Take care to **turn off the device** and recharge it when the **percentage reaches zero**!
+
+The top of the screen shows the currently selected setting and its value.
+Press the encoder to switch to the next available setting, or turn the encoder to change the value of the currently selected setting.
+
+The available settings depend on the current mode.
+Take a look at the sub-page for each mode for details.
+
+You can hold down some buttons while powering on the device to get different effects:
+
+* hold the `REC` button to skip the boot animation
+* hold the encoder `Click` button to show version information of the firmware
+* hold both `REC` and `Click` to enter the button test mode