README 

This sample is a simple monophonic mini organ. You have to attach an USB PC
keyboard to your Raspberry Pi to use it. The output signal is available on the
3.5mm headphones jack.

If you have a MIDI keyboard with USB interface, you can also use it to play. It
has to support the USB Audio Class MIDI device specification. This sample does
not support the MIDI velocity parameter.

Because this sample is enabled for USB plug-and-play, an USB (MIDI) keyboard can
be attached or removed at any time while or before the system is running. It
will be detected automatically and can be used, if it is attached. On the
Raspberry Pi 1-3 and Zero the system option USE_USB_SOF_INTR should be defined
in the file include/circle/sysconfig.h to use this sample program.

You can also attach a serial MIDI interface to your Raspberry Pi (31250 Baud).
The MIDI input data has to be feeded to GPIO15 (Broadcom numbering) at 3.3V
level. Because the MIDI standard uses a current loop, you need a converter
circuit. Serial MIDI input is enabled, if an other keyboard is not attached.
The MIDI output is not used by this sample.

The mini organ uses the PWM sound device to generate a square wave of the
required frequency for about one octave (note C3 to C4). A more complex sound
synthesis using the CPWMSoundBaseDevice class should be possible.

This sample can be built to be used with an I2S sound device too. It has been
tested with PCM5102A and PCM5122 DACs. You have to enable the #define USE_I2S in
the file miniorgan.h before building to use this. The DAC has to be connected to
GPIO18 (BCK), 19 (LRCK) and 21 (DATA, Broadcom numbering). For I2S operation
with a PCM5122-based sound device you may have to set the I2C slave address of
the DAC too. By default the addresses 0x4C and 0x4D will be auto-probed. I2S
sound cards with the WM8960 should work too (default I2C slave address is 0x1A).

This sample can be built to be used with a HDMI display with audio support too.
You have to enable the #define USE_HDMI in the file miniorgan.h before building
to use this. You may notice a delay between pressing a key and the start of the
tone. This latency is caused by the hardware and cannot be eliminated.

This sample can be built for the Raspberry Pi 4 to be used with an external USB
audio class streaming device. You have to enable the #define USE_USB in the file
miniorgan.h before building to use this. The USB sound card must be connected
before the sample program is started.

The PC keyboard mapping is as follows:

+--------+
|Key Note|
+--------+

   +-----+-----+     +-----+-----+-----+
   |S C#3|D D#3|     |G F#3|H G#3|J A#4|
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+-----+
|Z C3 |X D3 |C E3 |V F3 |B G3 |N A4 |M B4 |, C4 |
+-----+-----+-----+-----+-----+-----+-----+-----+

The inscription of the keyboard may vary, but the physical position of the
piano keys is the same on all keyboards. Using this scheme you should be able to
find them.


USB GADGET MODE

This sample works in USB gadget mode too. Normally the Raspberry Pi is the USB
host, but in gadget mode it is the peripheral device. This allows the Raspberry
Pi to be directly connected to a host computer. This is supported for the
Raspberry Pi (3)A(+), Zero (2) (W) and 4B only. The Raspberry Pi 4B uses the
USB-C power connector for that purpose. You need a suitable USB cable. The
Raspberry Pi is normally bus-powered via the USB host in this case. A separate
power supply need not be connected.

If you want to self-power your Raspberry Pi, you have to ensure that the supply
voltages at the USB connector and the power connector are exactly the same.
Otherwise there is a risk of damage. Be careful!

A configuration is required for this test. First you have to enable the define
USB_GADGET_MODE in the file kernel.cpp. Furthermore you have to define the macro
USB_GADGET_VENDOR_ID with your USB Vendor ID to be used for the USB device. See
the file include/circle/sysconfig.h for details!

Further general information about using the Raspberry Pi in USB gadget mode can
be found in this guide:

	https://github.com/thagrol/Guides/blob/main/ethernetgadget.pdf