This project shows you how
the hardware of a MIDI interface works and goes
through some simple examples to show how the
controller can be used in MIDI systems. First we
will start by explaining the details of the
hardware interface. Then we will look at the
MIDI protocol and see how simple it is to write
custom software for this MIDI controller.
The MIDI Hardware
There are only a couple
parts needed to build the MIDI connections. A
few resistors, a diode, and a small 8 pin IC and
that is it. The controller part of the system is
a 2051 microcontroller (a 20 pin version of the
popular 8051). The MIDI interface connects to
the serial port of the 2051 (pins 2 and 3). The
2051 also has a reset circuit (the 10 uF
capacitor and the 8.2k resistor) and an
oscillator circuit (the 24 MHz crystal and the
two 33 pF capacitors). For more information on
the 2051, The
only thing not shown is the 5 volt power supply
that creates Vcc (5 Volts DC) for the circuit.
LM7805 and 220 uF capacitor is included with
the kit to create 5 Volts DC from a 9 to 12 Volt
The 6N137 isolates one electrical system from
another. There is no electrical connection
between devices connected by MIDI. When a
current flows through the internal LED connected
between pins 2 and 3, the LED transmits light to
a light detecting diode. The diode is activated
and the Output at pin 6 is pulled low by the
internal transistor. If no current flows then
the Output floats. Rather than let it float, we
connect an 8.2k resistor to pin 6 to lightly
pull the voltage at the Output to 5 Volts when
the circuit in not active.
Sending a Signal
The microcontroller sends signals through the
MIDI port by controlling the voltage at its pin
3. The current is provided by the 5 volts (Vcc)
connected to pin 4 of the MIDI Out port. When
the MIDI Out port is connected to a MIDI In port
of another system the following circuit is
If we simplify this circuit a little bit then it
looks like the circuit below.
When Pin 3 of the controller goes to 0 volts
then a current flows through the circuit making
the internal LED in the 6N137 light up. Then
when Pin 3 of the controller goes to 5 Volts (Vcc),
the current stops flowing and the LED turns off.
As discussed above, the light detecting diode in
the receiving circuit detects when the LED turns
on and off, completing the reception of the
You should be able to build the
MIDI controller from the first schematic at the
top of the page. Use the MIDI connector with the
ground wire for the MIDI out port. You should
end up with something that looks like the
You can also add an LED
going from 5 volts to Pin 2 of the 2051, in
series with a 510 ohm resistor, to have a visual
indication when there is activity on the MIDI In
Port. You can do the same for Pin 3 to see
activity on the MIDI Out Port.
Using MIDI - The
Before we get into the
actual software for the controller, we will look
at a few simple MIDI commands. The first is a
Note On command. This tells the receiving system
to play a particular note. Then we send a Note
Off command to tell the system to stop playing
The commands are made
up of a series of bytes. The Note On command is
3 bytes. It looks like
9x yy zz
x is the channel
number. There are 16 possible channels (0, 1, 2,
3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F). yy is
the note number and can be anything between 00
and 7F (0 and 127 in decimal values). zz is the
attack velocity and can be anything between 01
and 7F (an attack velocity of 00 makes the Note
On command into a Note Off command).
The Note Off command is
about the same. It is
8x yy zz.
The values are the same
as before except zz is the release velocity.
For our first example
we will make the controller play some notes. The
software is midi1.asm. You only need to connect
a a MIDI cable from the MIDI out port of the
circuit to the MIDI IN port of a keyboard. Study
the documentation in the software file to learn
more about how the software works.
The basic process of
compiling an assembly language program and
loading it into the microcontroller was covered
first microcontroller project.
Compile the program and download it to the 2051.
Make sure the power is off
to the circuit you have built. Then move
the 2051 to the circuit. Turn on the power to
The second example monitors inputs on the
MIDI IN port of the controller. When a Note Off
command is detected, the controller sends out a
Note On and Note Off command for the same note.
This creates a sort of echo effect.