Digital Music Programming II: Basic Stamp MIDI output
This lab demonstrates how to get MIDI output using a
Basic Stamp IIsx microcontroller.
The following program is written for the Basic Stamp IIsx
microcontroller. The program emits a MIDI note once every second
which has a duration of 1/2 of a second.
outpin con 8
baudmode con 60
outpause con 0
serout outpin, baudmode, outpause, [$90, 60, 127]
serout outpin, baudmode, outpause, [$90, 60, 0]
The program consists of two sections: (1) a header which defines some
constants so that the rest of the program is easier to read, and
(2) an infinite loop which plays the same note over and over.
The header (lines 1-3) contains three constant variable
declarations. This variables are replaced with their numeric
values in the body of the program when the program is compiled, so
no memory storage space is used to hold these variables when the
program is running (except as constant numbers). The three constants
- outpin -- output pin from the microcontroller which is
used to send MIDI data. This value can range from 0 to 15, since
there are 16 programmable pins on the Basic Stamp IIsx. In the
example program, and pictures below, pin 8 is used to send the MIDI
data out of the microcontroller, but any other pin would work just as
- baudmode -- speed of serial data transmission. In this
case, the value is 60 which means a baud rate of 31250 bits per
second (standard MIDI data rate) translated according to this equation:
(2500000/rate)-20 for the Basic Stamp IIsx microcontroller.
- outpause -- how many units of 0.4 milliseconds
(0-65535) to wait before sending serial data.
The body of the program (lines 5-10) does all of the work.
There are three functions used in the body of the program:
serout, pause, and goto.
Here is an explanation of each line:
This is a label which is used to mark a location in the program. Line 10
will send the program control back to this point.
- serout outpin, baudmode, outpause, [$90, 60, 127]
This line sends a note-on message on MIDI channel 1 with note 60
(middle-c) at the loudest possible attack velocity (127). Note that
the MIDI command is in hexadecimal form: $90 this is
equivalent to the decimal number 144, but is in an easier form to
read the MIDI command and MIDI channel off of.
- pause 493
Pause for 493 milliseconds. Sending the MIDI data takes time, and
interpreting each command in the Basic Stamp program takes time as
well. This value was measured to keep the tempo at exactly 60 beats
per minute. Theoretically the value should be 500 for a pause of
- serout outpin, baudmode, outpause, [$90, 60, 0]
Send a note-off command.
- pause 492
Pause 492 milliseconds.
- goto top:
Go to the top: label, which in this case means to start
the program over from the beginning.
The circuit connected to the Basic Stamp for sending MIDI data is
shown below. The program uses I/O pin #8 (DIP pin #13) for the MIDI
output, but you can change the program to use any output pin for MIDI out.
The +5v power source for the circuit below can come from the DIP pin
#21 from the microcontroller.
- Set the pause between note-on and note-off messages to 1 or 2
milliseconds. Then examine the signal going through the MIDI cable
on an oscilloscope. Here is what you should see:
Notice that start bits and stop bits which are used to mark the data
being transmitted on the serial connection. The data is sent upside
down, since 1's are sent as 0 volts and 0's are sent as +5 volts.
Whenever there is no data being sent, the MIDI cable is set to 0
volts. When a new data value is sent on the cable, the first bit
sent is always a 0 (+5 volts), then the 8-bit data byte is sent
one bit at a time from highest bit to lowest bit. The a stop bit of
0 is sent.
- Output some other MIDI note or MIDI Data from the program.
- Output a glissando rather than repeating one note.
- generate a random walk melody (or rhythm) using the random
Basic Stamp function.