ee476 Lab 3

EE 476: Laboratory 3

One-octave keyboard.

Introduction.

You will construct a one-octave, single voice, square-wave output, keyboard synthesizer. The keyboard will produce the musical notes corresponding to:

        C4   262 Hz (middle C)
        D4   294
	E4   330
	F4   349
	G4   392
	A4   440
	B4   494
	C5   523

Procedure:

You will need to connect the switches to PortB of the mcu so that you can use all eight switches. One bit of PortD, perhaps bit 6 (which happens to be adjacent on the connector to ground), will be used to drive an earphone.

The interrupt-driven timer described in the first lab will be modified to produce one interrupt per 1/2 cycle of the desired audio tone. Be sure to use a pre-scaler value which maximizes the frequency accuracy of your tones. The interrupt routine should toggle the audio output bit. The output bit can drive the earphone directly, but is somewhat loaded by the earphone. You can make it louder by using a 74xx driver or an op-amp.

When you do part 2 of the assignment below, you will need to index into an array which contains the notes the human has entered. Use the instructions:

ld reg, Z	;copies the contents of (Z) to reg
st Z, reg	;copies the contents of reg to (Z)

to access the array. The indirect Z register is r30 on this mcu and contains the register number you wish to read/write. For instance if the Z register contains 0x0f, then the ld instruction will copy the contents of r15 to reg. Since r16 to r31 are somewhat more general than r0 to r15, you will probably want to use r0 to r15 to store the array of notes.

Assignment

Write a program which implements a one-octave keyboard synthesizer using eight switches connected to an input port and an earphone connected to PortD, bit 6.
When you demonstrate the keyboard to a staff member, you will play a scale and a short melody of your choice. You may use the melody shown below. The keyboard needs to produce only one note at a time (single voice).
```
Notes: E  E  E  F  F   G  F  E  D  E  F   G  C  F  E   D   C   C
Play the hollow notes twice as long as the filled notes.
```
Play the melody.
(performed by L. J. Buck)
Write a second program which will:
- record the first 16 notes played (first 16 button presses) after the RESET button is pushed
- play all the notes back whenever a button is pushed
- modify the note C5, played on button 0, to be "no note played". (During playback this modified key will produce silence)
- playback at about four notes/second. To set this rate, you will have to count interrupts.
Demo this program to a staff member.
Your written lab report should include:
- The scheme you used to detect the pushbutton state. (e.g. polling loop, interrupt)
- How you generated the freqencies and an estimate of how accurate they are.
- Other design aspects of the assignment (e.g. did you use table lookup for the frequencies or did you hardcode them).
- A heavily commented listing of your code.

Jan 1999