EE 476: Laboratory 5

Oscilloscope etch-a-sketch.

You will use the digital oscilloscope as an analog x-y display to simulate an etch-a-sketch. The device will be controlled by a 16-key keypad. As you push directional buttons on the keypad the device will leave a trail of dots on the 'scope screen.

You will need to build two simple digital-to-analog converters as shown below. One for the 'scope x input, one for the y input. On the 'scope press the Display button, then use the on-screen menu to change the format to XY.

The Keypad:

You will need to get user input from a keypad with the following configuration. Demonstration keypad scanning code is in ASM and in C.

Pin 1  -- row 1 2 3 A
Pin 2  -- row 4 5 6 B
Pin 3  -- row 7 8 9 C
Pin 4  -- row * 0 # D

Pin 5  -- col 1 4 7 *
Pin 6  -- col 2 5 8 0
Pin 7  -- col 3 6 9 #
Pin 8  -- col A B C D

(a) Each switch shorts one row to one column.
(b) Each pin should be connected to one bit of an i/o port.
(c) The i/o port pins will be used both as inputs and outputs. 
    When they are inputs, they should have active pullup resistors.

Write a program and build a circuit which will:

• When RESET, the program should accept an integer between 1 and 9999 from the keypad. This integer will be used to set the maximum number of points the program will display on the 'scope. The program should then center the point displayed on the 'scope and wait for the user to press the keypad to move the point (and add points).
• Use the keypad to move the point on the 'scope screen in one of eight directions using the 1-9 keys. Pressing a key briefly should cause one point to be added to the display list. When you hold a key down, points should be added at a rate of about 5/second.
• As you press buttons, the system should leave a trail of points with a length limited only by RAM. Clever compression schemes will be rewarded.
• The D key should erase the screen and center one point on the 'scope.
• The pattern being drawn on the 'scope should be refreshed at least 30 times/second.
• As you add points with the keypad, the system should display them all in order to produce an image. You will have to play with the relationship between keypresses and the increment/decrement value applied to the DAC.

When you demonstrate the program to a staff member, you should exercise the keypad commands to draw a picture. Full credit will be given only if you can store more points than you have bytes of RAM. Since you have 1k byte of RAM, you will need to draw at least 1000 points in 33 mSec. The code which blasts points from RAM to the DAC will probably have to be written in assembly language for adequate perfromance.

Your written lab report should include:

• The scheme you used to decode commands.
• A heavily commented listing of your code.
• A schematic of your circuit.
• An image captured from the oscilloscope showing a typical picture you generated.