EE 476: Laboratory 4

TV Oscilloscope

You will produce a digital oscilloscope which takes one voltage channel input and displays the resulting voltage trace on the TV.

You will probably want to review the code examples given in Video Generation with AVR.
Use this example as a basis for your program. Before compiling, under the Project menu, choose Configure... , then in the Compiler tab. Set "Optimize for Speed", reduce the Data Stack size to 100 bytes, and turn off the "char is unsigned" option.

Build the video DAC shown below and connect it to the yellow connector on the TV using clip leads and a RCA phone jack. Make sure the TV is set to video input. Test the TV connection using the example program linked above. The power supplies on the TVs and development boards are NOT interchangable.

The scope will be AC coupled. Use the following circuit to isolate the signal source from the DC level requird by the ADC. I suggest setting Vref to Vcc for easy connection to the STK500.

A comment on this assignment from a group in Spring 2004.

Oscilloscope Lab
Timing Very Critical
Complete and Correct

Write a program in C and assembler for the microcontroller with these specifications:

• Produces a video signal. At reset, the program should:
  • set the vertical voltage range to 5 volts full scale
  • set the time scale to about 8 mSec across the screen and display the time scale value.
  • set the trigger mode to free run and display the trigger mode.
  • set the run/stop mode to run.
• Samples an ADC channel at video rate (15.75 kHz) and displays a voltage trace as 128 dots across the screen. Since the input of the ADC is biased to 2.5 volts, it would make sense to center 2.5 volts vertically on the screen.
• All parameters should be set by using the 8 buttons on the STK500
• Time base should be settable to approximately 8, 16, 33, 65 mSec across the screen (128 samples taken every 63.625 microSec is 8.14 mSec).
• Trigger mode should be settable to free run or to trigger when the wave form passes 0.5 volt above the 2.5 volt bias (3 volts at input to A/D converter) with positive slope.
• You should be able to run/stop acqusition. When stopped, the last voltage trace should be displayed, along with the RMS voltage.
• There should be no video artifacts (rolling, tearing, flickering) during operation, except that video can stop during the computation of the RMS voltage.

When you demonstrate the program to a staff member, you should exercise all the scope functions and show that the voltage and time scales are accurately calibrated. One way to do this is to simultaneously display the output of the signal generator on your scope and on the Tek scope.

Your written lab report should include:

• The scheme you used to coordinate video and the ADC.
• The scheme you used to decode commands.
• An image from the TV.
• A schematic of the curcuit you built.
• A heavily commented listing of your code.