Design & Construction Rules
- Understand the requirements before you start your design.
Ask questions if it does not make sense to you. Try to determine
example input and output.
- Design the circuit, and make an accurate schematic. Errors
in schematics can cause a lot of trouble later. Omissions are
also errors. Enter pin numbers to simplify the wiring process.
- Plan the layout of the ICs on the test board. Make a layout
drawing. Identify the ICs as U1, U2, etc.
- Build the circuit exactly per the schematic.
- Put a piece of tape on each IC and write the Ui identifier
on it.
- Use a marker to line out on a copy of the schematic the
wire, the IC, or the discrete component you just installed. This
minimizes wiring errors and can be a big assist to help you keep
track of where you are in the construction process.
- When you can, test the circuit after adding one part but
before adding the next. Testing as you proceed helps immensely
in the debug process and the design-check process.
- If you detect a design error, take corrective action immediately.
- Design the change that corrects the error.
- Correct the schematic
- Rewire the circuit
- Test the completed project. There is much more to testing
than just testing as you go. Create a test plan that exercises
the design and demonstrates that it works. Make sure that your
tests include unusual input bit patterns as well as incorrect
inputs.
- Document your design.
Debugging Tips
Sometimes a circuit which is wired correctly will fail to work
because of faulty equipment. More often, the problem is a wiring
error. After the first few labs, the following things should
be checked before summoning the lab instructor. Until you are
comfortable with the steps listed, do not hesitate to ask for
assistance. If any component feels hot to the touch, turn off
the power immediately.
- Is the power turned on? Is each IC connected to +5 Volts and
to ground? Are the two halves of the outer superstrip "rails"
connected together if both halves are being used to supply power?
- Are the LEDs and switches connected to the circuit working?
Are the switches providing the voltage you expect for their physical
position?
- Are the vertical, horizontal, and triggering controls of the
CRO set to provide a picture of what you are trying to observe?
- Are unused IC inputs connected to logic 0 or 1?
- For which input combinations is the circuit misbehaving? Can
the above information be used to isolate a section of the circuit?
Is the wiring of that section correct? Is each IC providing the
expected output for its inputs?
Answering the above questions will isolate almost all bugs.
© Copyright 1996
Beginning of Design Rules
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