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Digital circuits design
System Switch
))) a
b
F
Digital Fire alarm system
Combinational circuit:A digital circuit whose outputs
depends solely on the present combination of the circuit inputs’ values.
Turn on alarm (F=1) when smoke sensed (a=1) and system switch is on (b = 1)
F = a AND bBuild using logic gates AND
a
bF
DetectorDetector
Project Design StepsStep 1: Define the problem
– Describe the problem to be solved by the circuit (i.e. what it is supposed to do). What are the inputs and outputs.
Step 2: Logic design• Capture the function
– Create a truth table or equations, whichever is most natural for the given problem, to describe the desired behavior of the combinational logic.
• Convert to equations– If you capture the function with a truth table you will need to do this step.
• Simplification – Boolean Algebra, Karnaugh Map, ..etc
If any errors appear at any stage, revise earlier phases.
Project Design Steps
Step 3: Implementation– Create a circuit corresponding to the output's equation. Use logic gates –
AND, OR, NAND, NOR, XOR …etc– Draw a circuit diagram (or diagrams) of the circuit showing all connections and
gates to be used. – You will use Altera Quartus II to draw your circuit.
Step 4: Circuit Simulation– Demonstrate that the circuit performs as expected. – Choices made regarding what to simulate.– You will be using Altera Quartus II for the simulation.
If any errors appear at any stage, revise earlier phases.
Project Design Steps
Step 5: Prototype Test Results– Download your circuit to the CPLD board – Wire the input and output to demo your circuit.
Step 6: Demonstration– Demo your circuit to other groups in the class– Produce a poster showing your project steps .
• Poster should contain a brief description of your testing procedure, outlining any significant problems encountered along the way.
– Peer-to-Peer evaluation, 3% of project mark.
If any errors appear at any stage, revise earlier phases.
Propagation delay
• The time required for any signal transition to travel between pins and/or nodes in a device.
• The total propagation delay time through a logic circuit is a major consideration.
• Propagation delays are additive, so the more gates or inverters between input and output, the greater the propagation delay time.
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