Lab #3: Asynchronous Circuits

ECE352F: Computer Organization

Fall 1999

Two industrious and energy conscious graduate students have hooked up the system shown in the figure below. Instead of light switches, the room has two photocells, one at each door. If either one or both students are in the office, the light is to be on. The students can enter or exit only as shown, with entrances and exits never occurring simultaneously. The photcells indicate a 1 when their beam is interrupted by a student entering or exiting and a 0 at all other times.

 

Assumptions:

• No one is thin enough that the signal generated by the photocell changes faster than the time that it takes for the control circuit to reach a stable state, i.e., your circuit can distinctly recognize the off to on and on to off transitions of the photocells.
• Once the photocell is triggered, the person is not allowed to back out.

Preparation

You are to do an asynchronous circuit design using the procedure described in class.

1.

Find a primitive and a minimum-row reduced flow table that describes the light-control operation.

2.

Show a valid assignment and find the excitation and output equations taking care of all possible races or glitches.

3.

Prepare a VHDL program that will implement your equations on the Xilinx board. You should only have some combinational logic equations in your program. There should be no use of statements like if or case.

Remember that you can use xsload to provide inputs to your program. You can use the display to simulate the light.

In the Lab

Implement and test your circuit.