Design of asynchronous sequential circuit

Thread Starter


Joined Jul 20, 2008

I'm given an output sequence of an asynchronous circuit(attached). I have to design the circuit and provide exitation functions, transition table and total state table. Hazards must be avoided.


To find excitation functions I need to make a flow table, then transition table. But thereis noinformation what happens when inputs are 01? Such values of inputs does not exist in the given sequence. I'm stuck.

1) I assumed that x and z0 are inputs, and z1 is an output. It is not specified in the task.
State table:

state x z0 z1
a 00 0
b 11 0
c 10 1

2) For total state table I assigned a=00, b=11, c=10 and an extra state d=01. I put don't care conditions for d:

xz0 (inputs)
###00# 01# 11 #10
00 00,0 _,_ 11,0 10,0
01 00,0 _,_ 11,0 10,0
11 00,0 _,_ 11,0 10,0
10 _,_ _,_ _,_ _,_
next state, z1(output)

I feel this is incorrect, but can't guess what to do either.

Thanks a lot!



Joined Nov 25, 2009
It's really weird that the question doesn't specify the inputs. My gut tells me X is your input and Z0 and Z1 your outputs.
See, if X and Z0 are your inputs, you don't have much of a sequential circuit, as your output can be defined simply as X*Z0' (where ' is for complementation).
It makes more sense to be given a pulse wich drives two flip flops through 4 states. Note also that you must specify the type of flip flop you use before trying to build the excitation table. This is because each ff type requires different excitation to be driven to a certain state.
So, how do you want to do this?

Thread Starter


Joined Jul 20, 2008
Thank you very much, Georacer.
I think I have to use SR latch. At least in lecture notes only those are discussed. I'll try with x input and z0z1 output again and come back.