4 Sequential circuits.ppt

Flip-flops
Chapter4
Sequential circuits
Combinational logic
X0
Xn
Y0
Ym
binational circuit model
No memory units
No feedback from logic outputs back to the inputs
Combinational logic
System Input Variables
System output Variables
I0
In
O0
Om
S0
Sx
M0
Ex
State Variables
Excitation Variables
MX
E0
Universal sequential circuit model
Sequential implies that events are ordered in time, that one event then another occurs, separated by time.
Two types of sequential logic exist:
Synchronous sequential circuit
Synchronous sequential circuit is clocked
Asynchronous sequential circuit
Asynchronous sequential circuit is not clocked
Combinational logic
System Input Variables
System output Variables
I0
In
O0
Om
S0
Sx
M0
Ex
State Variables
Excitation Variables
MX
E0
Set I{ I0……In } are external input variables presented to the system.
Set O{ O0……Om } are external output variables.
Combinational logic
System Input Variables
System output Variables
I0
In
O0
Om
S0
Sx
M0
Ex
State Variables
Excitation Variables
MX
E0
Internally, the system generate the excitation variables,E0 to Ex, and state variables, S0 to Sx.
internal output variables
internal input variables
The memory excitation input relationship can be written as
E = f( I, S ). Excitation E force the memory elements to the desired state variables.
Output O is generated by binational logic transform, (g), operating on input variables I and state variables S.
O = g( I, S )
Combinational transform (f)
Memory
M
Combinational logic (g)
Input (I)
(E)
CLK
State variables
(S)
Excitation variables
Output (O)
E = f( I, S )
O = g( I, S )
The clock input is not a binary value representing the time of day, but rather a ”synchronous” train of pulses. Synchronous memory changes its data only at certain time intervals.
Combinational transform (f)
Memory
M
Combinational logic (g)
Input (I)
(E)
CLK
State variables
(S)
Excitation variables
Output (O)
State varia