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Combinatorial logic
AND-OR logic  
The logic-functions NOT, AND, OR can be used to form your digital design

Actually are all you need, NAND gates

Output = D0 ● /S0 + D1 ● S0
           = /( /(D0●/S0)●/( D1●S0))

Standard functions

 The Multiplexer

Instead of creating all logic from scratch, with AND-OR logic, will it be useful to work with Standard functions like Decoders and Multiplexers

Function table - M2_1
S0 Output
0 D0
1 D1
Sequential Logic
Asynchronous feedback loops


A combinatorial circuit may (normally) not contain Asynchrony feedback loops.

However is sequential logic based on this type of feedback loop

Note: Don't use this in "real-life" projects.
Even it works correct in the exercises

  Gate  Data
Q 00 01 11 10
0 0 0 1 0
1 1 1 1 0
  Q* = Next value of Q

Red values 0/1 are stable



The D-Latch seldom used in State-Machines, more often will it be found in computersystems used to de-multiplex a multiplxed Address / Data-bus  

Gate Q
0 Last Q
1 Data

A D-Latch level trigged (0 or 1)
In this case, active high (1-trigged)

D-Flip/Flop principle  
If an active low and an active high D-Latch connected serial (like shown) will an edge-trigged circuit be formed.

The first (Leftmost) D-Latch will be open when Gate=0 and the rightmost D-Latch will be closed.

Whenever the Gate change value from 0 >1 will the Data value be transferred to the output Q.

If Gate = 1 will the value be kept stable on output Q.

Question:  What happens when Gate changes from 1 > 0 ?

D-F/F's are trigged at level-shifts opposite the D-Latch which opens at specific level.

D-F/F's which trigged by 0>1 shift called positive-edge trigged.

D-F/F's which trigged by 1>0 shift called negative-edge trigged.

Clock Q
0 Last Q
0 >1 shift Data
1 Last Q
1 >0 shift Last Q
Synchronous State Machines  

State-Machines contains a memory which remembers the Current state.

The Next State Logic generates a new value for the State memory - Next state, from the input values and the Current state.

Each time the Clk make a 0>1 change will the Next state become Current.

Standard Sequential Logic   Counters and Shift registers
CPU   "Just" a State Machine designed to execute a program.