数字电路教学课件：chapter4-1.ppt

文档介绍：Flip-flops
Chapter4
Flip-flops
Flip-flops are digital devices that have the ability to store binary information after the excitation input has changed.
They are considered to be the basic memory cell for the majority of electronic binary data storage applications.
This section explores the flip-flop from a functional perspective.
R
S
Q
Q’
Two output Q and Q’. And Q’ is the complement of Q.
Two stable state, either a 0 or a 1. “Q=1” is the state-1 and “Q=0” is the state-0. The state would be remained without the changing of input variables .
By the excitation of certain input variables, flip-flop changes from one stable state to another stable state. The state prior to the excitation of input is present-state, represented by symbol “Qn(Q)”, the state after the excitation of input is next-state, represented by symbol “Qn+1”.
Four types of flip-flops are commonly considered: JK, SR, D and T.
Each store binary data but have a unique set of input variables.
Each flip-flop can be described in terms of input and output characteristics by writing special truth table ,called an excitation table.
A characteristics equation can be generated from the excitation tables.
Flip-flops vs Latches
The names flip-flops and latches are sometimes used interchangeably;
Flip-flops are clocked and latch are not.
The term “flip-flop” is more appropriately associated with devices change state only on a clock edge or pulse, whereas latches change state without being clocked.
R-S NAND Latch
Analysis
R=1, S=1 Qn+1=(SQ’)’=Q;
Qn+1’=(RQ)’=Q’;
R=1, S=0 Qn+1=1;
Qn+1’=0;
R=0, S=1 Qn+1=0;
Qn+1’=1;
R=0, S=0 Qn+1=1;
Qn+1’=1;
R
S
Q
Q’
R
S
Q’
Q
&
a
&
b
R=0, S=1
1
1
0
0
1
0
1
0
Output next state
Present state
&
a
&
b
R=0, S=1
0
1
1
1
1
0
1
0
Output next state
Present state
R=1, S=0
1
0
1
0
1
0
1
1
Output next