数字逻辑设计及应用：chap7 Sequential Logic Design Principles.ppt

Latches and Flip-flops
Analysis of Synchronous Sequential Circuits
Design of Synchronous Sequential Circuits
Chapter 7 Sequential logic design principles
Sequential Circuit
Sequential Circuit
– Output depends on current and previous inputs
– Hold/store previous information
storing bits or known information as states
Storing One Bit Example Requiring Bit Storage
Press call: light turns on
Stays on after button released
Q
Call
Cancel
Doesn’t work. Q=1 when Call=1, but doesn’t stay 1 when Call returns to 0
Need some form of “feedback” in the circuit
a
Bit
Storage
Blue light
Call
button
Cancel
button
1. Call button pressed – light turns on
Bit
Storage
Blue light
Call
button
Cancel
button
2. Call button released – light stays on
Bit
Storage
Blue light
Call
button
Cancel
button
3. Cancel button pressed – light turns off
1
1
0
Flight attendant call button
Can combinational logic circuit implement this?
Press cancel: light turns off
Stays off after button released
Latches（锁存器）
Q
QN
1
1
0
0
Q
QN
Two Stable State:
Q = 1 or Q = 0
QN=0 or QN=1
0
0
1
1
When Power is first Applied to the circuit, it Randomly Comes up in One State or the Other and Stays there Forever.
Bistable Elements (双稳态元件)
Vin1
Vout1
Vin2
Vout2
Vout2
Vin2
= Vin2
= Vout2
Stable 稳定的
Metastable 亚稳稳定的
Q
QN
Vin1 Vout1
Vin2 Vout2
V
V
V
V
V
V
V
V
V
V
V
V
Metastability（亚稳态）
Metastability is inherent in any bistable circuit
Metastability can’t stay forever
V
QN
Another look at metastability
Even though Metastability can’t stay forever, but it would lead to an unknown stable state
Apply a definite Pulse Width from a Stable state to the Other
(从一个“稳态”转换到另一个“稳态”需加一定宽度的脉冲（足够的驱动