Transcript Digital Flip-Flops 2

Latches and Flip-Flops 2
©Paul Godin
Created September 2007
Flip2013
Flops 2.1
Last Edit Aug
Review: Steering Gates
◊ The flow of logic can be controlled with a logic
gate.
1
0
Signal
10
Control 1
◊ The NAND as a steering gate inverts the input.
ANIMATED
Flip Flops 2.2
Review: Steering Gates
◊ A control input ‘0’ inhibits the signal.
1
0
Signal
10
1
Control 0
ANIMATED
Flip Flops 2.3
Steering Gates in Pairs
1
0
Signal
Control
10
1
0
Signal1
10
Output Enabled (inverted)
ANIMATED
Flip Flops 2.4
Steering Gates in Pairs
1
0
Signal
Control
10
1
0
0
Signal1
1
10
Output Disabled
ANIMATED
Flip Flops 2.5
Transparent SR Latch
◊ A Transparent latch has an additional “control”
input that enable or disable the input signals.
S
S
Q
Control
Q
R
R
Flip Flops 2.6
Transparent SR Latch
◊ The Transparent SR Latch is also known as:
◊ Gated SR Latch
◊ Enabled SR Latch
◊ The Control (Enable) input of the Transparent
SR Latch
◊ Controls when the SR inputs are read
◊ Places the Latch in a hold state
Flip Flops 2.7
Transparent Latch
◊ The control (steering) gates invert the input.
The latch portion is active-low but the input to
the transparent latch is active high.
S
S
Q
Control
Q
R
R
Flip Flops 2.8
Gated SR Latch
◊
The enable input can be used to disable further SR
inputs to the device, leaving it in a hold state.
En S
S
Q
En
R
Q
R
Q
Q’ Mode
1
0
0
Q
Q’ Hold
1
0
1
0
1
Reset
1
1
0
1
0
Set
1
1
1
?
?
Invalid
0
X
X
Q
Q’ Disabled
“X” is “don’t care”
or “doesn’t matter”
Flip Flops 2.9
Exercise 1: Complete the timing diagram
S
Q
En
R
Q
En
S
R
Q
Q
Flip Flops 2.10
Exercise 2: Complete the timing diagram
S
Q
En
R
Q
En
S
R
Q
Q
Flip Flops 2.11
D Latch
◊ The problem with the SR Latch is that it
requires two inputs to store one value.
◊ A latch is needed where one input is applied to
store one value.
◊ A control input is also required to place the
device in a hold state.
Flip Flops 2.12
D Latch
◊ This device has one input to store one value
(D), and an input to hold that value (Enable).
D
S
Q
Enable
Q
R
Flip Flops 2.13
Gated D Latch
◊
“When enabled, whatever appears on D appears on Q”.
En
D
Q
Q’ Mode
Q
1
0
0
1
Reset
En Q
1
1
1
0
Set
0
X
Q
Q’ Disabled
D
This device is the foundation of digital memory.
Flip Flops 2.14
Exercise 3: Complete the timing diagram
D
Q
En Q
En
D
Q
Q
Flip Flops 2.15
END
©Paul R. Godin
prgodin°@ gmail.com
Flip Flops 2.16