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# =Electronics Hardware Questions=

Q: Flip flops

A:

```
a)The simplest form of FF is Set-Reset FF.
See the drawing and gate level schematic below.

_______
S	|	|   Q
-----|	|----
|	|
|	|   _
R	|	|   Q
-----|	|----
|_______|

_
S	R	Q	Q	Comment
----------------------------------------
0	0	Q	!Q	Hold condition
1	0	1	0
0	1	0	1
1	1	x	x	Not used

b)Gated RS FF

It has two more NAND gates on R and S lines.

_______
S	|	|   Q
-----|	|----
enable |	|
-----|	|   _
R	|	|   Q
-----|	|----
|_______|

A signal gate_enable=1 enables the work of RS FF,
and gate_enable=0 disables it.

c) D-latch

Gated RS FF can be  transformed into D latch by addition of an inverter.

_______
D		|	|   Q
___________|S	|----
enabl__|________|	|
__|__      |	|   _
\   / 	|	|   Q
\ /    ___|R	|----
v    |	|_______|
o    |
|____|

When gate_enable=1 D latch is transparent (Q=D),
and when gate_enable =0  the data at the output remains latched.

d) Master-Slave FF

_______		 _______
|	|		|	|
____|D    Q1|_______________|S    Q2|---
|	|		|	|
|	|		|	|
|	|_______________|R	|
|   c	|		|   c	|
|_______|		|_______|
clock	   |			     |
___________|_______________o_________|

It contains 2 FF(D-latch and RS) and inverter and works this way:
When the clock = HIGH, Q1=D ; FF2 does not change it's state since it
waits for a negative clock polarity.
On the falling edge of clock master FF1 transfers it's state to the
slave FF2.

e)Edge-triggered FF

_______
D	|	|   Q
-----|	|----
clock |	|
----->	|   _
|	|   Q
|	|----
|_______|

_______
D	|	|   Q
-----|	|----
clock |	|
----o>	|   _
|	|   Q
|	|----
|_______|

Unlikely to the D-latch these FF are edge triggered. First works
on the positive edge and second on negative edge. Input data must
present T_setup time before clock edge and remain T_hold time after.

f) JK FF

_______
J	|	|   Q
-----|	|----
clock |	|
----->	|   _
K	|	|   Q
-----|	|----
|_______|

J	K	Q	Comment
----------------------------------
0	0	Q-1	Hold previous state
1	0	1
0	1	0
1	1	!(Q-1)

when J=K=1 the output will change its state on every clock pulse

Every transaction takes place when the clock is active (level or edge).
Usually JK FF are edge triggered.JK FF can be easily transformed into
D FF by adding an inverter between J and K.

_______
D		|	|   Q
___________|J	|----
|        |       |
clk  --|-------->	|
__|__      |	|   _
\   / 	|	|   Q
\ /    ___|K	|----
v    |	|_______|
o    |
|____|

```