Rangkaian Sekuensial •Rangkaian Sekuensial •Flip-flop
Combinational versus Sequential
Functions Logic functions are categorized as being either
combinational (sometimes referred to as
combinatorial) or sequential.
In the case of a combinational function, the logic
values on that function’s outputs are directly
related to the current combination of values on its
inputs.
In the case of a sequential function, the logic values
on that function’s outputs depend not only on its
current input values, but also on previous input
values.
Sequential Functions
That is, the output values depend on a
sequence of input values.
Because sequential functions remember
previous input values, they are also
referred to as memory elements.
RS Latches
One of the simpler sequential functions is that of an RS
latch, which can be implemented using two NOR gates
connected in a back-to-back configuration
RS Latches
The secret of the RS latch’s ability to
remember previous input values is based
on a technique known as feedback.
This refers to the feeding back of the
outputs as additional inputs into the
function.
D-Type Latches
A more sophisticated function called a D-type
(“data-type”) latch can be constructed by attaching
two ANDs and a NOT to the front of an RS latch
D-Type Flip-flops
In the case of a D-type flip-flop (which may also be referred to as a register), the data appears to be loaded when a transition, or edge, occurs on the clock input
A transition from 0 to 1 is known as a rising-edge or a positive edge, while a transition from 1 to 0 is known as a falling-edge or a negative-edge
A D-type flip-flop’s clock input may be positive-edge or negative-edge triggered
JK and T Flip-flops
T flip-flop doesn’t have any data inputs; the outputs
simply toggle to the inverse of their previous values
on each active edge of the clock input.
Flip Flop
Flip-flops are synchronous bistable devices.
The term synchronous means the output changes state only when the clock input is triggered. That is, changes in the outputoccur in synchronization with the clock.
Flip-flop is a kind of multivibrator. There are three types of multivibrators:
◦ Monostable multivibrator has only one stable state. It produces a single pulse in response to a triggering input.
◦ Bistable multivibrator exhibits two stable states. It is able to retain the two SET and RESET states indefinitely. It is commonly used as a basic building block for counters, registers and memories.
◦ Astable multivibrator has no stable state at all. It is used primarily as an oscillator to generate periodic pulse waveforms for timing purposes.
Edge-Triggered Flip-flops
An edge-triggered flip-flop changes states
either at the positive edge (rising edge) or
at the negative edge (falling edge) of the
clock pulse on the control input.
The three basic types are introduced
here: S-R, J-K and D.
Edge-triggered J-K flip-flop
The J-K flip-flop works very similar to S-R flip-
flop. The only difference is that this flip-flop has
NO invalid state.
The outputs toggle (change to the opposite state)
when both J and K inputs are HIGH. The truth
table is shown below.
Edge-triggered D flip-flop
The operations of a D flip-flop is much more
simpler. It has only one input addition to the clock.
Pulse-Triggered (Master-Slave) Flip-flops
The term pulse-triggered
means that data are
entered into the flip-flop
on the rising edge of the
clock pulse, but the
output does not reflect
the input state until the
falling edge of the clock
pulse.
Data Lock-Out Flip-flops
The data lock-out flip-flop is
similar to the pulse-triggered
(master-slave) flip-flop except it
has a dynamic clock input.
The dynamic clock disables (locks
out) the data inputs after the
rising edge of the clock pulse.
Therefore, the inputs do not
need to be held constant while
the clock pulse is HIGH.