TTL stands for Transistor Transistor Logic. According to the TTL standards,
voltage of LOGIC HIGH state ranges from 2V to 5V and the voltage range of
LOGIC LOW state is between 0V and 0.8V,
CMOS stands for Complementary Metal Oxide Semiconductor. In CMOS logic,
voltage of HIGH state ranges from 3.5V to 5V while the voltage range of LOW
state is 0V to 1.5V.
A flip flop is a single bit storage element which can store one bit of
information when given power supply.As long as power is fed in, the storage
will n. Else, the data is lost.
Flip flops are different from latch.The output of the latch is affected
consistently by the inputs when the enable is asserted. But, when we take
flip flop into consideration, the output will be changed during rising or
falling edge of the clock pulse.
Setup time is defined as the minimum amount of time the data signal should be
held steady before the clock event so that the data are reliably sampled by
the clock.
Hold time is the minimum amount of time the data signal should be held steady
after the clock event so that the data are reliably sampled.
It may be visualized as a circulating shift register. The output of the MSB
(last flip flop in the circuit) will be fed back to the input of the first
flip flop, i.e. LSB.A 4 bit ring counter would be sufficient to understand
the concept. The circuit is designed in such a way that only one | is kept
circulated and it is done in synchronization with the clock pulses.
In a Johnson counter, the output from the last flip flop is inverted and fed
into the first one, and then shifting is carried out, whereas this inversion
is not carried out in case of normal ring counter.
If there is an asynchronous loop in the design the circuit becomes
oscillatory or it may reach a stable state where it might get hung and it
could not get out.