Microsoft Word - Digital Logic Design v_4_6a

5.3. State Assignment Encoding, Shift Register Counters, and Adding an Enable Input

 Full-encoding compared to one-hot encoding

 Full encoding uses all possible combinations of flip flop outputs to represent states, so the

equation 2 #flip-flop ≥ #states is used to determine the number of flip-flops required. Full encoding:

 leads to minimum number of Flip Flops.

 best used with Simple Programmable Logic Devices (SPLDs) and Complex Programmable

Logic Devices (CPLDs).

 One-hot encoding, on the other hand, allows only one flip-flop outputs to be active (or “hot”) at

any one time. So the equation #flip-flop = #states is used to determine the number of flip-flops

required. One-hot encoding:

 leads to larger number of flip-flops.

 best used with Field Programmable Gate Arrays (FPGAs). FPGAs, which are sometime

referred to as “a sea of flip-flops”, has made the use of one-hot encoding a viable approach

due its overabundance of flip-flops.

 Power-on Reset Circuit
With either type of encoding there may be illegal and/or unreachable states. Additionally, when your
system is turned on initially or regains power after an interruption, it is important for it to recover in a
predefined state.

A power-on reset circuit ensures that a reset is generated immediately after a power up condition.

This could be used to preset or reset flip flops into the desired state.

Using RC circuits, we can design circuits that generate active high or low signals, depending on our

needs, as shown below: