Tài liệu User Defined Primitives part 2 pptx

[ Team LiB ]

12.3 Sequential UDPs

Sequential UDPs differ from combinational UDPs in their definition and behavior.

Sequential UDPs have the following differences:

• The output of a sequential UDP is always declared as a reg.

• An initial statement can be used to initialize output of sequential UDPs.

• The format of a state table entry is slightly different.

• <input1> <input2> ..... <inputN> : <current_state> : <next_state>;

• There are three sections in a state table entry: inputs, current state, and next state.

The three sections are separated by a colon (:) symbol.

• The input specification of state table entries can be in terms of input levels or edge

transitions.

• The current state is the current value of the output register.

• The next state is computed based on inputs and the current state. The next state

becomes the new value of the output register.

• All possible combinations of inputs must be specified to avoid unknown output

values.

If a sequential UDP is sensitive to input levels, it is called a level-sensitive sequential

UDP. If a sequential UDP is sensitive to edge transitions on inputs, it is called an edge￾sensitive sequential UDP.

12.3.1 Level-Sensitive Sequential UDPs

Level-sensitive UDPs change state based on input levels. Latches are the most common

example of level-sensitive UDPs. A simple latch with clear is shown in Figure 12-3.

Figure 12-3. Level-Sensitive Latch with clear