BCD to seven-segment display converter

This section presents a practical example of some of the language constructs introduced in section 4.2. The same approach is followed to the design 

As with the example in section 4.2, all signals are of type BIT (two-value logic). 

This circuit contains two identical components. One drives the HI-DRIVER bus and the other the LO-DRIVER bus. The former decodes the upper 4 bits or nibble of the input and the latter the lower nibble. This suggests that a structural style would be an appropriate approach to adopt for a top-level architecture. This modular approach has the benefits of easier design and debugging, and also a simple expansion capability. Within each component, a straightforward transform is required from a 4-bit BCD to a seven-segment display bit pattern. Of the possible language constructs studied so far, it would seem that a Case statement is most appropriate for this task and therefore a behavioural style architecture is required. 

In the entity CONVERTER, notice that the output buses HIJDRIVER and LO DRIVER have been declared as type BIT VECTOR with ascending ranges. This method is used because these buses carry a bit pattern and not a binary value. In such a situation it is not as important to stick with the convention that bit 0 is the least significant. In fact, in this case it is ad van- 

The component is compiled into the working directory in this case and therefore no explicit references to libraries are required in the CONVERTER design xmit. Component instantiation has been made indirectly and so a component declaration is required. This has been placed in the declarative part of the architecture but could have appeared in a referenced package. 

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Figuie 4<34 Top-level structural description of the BCD to seven-segment display converter. 

The BCD to seven-segment display converter designed in the previous chapter was optimized in two ways. One involved the global optimization of the circuit and the flattening of its component hierarchy. The other preserved the hierarchy and optimized individual components. There was shown to be littie to choose between either approach. 

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