TABLE OF CONTENTS Throughout the previous chapters we have studiously avoided considering the case of multiple output ports connecting one signal. The problem that arises in such a case is determining the final value of the signal when multiple sources drive it. In this chapter we discuss resolved signals, the mechanism provided by VHDL for modeling such cases.
If we consider a real digital system with two outputs driving one signal, we can fairly readily determine the resulting value based on some analog circuit theory. The signal is driven to some intermediate state, depending on the drive capacities of the conflicting drivers. This intermediate state may or may not represent a valid logic state. Usually we only connect outputs in a design if at most one is active at a time, and the rest are in some high-impedance state. In this case, the resulting value should be the driving value of the single active output. In addition, we include some form of "pull-up" that determines the value of the signal when all outputs are inactive.
While this simple approach is satisfactory for some models, there are other cases where we need to go further. One of the reasons for simulating a model of a design is to detect errors such as multiple simultaneously active connected outputs. In this case, we need to extend the simple approach to detect such errors. Another problem arises when we are modeling at a higher level of abstraction and are using more complex...
