Dedicated Digital Processors: Methods in Hardware/Software Co-Design

5

Sequential Control Circuits

After discussing the data paths of several circuits designed to be used sequentially during an algorithm we now discuss the auxiliary circuits needed to orderly run through the sequential steps, to supply the necessary function codes, and to implement the control flow and synchronization, starting from a schedule for the operations to be executed on the circuit. The circuits in sections 4.5–7 and 4.9 require the generation of trains of clock pulses to go through the steps of the computation after some starting event, and of control signals to redirect the data during some clock cycles. The times at which the control signals change can be defined by the L-H transitions of some clock signal, e.g. a free-running periodic waveform. Elementary circuits that change their outputs in response to a clock event are the register or, more generally, the feedback circuit built around a register as already shown in Figure 2.20 and called an automaton.

Circuits executing a moderate number of steps or repeating their operation periodically can be controlled by simple automata that use an application-specific transition function implemented by a minimized algorithm. For more complex circuits that need long, irregular sequences of control and operand select codes, methods to implement complex automata are required. Complex automata can be composed from simpler ones. A general method to generate arbitrary sequences is by reading them from a memory table. This approaches ...