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

2

Hardware Elements

2.1 TRANSISTORS, GATES AND FLIP-FLOPS

2.1.1 Implementing Gates with Switches

Elementary building blocks implementing the Boolean operations AND, OR, NOT or SEL from which all Boolean functions can in turn be constructed (see section 1.3.1) are realized quite easily by electronic means. A simple solution that was actually applied in the early days of computing is the use of electrically controlled, mechanical switches. The single basic component is the controlled switch with a control input G, a coil connected between G and a ground reference C, and the poles S, B, M of the mechanical switch (Figure 2.1). If a sufficiently high voltage is applied to G w. r. t. C, the magnetic force of the coil breaks the connection from S to B and makes the one from S to M, and hence performs a selection between the voltage levels applied to B and M, depending on the control input. An interval of voltage levels that cause the switch to be actuated is used to present the Boolean 1 while the voltages near zero represent the 0, all voltages being referenced to C. This SEL building block fulfills the requirement that the input and output signals be compatible. It can thus be composed with others. The high voltage level can be selected from a power supply (denoted ‘+’ in Figure 2.1). To output a zero voltage to another coil, the corresponding select switch input can be left open, as the unconnected coil will assume the zero level by itself. Thus, the switch can be simplified to a ...