(BQ) Part 2 book "Digital design" has contents: Registers and counters, memory and programmable logic, design at the register transfer level, laboratory experiments with standard ICs and FPGAs, standard graphic symbols. | Chapter 6 Registers and Counters REGISTERS A clocked sequential circuit consists of a group of flip‐flops and combinational gates. The flip‐flops are essential because, in their absence, the circuit reduces to a purely combinational circuit (provided that there is no feedback among the gates). A circuit with flip‐flops is considered a sequential circuit even in the absence of combinational gates. Circuits that include flip‐flops are usually classified by the function they perform rather than by the name of the sequential circuit. Two such circuits are registers and counters. A register is a group of flip‐flops, each one of which shares a common clock and is capable of storing one bit of information. An n‐bit register consists of a group of n flip‐flops capable of storing n bits of binary information. In addition to the flip‐flops, a register may have combinational gates that perform certain data‐processing tasks. In its broadest definition, a register consists of a group of flip‐flops together with gates that affect their operation. The flip‐flops hold the binary information, and the gates determine how the information is transferred into the register. A counter is essentially a register that goes through a predetermined sequence of binary states. The gates in the counter are connected in such a way as to produce the prescribed sequence of states. Although counters are a special type of register, it is common to differentiate them by giving them a different name. Various types of registers are available commercially. The simplest register is one that consists of only flip‐flops, without any gates. Figure shows such a register constructed with four D‐type flip‐flops to form a four‐bit data storage register. The common clock input triggers all flip‐flops on the positive edge of each pulse, and the binary data available at the four inputs are transferred into the register. The value of (I3, I2, I1, I0) immediately before the clock edge determines the value
