A sequential circuit has stored state. Its future output can therefore depend on earlier inputs as well as the present inputs.
A directed graph with one vertex for each state and labelled edges for the permitted transitions. A finite counter has a directed cycle when every state has one successor.
A counter stores a binary representation of a state, usually advancing by one increment or decrement modulo when it has bits.
A cascade in which an output transition of one flip-flop clocks the next. With positive-edge-triggered toggle stages and uncomplemented connections, it decrements modulo : a zero bit rises to one and propagates a borrow, while a one bit falls to zero and stops propagation. Ripple propagation takes time, so intermediate transient states are not the settled counter state.
A flip-flop stores one bit and updates it at a specified clock edge; an asynchronous clear can override that clocked update. Signal polarity is part of the specification, not something inferred from the name alone.
The data input must remain stable for a specified interval before the sampling clock edge and a specified interval after it. Violations can prevent a well-defined sampled bit.
A control that resets stored state independently of the clock edge. An active-low clear is asserted by a low signal; clocked sampling resumes after the clear is released.
At the active clock edge, . An asynchronous clear, when provided and asserted, takes precedence over this data-sampling rule.
At the active clock edge, the next state is : inputs hold, reset, set, and toggle. In particular, gives , whereas gives . The standard function table is illustrated by a positive-edge device in the [ref]. Its asynchronous reset is active high; another circuit may specify an active-low clear.
Articles by others on the same topic
Sequential logic is a type of digital logic circuit whose output depends not only on the current inputs but also on the history of past inputs. This means that the output state of a sequential logic circuit can change based on a sequence of inputs and the current state of the system. Unlike combinational logic, where the outputs are determined solely by the present inputs, sequential logic incorporates storage elements (memory), allowing it to maintain a state over time.