Editing Counter (digital) (section)

===Signals===
[[File:Digital counter signals.jpg|thumb|Common signals on digital counters]]
Every counter has a fundamental group of signals common to state machines:
* Clock (input) - triggers state change upon rising or falling edge (known as the ''active edge''<ref name="Keslin">{{cite book |last1=Keslin |first1=Hubert |title=Top-down Digital VLSI Design: From Architectures to Gate-Level Circuits and FPGAs |date=2014 |publisher=Morgan Kaufmann |isbn=0128007729}}</ref>). 
* Reset (input) – sets count to zero. Some IC manufacturers name this signal "clear" or "master reset (MR)". Depending on the counter design, this signal may be asynchronous or synchronous.
* Count (output) - bit vector representing the accumulated count. Depending on the counter design, this may be the current state (flip-flop outputs) or an encoding of the current state.

In addition to Clock and Reset, many counters provide other input signals such as:
* Enable – allows or inhibits counting. Sometimes this is labeled ''CE'' (count enable).
* Direction – determines whether count will increment or decrement.
* Data – parallel input data which represents a particular count value.
* Load – copies parallel input data to the counter. This typically takes precedence over Enable if both Load and Enable are asserted.<ref name="AMD"/>

Counter inputs are in many cases synchronous, meaning that they only affect counter operation upon active clock edges. For any particular counter, each synchronous input signal must satisfy the setup and hold times required for proper operation (i.e., it must be stable before and after active every clock edge for specified minimum times).<ref name="Keslin"/>

Some counters provide a Terminal Count output which indicates that the next clock will cause overflow or underflow. This is used in various ways, including:

* to implement [[Counter (digital)#Cascading|counter cascading]] (combining two or more counters to create a single, larger counter) by connecting the Terminal Count output of one counter to the Enable input of the next counter.
* to dynamically change the counter modulus, by connecting Terminal Count to the counter’s Load input and applying an appropriate value to the Data inputs.