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<title>Counter</title>
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<h1><img  align="center" src="../../../../en/icons/counter.gif" width="32" height="32">
<em>Counter</em></h1>

<p><table>
<tr><td><strong>Library:</strong></td>
	<td><a href="index.html">Memory</a></td></tr>
<tr><td><strong>Introduced:</strong></td>
	<td>2.3.0</td></tr>
<tr><td valign="top"><strong>Appearance:</strong></td>
	<td valign="top"><img src="../../../../en/img-libs/counter.png" width="34" height="44"></td></tr>
</table></p>

<h2>Behavior</h2>

<p>The counter holds a single value, whose value is emitted on the output
<var>Q</var>. Each time the clock input (diagrammed with a triangle on the
component's south edge) triggers according to its Trigger attribute,
the value in the counter may update based on
the two inputs on the component's west edge: The upper input is called
<var>load</var> and the lower is called <var>count</var>, and they are
interpreted as follows.</p>

<center><table><thead>
<tr><td><var>load</var></td><td><var>count</var></td><td>trigger action</td></tr>
</thead><tbody>
<tr><td align="center">0 or z</td><td align="center">0</td>
    <td>The counter remains unchanged.</td></tr>
<tr><td align="center">0 or z</td><td align="center">1 or z</td>
    <td>The counter increments.</td></tr>
<tr><td align="center">1</td><td align="center">0</td>
    <td>The counter loads the value found at the
    <var>D</var> input.</td></tr>
<tr><td align="center">1</td><td align="center">1 or z</td>
    <td>The counter decrements.</td></tr>
</tbody></table></center>

<p>The range of counting can be configured using the Maximum
Value attribute. When the counter reaches this value, the next
increment wraps the counter back to 0; and if it is at 0,
then a decrement will wrap the counter around back to its maximum value.</p>

<p>In addition to the output <var>Q</var>, the component also includes a
single-bit output <var>carry</var>. This is 1 whenever the counter is at its
maximum and the <var>load</var> and <var>count</var> inputs indicate that the
component should increment on the next step - or when the counter is at 0
and the <var>load</var> and <var>count</var> inputs indicate to decrement at
the next step.</p> 

<p>The <var>clear</var> input resets the counter's value to 0 (all
zeroes) asynchronously; that is, as long as the <var>clr</var> input
is 1, the value is pinned to 0, regardless of the clock input.</p>

<h2>Pins</h2>

<dl>

<dt>East edge, labeled <var>Q</var> (output, bit width matches Data Bits attribute)</dt>
<dd>Outputs the value currently stored by the counter.</dd>

<dt>East edge, lower pin (output, bit width 1)</dt>
<dd>Carry: When <var>load</var> and <var>count</var> indicate to increment,
this output is 1 whenever the counter is at its maximum. When <var>load</var>
and <var>count</var> indicate to decrement, this output is 1 whenever the
counter is at 0. At all other times, this output is 0.</dd>

<dt>West edge, top pin (input, bit width 1)</dt>
<dd>Load: When this is 1 while the <var>count</var> input is 0, the counter
will load the value found at the <var>data</var> input at the next clock
trigger - or, if the <var>count</var> input happens to be 1, the counter's
value will decrement.</dd>

<dt>West edge, middle pin labeled <var>D</var>
  (input, bit with matches Data Bits attribute)</dt>
<dd>Data: When the clock triggers while <var>load</var> is 1
and <var>count</var> is 0, the counter's value changes to the value found at
this input.</dd>

<dt>West edge, lower pin labeled <var>ct</var> (input, bit width 1)</dt>
<dd>Count: When this is 1 or unconnected, the value in the counter increments
whenever the clock input is triggered - or it decrements if the <var>load</var>
input happens to also be 1.</dd> 

<dt>South edge, indicated with a triangle (input, bit width 1)
<dd>Clock: At the instant that this is triggered as specified by the Trigger
attribute, the counter updates as indicated by the <var>load</var> and
<var>count</var> inputs.</dd>

<dt>South edge, labeled 0 (input, bit width 1)
<dd>Clear: When 0 or undefined, this input has no effect.
As long as it is 1, the counter's value is asynchronously pinned to 0. This
occurs asynchronously - that is, without regard to the current clock input
value. As long as this is 1, the other inputs have no effect.</dd>

</dl>


<h2>Attributes</h2>

<p>When the component is selected or being added,
Alt-0 through Alt-9 alter its <q>Data Bits</q> attribute.</p>

<dl>

<dt>Data Bits</dt>
<dd>The bit width of the value emitted by the component.</dd>

<dt>Maximum Value</dt>
<dd>The maximum value, at which point the counter will set its carry output.</dd>

<dt>Action On Overflow</dt>
<dd>The behavior when the counter attempts to increment beyond the maximum value
or decrement beyond 0. Four possible actions are supported:
<dl>
<dt>Wrap around</dt>
<dd>The next value is 0 (if incrementing - the maximum value if decrementing)</dd>
<dt>Stay at value</dt>
<dd>The counter's value remains at the maximum (or 0 if decrementing)</dd>
<dt>Continue counting</dt>
<dd>The counter continues incrementing/decrementing, keeping the number of bits
as provided by the Data Bits attribute</dd>
<dt>Load next value</dt>
<dd>The next value is loaded from the D input.</dd>
</dl></dd>

<dt>Trigger</dt>
<dd>Configures how the clock input is interpreted. The value <q>rising edge</q>
indicates that the counter should update its value at the instant when the
clock rises from 0 to 1. The <q>falling edge</q> value indicates that it should
update at the instant the clock falls from 1 to 0.</dd>

<dt>Label</dt>
<dd>The text within the label associated with the component.</dd>

<dt>Label Font</dt>
<dd>The font with which to render the label.</dd>

</dl>

<h2>Poke Tool Behavior</h2>

<p>Clicking the counter brings keyboard focus to the component
(indicated by a red rectangle), and typing hexadecimal digits will
change the value stored in the counter.</p>

<h2>Text Tool Behavior</h2>

<p>Allows the label associated with the component to be edited.</p>

<p><a href="../index.html">Back to <em>Library Reference</em></a></p>

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