Priority encoder

<p>A priority encoder is a <a href="/facts/Electronic_circuit/IRfQ6Cc2">circuit</a> or <a href="/facts/Algorithm/fnl5NmRt">algorithm</a> that compresses multiple <a href="/facts/Binary_code/v5YF0sQl">binary</a> inputs into a smaller number of outputs, similar to a <a href="/facts/Encoder_(digital)/XhpVpWL3">simple encoder</a>. The output of a priority encoder is the binary representation of the <a href="/facts/Zero-based_numbering/DITPGFHR">index</a> of the <a href="/facts/Bit_numbering/94h4P3qr">most significant</a> activated line. In contrast to the simple encoder, if two or more inputs to the priority encoder are active at the same time, the input having the highest priority will take precedence. It is an improvement on a simple encoder because it can handle all possible input combinations, but at the cost of extra logic.
</p><p>Applications of priority encoders include their use in <a href="/facts/Programmable_interrupt_controller/0ukSAuF1">interrupt controllers</a> (to allow some <a href="/facts/Interrupt_request/n2VvmL4K">interrupt requests</a> to have higher priority than others), decimal or <a href="/facts/Binary_encoding/v5YF0sQl">binary encoding</a>, and <a href="/facts/Analog-to-digital_converter/8YYNsMFg">analog-to-digital</a> / <a href="/facts/Digital-to-analog_converter/5loEvAeb">digital to-analog</a> conversion.
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<p>A <a href="/facts/Truth_table/4KsmArDW">truth table</a> of a single bit 4-to-2 priority encoder is shown, where the inputs are shown in decreasing order of priority left-to-right, and "x" indicates a <a href="/facts/Don%2527t_care_term/kvQuwYMK">don't care term</a> - i.e. any input value there yields the same output since it is superseded by a higher-priority input. The (usually-included) "v" output indicates if the input is valid.
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4 to 2 Priority Encoder<table><tbody><tr><td colspan="4">Input</td><td colspan="3">Output</td></tr><tr><td>I3</td><td>I2</td><td>I1</td><td>I0</td><td>O1</td><td>O0</td><td>v</td></tr><tr><td>0</td><td>0</td><td>0</td><td>0</td><td colspan="2">x</td><td>0</td></tr><tr><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>1</td></tr><tr><td>0</td><td>0</td><td>1</td><td>x</td><td>0</td><td>1</td><td>1</td></tr><tr><td>0</td><td>1</td><td colspan="2">x</td><td>1</td><td>0</td><td>1</td></tr><tr><td>1</td><td colspan="3">x</td><td>1</td><td>1</td><td>1</td></tr></tbody></table>
<p>Priority encoders can be easily connected in arrays to make larger encoders, such as one 16-to-4 encoder made from six 4-to-2 priority encoders – four 4-to-2 encoders having the signal source connected to their inputs, and the two remaining encoders take the output of the first four as input. 
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Priority encoder open-in-new

Priority encoder