Priority Encoder
Priority Encoders take all of their data inputs one at a time and converts them into an equivalent binary code at its output
Unlike a multiplexer that selects one individual data input line and then sends that data to a single output line or switch, Digital Encoder more commonly called a Binary Encodertakes ALL its data inputs one at a time and then converts them into a single encoded output. So we can say that a binary encoder, is a multi-input combinational logic circuit that converts the logic level “1” data at its inputs into an equivalent binary code at its output.
Generally, digital encoders produce outputs of 2-bit, 3-bit or 4-bit codes depending upon the number of data input lines. An “n-bit” binary encoder has 2n input lines and n-bitoutput lines with common types that include 4-to-2, 8-to-3 and 16-to-4 line configurations.
The output lines of a digital encoder generate the binary equivalent of the input line whose value is equal to “1” and are available to encode either a decimal or hexadecimal input pattern to typically a binary or “B.C.D” (binary coded decimal) output code.
One of the main disadvantages of standard digital encoders is that they can generate the wrong output code when there is more than one input present at logic level “1”. For example, if we make inputs D1 and D2 HIGH at logic “1” both at the same time, the resulting output is neither at “01” or at “10” but will be at “11” which is an output binary number that is different to the actual input present. Also, an output code of all logic “0”s can be generated when all of its inputs are at “0” OR when input D0 is equal to one.
One simple way to overcome this problem is to “Prioritise” the level of each input pin. So if there is more than one input at logic level “1” at the same time, the actual output code would only correspond to the input with the highest designated priority. Then this type of digital encoder is known commonly as a Priority Encoder or P-encoder for short.
The Priority Encoder solves the problems mentioned above by allocating a priority level to each input. The priority encoders output corresponds to the currently active input which has the highest priority. So when an input with a higher priority is present, all other inputs with a lower priority will be ignored.
The priority encoder comes in many different forms with an example of an 8-input priority encoder along with its truth table shown below.
Priority encoders are available in standard IC form and the TTL 74LS148 is an 8-to-3 bit priority encoder which has eight active LOW (logic “0”) inputs and provides a 3-bit code of the highest ranked input at its output.
Priority encoders output the highest order input first for example, if input lines “D2“, “D3” and “D5” are applied simultaneously the output code would be for input “D5” (“101”) as this has the highest order out of the 3 inputs. Once input “D5” had been removed the next highest output code would be for input “D3” (“011”), and so on.
The truth table for a 8-to-3 bit priority encoder is given as:
Where X equals “dont care”, that is logic “0” or a logic “1”.
From this truth table, the Boolean expression for the encoder above with data inputs D0to D7 and outputs Q0, Q1, Q2 is given as:
Output Q0
Output Q1
Output Q2
Then the final Boolean expression for the priority encoder including the zero inputs is defined as:
In practice these zero inputs would be ignored allowing the implementation of the final Boolean expression for the outputs of the 8-to-3 priority encoder. We can constructed a simple encoder from the expression above using individual OR gates as follows.
