Question

Design a four-input priority encoder. Assume that the encoder does not have an enable input.

It has an additional IDLE output that will be asserted when none of the inputs to the priority encoder
are asserted. You need to draw the circuit using AND, OR gates and inverters.

Answer 1

To design a four-input priority encoder without an enable input, create a circuit with four inputs that outputs two encoded signals representing the highest-priority input and an IDLE signal when all inputs are inactive, using AND, OR gates, and inverters.

Step-by-step explanation:

The student has asked for help in designing a four-input priority encoder with an additional IDLE output. A priority encoder is a circuit that includes multiple inputs but only the highest-priority input is encoded as the output. In our case, we have four inputs, let's label them I3 (highest priority), I2, I1, and I0 (lowest priority).

Here is a step-by-step guide to designing the circuit:

1. Create an output for IDLE which is an OR gate that takes the inverse of all inputs. This output will be high only when all inputs are low.
2. We will have two outputs for the priority encoder, let's call them Y1 (MSB) and Y0 (LSB).
3. Design the output Y1 such that it will be high if either I3 or I2 is high, which can be achieved using an OR gate for inputs I3 and I2.
4. Output Y0 will be a little complex as it must be high when I3 is low and I2 is high or when I3 is low, I2 is low and I1 is high. This can be achieved using an AND gate for the inverse of I3 and original I2, then another AND gate for the inverse of I3, the inverse of I2 and original I1, and finally an OR gate to combine these two AND gates' outputs.
5. Connect all components correctly with AND, OR gates and inverters accordingly, ensuring that the logic implements the desired encoding scheme.

The IDLE output, alongside the Y1 and Y0 outputs, will correctly represent the highest-priority active input or indicate that all inputs are inactive.