Question 3.1: Compute the size of the ROM required to implement an 8-to-3 ...
Compute the size of the ROM required to implement an 8-to-3 priority encoder.
An encoder performs the inverse function of a decoder. An 8-to-3 priority encoder is illustrated in Figure 3-6. If input y_{i} is 1 and the other inputs are 0, then the abc outputs rep- resent a binary number equal to i. An additional output d is used to indicate invalid outputs. A value of 1 on bit d indicates that the output bits a, b, and c are valid. If more than one input is 1 in a priority encoder, the highest numbered input determines the output. The truth table in Figure 3-6 illustrates the output combinations for each input combination. The X_{s} in the truth table indicate “don’t cares.” As illustrated, the 8-to-3 priority encoder has 8 inputs and 4 outputs. Hence, it needs a 2^{8}\times 4 bit ROM.
Comment: There will be 256 entries in this ROM. When all the “don’t cares” in the truth table in Figure 3-6 are expanded, it does result in 256 entries.
