The parallel bus arbitration technique uses an external priority encoder and a…
2019
The parallel bus arbitration technique uses an external priority encoder and a decoder. Suppose, a parallel arbiter has 5 bus arbiters. What will be the size of priority encoder and decoder respectively?
- A.
4×2,2×4
- B.
2×4,4×2
- C.
3×8,8×3
- D.
8×3,3×8
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Correct answer: D
Answer: Priority encoder = 8×3, Decoder = 3×8.
Reason: For 5 bus arbiters we need enough output bits to uniquely represent each requester. Compute the number of output bits as ceil(log2(5)) = 3. Therefore the encoder must produce 3 output lines.
Step 1: Number of output bits = ceil(log2(5)) = 3.
Step 2: Choose encoder inputs as the next power of two ≥ 5, which is 8. So use an encoder with 8 inputs and 3 outputs (8×3).
Step 3: The decoder is the inverse mapping of those 3 bits back to outputs, so it is 3 inputs to 8 outputs (3×8). Only 5 of the 8 decoded lines will be used in practice.
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