Embedded Controller Design, W00 Problem Solutions 9 Solutions to Chapter 5 Problems 1. If the design of Figure 5-7 needs to be changed to eliminate the duplication of addresses caused by partial address decoding, how many additional input signals would be required for the decoder? In order to implement complete address decoding, using a single address for both the input (switch) port and the output (LED) port, all 16 address lines must be used to select the I/O port. Since three lines are used (A13, A14, A15 to the A,B,C inputs of the '138 decoder), and there are 16 address lines total (A0-15), 16-3 = 13 additional address lines must be decoded in order to change the design so that one and only one address is used for the memory mapped I/O port in Figure 5-7. If the address FFFF hex is decoded using a single gate, one would need a 16 input NAND gate, which has an active low output only when all 16 address line inputs are high (address = FFFF hex). 2.
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