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An 8 Kbyte ROM with an active low Chip Select input (CS) is to be used in an 8085 microprocessor based system. The ROM should occupy the address range 1000H to 2FFFH. The address lines are designated as A15 to A0, where A15 is the most significant address bit. Which of the following logic expressions will generate the correct CS signal for this ROM ?
- A15 + A14 + (A13 · A12 + A13̅ · A12̅)
- A15̅ · A14̅ · (A13 + A12̅)
- A15̅ · A14̅ · (A13̅ · A12̅ + A13̅ · A12)
- A15̅ + A14̅ + A13̅ · A12
Correct answer: A15 + A14 + (A13 · A12 + A13̅ · A12̅)
Solution
The 8KB span 1000H-2FFFH requires A15=0, A14=0 and A13 different from A12 (01 or 10). The active-low CS is A15+A14+(A13.A12 + A13'.A12'), which equals 0 (selected) exactly over that range; verified by full truth table.
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