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📝 N key Rollover of 8279

📖 From An Introduction To Microprocessor 8085 • 10. Programmable Keyboard and Display Interface: 8279 • 4 questions available

Practice MCQs for N key Rollover of 8279. Test your knowledge with carefully crafted questions across easy, medium, and hard difficulty levels.

🔄 Last updated: 2026-07-30

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Easy Questions
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Medium Questions
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Hard Questions

📝 Sample Questions

Q1. A designer connects the 8253 to a keyboard matrix. Pin 11 (GATE 0) is wired to a row line, and pin 10 (OUT 0) drives an LED. Which condition will cause the LED to change state when a key in that row is pressed?

🔹 A. The LED toggles whenever the GATE 0 line goes low, regardless of clock input.
🔹 B. Pressing a key that generates a clock pulse on CLK 0 will increment the counter, causing OUT 0 to toggle.
🔹 C. The LED changes only when the counter reaches its terminal count, independent of key presses.
🔹 D. The LED never changes because OUT 0 is unrelated to the keyboard matrix.

💡 Difficulty: easy | ✅ Correct: B

Q2. To write a mode word for counter 2, the CPU must assert specific control signals. Which combination of CS, RD, WR, A1, and A0 will correctly load the mode word into counter 2 without affecting counters 0 or 1?

🔹 A. CS = 0, RD = 1, WR = 0, A1 = 1, A0 = 0
🔹 B. CS = 0, RD = 1, WR = 0, A1 = 1, A0 = 1
🔹 C. CS = 0, RD = 0, WR = 1, A1 = 1, A0 = 0
🔹 D. CS = 0, RD = 1, WR = 0, A1 = 0, A0 = 1

💡 Difficulty: medium | ✅ Correct: D

Q3. Examine Table 9.2. If CS is low, RD is high, WR is low, and both A1 and A0 are high (an undefined state), what operation will the 8253 perform?

🔹 A. It will load counter 2 with the supplied data.
🔹 B. It will write the mode word to the control register.
🔹 C. It will perform no operation and enter a three‑state condition.
🔹 D. It will read the current count from counter 2.

💡 Difficulty: hard | ✅ Correct: A

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🔗 Related Topics

📝 INTEL Programmable Keyboard/Display Interface 8279📝 Block Diagram of 8279📝 Functional Description of 8279📝 Key Board San of 8279📝 Scanned Keyboard of 8279
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