OR Gate

The OR gate outputs HIGH (1) when at least one of its inputs is HIGH. The output is LOW only when all inputs are LOW. Think of it like a parallel circuit with two switches: the light turns on when either switch is closed.

Objectives

• Understand the OR gate truth table
• Recognize OR gate schematic symbols
• Write Boolean expressions using OR notation
• Apply OR gates for combining signals
• Understand De Morgan's theorem relating OR and AND

Key Takeaways

• OR outputs HIGH when ANY input is HIGH
• Output is LOW only when ALL inputs are LOW
• Notation: A + B, A | B, A OR B
• Behaves like switches in parallel
• Common IC: 74HC32 (four 2-input OR gates)
• OR and AND are duals — related by De Morgan's theorem

Applications

• Interrupt Combining: Multiple interrupt sources ORed to single CPU interrupt
• Alarm Systems: Any sensor trigger sets off alarm
• Bit Setting: Set specific bits by ORing with a mask
• Default Values: Use primary signal OR fallback signal

Practice Problems

Problem 1: What is 0 OR 1?

Problem 2: How many rows have output=0 for a 4-input OR?

Problem 3: Apply De Morgan to NOT(A + B)