NOR Gate
A NOR gate inverts the OR function: output is 1 only when every input is 0, otherwise 0. Like NAND, NOR is functionally complete and can build any other gate. The 74HC02 is the canonical CMOS quad 2-input NOR.
Two PMOS transistors stack in series between the output and Vₒₒ, with two NMOS in parallel between the output and ground. Both inputs LOW conducts the PMOS chain and pulls the output high; any input going high turns on at least one parallel NMOS and pulls the output to 0. The PMOS stack means NOR's HIGH output drive is weaker than NAND's, which is why CMOS standard-cell libraries usually prefer NAND for critical paths.
In plain terms
A motion-sensor light that stays ON only while the room is completely empty: any motion at all turns it off. Its output is 'all clear' just when no input is asserting itself.
Why designers use it
- Build the second universal-gate version of any Boolean expression, often used as the de Morgan dual of a NAND-based design.
- Form the SR latch alternative built from two cross-coupled NORs (used in the original NMOS 6502 microprocessor).
- Detect 'all inputs idle' on a quiet bus, asserting a low-power-mode signal when no traffic is present.
- Implement a CMOS reset network where an OR-then-invert is replaced by a single NOR for fewer transistors.
Best for
- Universal logic
- SR latches
- Idle detection
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- For high-fan-in designs — the series PMOS stack is much slower than series NMOS in the same process, so wide NORs hurt timing more than wide NANDs.
- When positive-true OR is the natural sense — don't add an inverter just to cancel an unwanted polarity.
Common mistakes
- Pulling an unused NOR input to Vₒₒ instead of GND — the NOR identity is 0, so unused inputs must tie to ground or the gate locks output low.
- Replacing a 74HC32 OR with a 74HC02 NOR plus an inverter ‘because NOR is faster' — the savings vanish once the inverter delay is added back.
Where you will find it
- The MOS Technology 6502 microprocessor (1975) used cross-coupled NMOS NOR latches as its register file flip-flops: every bit in the accumulator and X/Y registers was held by a pair of NORs, a topology copied directly into the Apple II, Atari 2600, and Commodore 64 — making NOR latches arguably the most-deployed flip-flops of the 8-bit era.
- A satellite power-conditioner board uses a wide NOR to detect 'all rails are at 0 V': during launch the spacecraft must remain electrically silent, and the NOR output is the only signal allowed to go HIGH if and only if every rail reads zero, gating the wakeup beacon.
- A modular synthesiser's gate-combine module uses 74HC02 NORs to detect rests in a sequence: when no key on any of four channels is pressed, the NOR output triggers a release envelope, producing musical phrasing with discrete logic instead of a microprocessor.
A short history
A NOR gate performs the inverse of OR: its output is HIGH only when all its inputs are LOW. If any input is HIGH, the output is LOW. NOR gates can be combined to implement other logic functions, making them useful building blocks for larger digital circuits.